Chapter 57 Small Intestine

Structure and Function

The small intestine (SI) is, in essence, an interface between the external environment and the body, and is both an absorptive surface and a barrier; it must digest and absorb nutrients while excluding antigens and microbes and eliminating fecal waste. It faces a frequently changing dietary and bacterial intake, and yet has to maintain a dynamic but balanced microflora within its lumen while being intermittently exposed to pathogens. All of its functions—mixing and propulsion, secretion, digestion, absorption, regulation of blood flow, immunologic reaction and tolerance, and elimination—are fully integrated through both local and remote neuroendocrine and immunologic mechanisms (see Chapter 1). It thus has a complex task and requires specialized anatomic arrangements to perform them.

Gross Structure

Anatomic Regions

The SI is basically a tube, beginning at the pylorus of the stomach and ending at the ileocolic valve. However, this tube is ultimately in continuity with the external environment, proximally from the mouth via the esophagus and stomach, and distally to the anus via the large intestine (Figure 57-1, A).¹^–³ It is relatively short, reflecting the typical dietary intake of cats and dogs. It is approximately 1 to 1.5 meters long in adult cats and ranges from 1 to 5 meters in adult dogs, in proportion to the size of the individual. It is divided arbitrarily into three anatomic segments: the duodenum proximally, then the jejunum, and finally the ileum distally (see Figure 57-1, A).

Figure 57-1 Functional anatomy of the small intestine.

A, Anatomic arrangement of the small intestine. B, The small intestine is basically a tube with a serosal surface covered by visceral peritoneum and an inner absorptive and digestive surface, the mucosa. C, Beneath the outer serosa, longitudinal and circular muscle layers produce peristaltic and segmental contractions for propelling and mixing the luminal contents. The submucosa is rich in blood and lymphatic vessels. The mucosa comprises the thin muscularis mucosa, the lamina propria, and the columnar epithelium; it is thrown into folds and is covered by finger-like villi to increase the digestive and absorptive surface area. D, Enterocytes, which are shed from the villus tip but are continually replaced through division of crypt cells, are the site of nutrient digestion and absorption. Goblet cells secrete protective mucus. Water-soluble nutrients pass into the rich capillary network of the lamina propria, and fat is passed as chylomicrons into the lacteals. Immunocytes in the lamina propria are involved in maintaining tolerance to luminal antigens. E, The luminal membrane of the enterocyte is thrown into processes called microvilli, which increase the luminal surface area. Tight junctions between enterocytes maintain epithelial integrity. Absorbed nutrients are passed from the enterocyte into the intercellular space for distribution to the body. F, Schematic of a microvillus showing digestive hydrolases anchored in the phospholipid cell membrane and protruding into the intestinal lumen. Carrier proteins in the membrane are believed to act as “pores,” shuttling nutrients across the membrane by means of conformational changes in their structure often induced by sodium influx at the expense of energy utilization through Na/K-adenosine triphosphatase (ATPase) on the basolateral membrane.

(From Hall EJ: Small intestinal disease. In: Gorman NT, editor: Canine Medicine and Therapeutics, ed 4, Oxford, UK, 1998, Blackwell Science, p 488.)

Duodenum

The first part of the SI, the duodenum, comprises approximately 10% of its total length. It passes from the pylorus dorsally and to the right, at the level of the ninth intercostal space, and is immobilized by the hepatoduodenal ligament. It then turns caudally into the descending duodenum in contact with the right flank, turning again at the caudal flexure near the pelvic brim. It is in close association with the common bile duct and the head and right limb of the pancreas, which lie in its mesentery.

The common bile duct and one pancreatic duct enter the duodenum via the major papilla. In dogs an accessory pancreatic duct often enters at a minor papilla more distally and slightly more ventrally (Figure 57-2, A), but there is a range of variations in the actual number of ducts and their drainage pattern from the pancreas (see Chapter 60). The papillae are notable endoscopic landmarks in dogs, but may not be obvious in cats.

Figure 57-2 Videoendoscopic appearance of the normal upper small intestine.

A, The major duodenal papilla (M) in the duodenum of the dog is the site of entry of the common bile duct and major pancreatic duct. The minor duodenal papilla (m) is seen in some but not all dogs distal to the major papilla and approximately 100 degrees clockwise from it. B, Normal descending duodenum in a dog; the distal flexure is visible in the distance. C, Peyer patches (lymphoid aggregates) in the duodenum appear as pale oval depressions along the antimesenteric border of the descending duodenum.

(Reprinted with permission from Lhermette P, Sobel D: BSAVA Manual of Canine and Feline Endoscopy and Endosurgery. Gloucester, UK, 2008, BSAVA Publications.)

The distal duodenal flexure, where the duodenum courses to the left side of the abdomen (see Figure 57-2, B) is often at the limit of the reach of a standard 1-meter gastroscope, except in cats and small dogs. In dogs the antimesenteric side of the duodenum is marked by a line of whitish, mucosal depressions signifying the presence of specialized lymphoid areas, the Peyer patches (see Figure 57-2, C). Secretory Brunner glands and annular mucosal folds are features of the human proximal duodenum, but are not present in dogs and cats. After the distal duodenal flexure, the ascending limb of the duodenum crosses the midline and ends at the level of L6 close to the root of the mesentery near the left kidney, with a mesenteric attachment to the colon, the duodenocolic ligament.

Intestinal Compartments

Microflora

The microflora of the SI is an integral part of its structure and function. There is a gradual increase in bacterial numbers and a shift from aerobic to anaerobic organisms progressing distally down the SI. Chapter 2 provides a more detailed description of the composition of the microflora and its interaction with the mucosa.

Mucosa

The SI mucosa performs the intestinal barrier and absorptive functions, and is comprised of an intestinal epithelium covering the lamina propria that hosts the local mucosal immune system, and is surrounded by the submucosa and the outer muscle layers.

One of the most important structural modifications of the mucosa is a vast increase in its surface area relative to the size of the animal, with an almost 600-fold increase compared with the basic tubular structure of the intestine. The surface area of the human intestine has been estimated at 175 m², and although the adult human intestine is longer than in even the largest dog, the villi in cats and dogs are almost twice as long (approximately 1 mm) compared with those of humans. The increase in surface area is created by folds in the mucosal wall (tripling the surface area), villus projections into the intestinal lumen (providing an approximate 10-fold increase), and microvilli on the surface of each epithelial cell (providing a further 20-fold increase in area) (see Figure 57-1, C). Diseases causing villus atrophy or even just microvillus damage are likely to produce profound malabsorption and diarrhea.

Gut-Associated Lymphoid Tissue

The GI tract is the largest immunologic organ in the body, and the SI comprises a large component of the mucosal immune system. Within the SI, the Peyer patches (see Figure 57-2, C) act as inductive sites and are covered with a specialized epithelium containing microfold (M) cells, which sample luminal antigens. Activated lymphocytes migrate via mesenteric lymph nodes to the circulation, from where they home to their effector sites, the lamina propria and epithelium. Chapter 3 details the structure and role of the gut-associated lymphoid tissue.

Microstructure

An identical, basic, tubular, cross-sectional structure is present throughout the length of the SI (see Figure 57-1, C): the external serosa surrounds the muscularis, submucosal and mucosal layers which are present throughout, and can be detected ultrasonographically (Figure 57-3).⁴^–¹⁰ A very narrow hyperechoic interface between the lumen and mucosal surface is usually visible above the four true layers: (a) a slightly hypoechoic mucosa, (b) hyperechoic submucosa, (c) hypoechoic muscularis, and (d) brightly hyperechoic serosa.

Figure 57-3 Ultrasound Image of the Small Intestine.

Abdominal ultrasound image showing transverse image of three loops of bowel in a dog, with normal layering of the small intestinal wall.

(From Ettinger SJ and Feldman EC, editors: Textbook of Veterinary Internal Medicine, ed 7, Philadelphia. 2010, Saunders, p 1541.)

Regional variations in the relative proportions of each layer reflect differences in the functions of the proximal, middle, and distal regions. The mucosa is thickest in the duodenum (normal dog ≤6 mm) and thinnest in the ileum (normal dog ≤4.7 mm). Variations in the microstructure also occur with species and age and within individual animals depending on their dietary intake, as well as disease. Submucosal Brunner glands are found in the human duodenum but not in dogs and cats. Loss of normal ultrasonographic layering is suggestive of neoplastic infiltration, and echogenic mucosal striations may indicate lymphatic dilation.

The mucosal layer is responsible for secretion and absorption as well as being a barrier to the luminal environment. The submucosa, between the muscularis mucosa and muscularis, provides connective tissue support and delivers blood vessels, nerves, and lymphatics. Within the muscularis, the outer longitudinal and inner circular muscular layers provide propulsive and segmental peristaltic contractions that mix chyme and ultimately propel it aborally. Neural plexuses are found between the muscle layers (the myenteric or Auerbach plexus) and in the submucosa (Meissner plexus), and communicate with all layers of the intestinal wall. They help coordinate intestinal motility and secretory activity, and even mucosal immune responses (see Chapters 1 and 3).

Mucosa

This is the most important layer of the intestine clinically. It is comprised of the epithelium and lamina propria overlying the muscularis mucosa, and is modified by gross folds and the villi (see Figure 57-1, C). The muscularis mucosa is a thin sheet of smooth muscle, from three to 10 cells thick, separating the mucosa from the submucosa. Smooth muscle branches within the villus lamina propria enable shortening and lengthening movements of the villi.

The lamina propria is a continuous connective tissue space bounded by the muscularis mucosa below and the epithelium above, and contains aggregates of lymphoid tissue, and nonaggregated immunocytes (see Chapter 3), enteric neurons, and blood and lymphatic vessels. A central lymphatic vessel (lacteal) within each villus drains chylomicrons into intestinal lymphatics and ultimately to the cisterna chyli.

Blood flow to a villus is provided by an arteriole that passes to the tip of the villus where it arborizes and forms a subepithelial capillary network that drains into veins. Crypts are supplied by separate arterioles and blood flow in the two regions can be controlled independently. Mucosal capillaries are fenestrated, and in conjunction with the lacteal, carry away protein-rich tissue fluid. Loss of epithelial integrity permits leakage of the protein-rich fluid and the development of a protein-losing enteropathy (PLE).

Crypt-Villus Unit

A group of crypts and their associated villus comprise the functional unit of the SI (see Figure 57-1, D).¹¹ Crypts are continually replenished by cell division, producing undifferentiated epithelial cells. It is estimated that there are between four and 40 stem cells per crypt in the adult intestine, with further division of daughter cells occurring as the cells pass up the crypt. As the crypt cells pass through a maturation zone they undergo a final division and differentiate into immature epithelial cells. The predominant epithelial cell type is the enterocyte, but as a number of crypts may supply the enterocytes to one villus, each villus epithelium may consequently represent a polyclonal cell population.

Mucosal Epithelium

The intestinal surface is covered by a monolayer of polarized epithelial cells; their luminal surface is structurally and functionally distinct from their basolateral membrane.¹²^–¹⁸ The epithelial basement membrane is readily permeable to nutrients, but has an important role as the structural matrix on which the epithelium grows. It expresses glycoproteins, called laminins, that interact with integrins, transmembrane recognition molecules expressed by epithelial cells. These interactions promote cell adhesion, growth, polarization, and differentiation. Enterocyte differentiation during migration up the villus may be programmed, but is likely also to be modulated by the expression of different integrins at different sites on the crypt–villus axis. Communication between epithelial cells is mediated by E-cadherin, a transmembrane molecule, linked to intracellular catenins, proteins that transmit signals to the actin cytoskeleton and to intracellular growth control pathways.

A mucosal barrier is formed by the intestinal epithelium (Box 57-1). This barrier depends on intercellular tight junctions between enterocytes, encircling their lateral aspects and excluding antigens and bacteria. Effete enterocytes are shed from the villus tip by a mechanism that maintains the mucosal barrier (see Figure 57-1, D). Studies in rodents suggest intercellular bridges develop between neighboring enterocytes below the effete cell before it is shed, thus maintaining mucosal integrity. However, epithelial integrity is likely to be altered in some intestinal diseases, and the integrity of the tight junctions is actually least in the crypts, where fluid secretion occurs. There is an association of cryptal lesions with the development of PLEs.

Box 57-1

Components of the Intestinal Mucosal Barrier

Protein denaturation by gastric acid

Protein degradation by proteolytic enzymes and bacteria

Clearance of waste by peristalsis

Unstirred water layer

Surface mucus layer

Secretory immunoglobulin A

Enterocyte microvillus membrane

Epithelial tight junctions

Mucosa-associated lymphoid tissue

Crypt cells have a potent secretory capacity and the crypts are the site of most mucosal fluid secretion. As enterocytes migrate to the villus tip, maturation involves loss of secretory activity and the expression of digestive and absorptive molecules in the apical (luminal) cell membrane. Some enterocytes undergo stochastic (random) cell death, but the majority undergoes apoptosis via a caspase-dependent process, and exfoliates at the tips of the villi. The duration of migration from crypt to villus tip is believed to be 3 to 5 days in dogs and cats. More rapid transit may occur in diseases where cells are lost and compensatory crypt activity occurs, but the new enterocytes tend to be functionally immature.

Enterocytes predominate in the epithelium, representing approximately 80% of all cells, with interspersed mucus-secreting goblet cells. Goblet cell density in the SI mucosa varies, being highest in the ileum. These cells secrete protective mucus and some novel clover leaf–shaped peptides (trefoil peptides) that act as growth factors. Paneth cells, a population of cells found in some species below the proliferation zone in crypts and that secrete antibacterial peptides, are not recognized in dogs or cats. Endocrine- and paracrine-secreting cells (synonyms: enteroendocrine, enterochromaffin, argentaffin, argyrophil cells) are also present in the mucosal surface layer, and have important trophic and functional activities.

In addition to locally produced growth factors, a variety of luminal and humoral factors act as physiologically active growth regulators. Receptors for epidermal growth factor (EGF) are found on the luminal and basolateral surfaces of enterocytes, suggesting that they may respond to bloodborne EGF and to EGF secreted into the lumen by salivary and pancreatic tissue or delivered in milk. Transforming growth factor (TGF)-α, a polypeptide related to EGF and expressed throughout the mucosa, has growth regulatory properties. However, EGF and TGF-α are also probably important in repair of damaged epithelium as they stimulate repair without fibroblast activity, unlike TGF-β, which inhibits epithelialization and stimulates fibroplasia, and is important in deeper wound repair.

In the Peyer patches, enterocytes overlying lymphoid aggregates are modified into follicle epithelium and M cells, probably in response to signals from underlying lymphoid cells. The M cells sample the luminal contents and help present antigen to the mucosal immune system (see Chapter 3).

Enterocytes

Enterocytes contain the intracellular organelles, such as mitochondria, lysosomes, and endoplasmic reticulum, common to all cells, and which support normal cellular functions. However, enterocytes also perform specific digestive and absorptive functions.^19,²⁰ Enzymes expressed on the surface of enterocytes perform terminal digestion of polysaccharides and peptides in conjunction with luminal hydrolysis of food polymers by pancreatic enzymes. The enterocytes then absorb the simple nutrients. These functions depend on the polarity of the enterocyte, involving a specialized portion of the cell membrane on the luminal surface, the microvillar membrane (MVM). The microscopic appearance of the MVM is the basis of its alternative name, the “brush-border” (see Figure 57-1, E and F). It consists of thousands of parallel cylindrical processes (microvilli) bearing the digestive enzymes and specific carrier proteins.

The MVM is a phospholipid bilayer that has specific proteins inserted into it. Enzymes responsible for the terminal stages of carbohydrate and protein digestion are usually anchored in the MVM by a small hydrophobic terminal and have an active site exposed to the intestinal lumen (see Figure 57-1, F). Specific carrier proteins traverse the MVM or basolateral membrane and, through conformational changes, shuttle nutrients into and out of the enterocyte across the cell membrane. The maximal brush-border enzyme and transport activities are expressed in the mid-villus region. Diseases damaging enterocytes often accelerate cell production and the more immature enterocytes are not as effective functionally.

Brush-border enzyme activities are highest in the proximal SI and decline in an aborad gradient. Digestive enzymes, especially disaccharidases and transport proteins, may be inducible in response to changes in the composition of the diet. This has been shown in dogs but not in cats, perhaps reflecting the obligate carnivore status of cats. Indeed dogs, which are omnivorous, show increased sucrase and maltase in response to increased dietary carbohydrate and, conversely, when fed a cereal-free diet develop reduced activities of brush-border sucrase and maltase but not lactase. Thus a sudden change in diet in dogs may cause diarrhea through transient intolerance until either existing enterocytes upregulate expression of specific enzymes and carriers, or new enterocytes expressing induced proteins differentiate, thus rendering the diarrhea self-limiting.

Enterocyte metabolism is geared toward the production of brush-border proteins and the transfer of nutrients and water from the lumen to the blood. Basolateral cell membranes export sodium from the cell via an energy-dependent N⁺-K⁺-ATPase. Water can follow osmotically, or compensatory sodium influx at the luminal surface can drive carrier-mediated nutrient absorption. Natural inhibition of glycolysis through expression of an alternate phosphofructokinase isoenzyme in enterocytes facilitates the transfer of glucose from the lumen to the blood. Gluconeogenesis is also inhibited, and so enterocytes can utilize ketone bodies. However their major energy source is actually glutamine (Figure 57-4). A surge in enterocyte glutamine metabolism during digestion is probably partly responsible for the postprandial rise in blood ammonia seen in some patients with hepatic dysfunction.

Figure 57-4 Metabolism of glutamine by enterocytes, and one potential mechanism for postprandial increases in endogenous ammonia.

Both major nutrients for enterocytes (glutamine and ketone bodies) are largely derived from the lumen, which helps explain the decline in villus structure, epithelial integrity, immune function, and absorptive function in starvation and anorexia. Consequently, attempting to maintain enteral nutrition, often by using glutamine-containing products, may be of clinical benefit.

Digestive and carrier proteins are synthesized by enterocytes and inserted in the MVM. This mechanism has been demonstrated for the synthesis of the enzyme complex of sucrase–isomaltase in pigs, but a similar process is likely to occur for this and other enzymes in dogs and cats. The sucrase–isomaltase complex is synthesized as a single polypeptide by ribosomal translation of its messenger RNA (mRNA). A terminal signal peptide extension that is ultimately cleaved, directs the intracellular trafficking of the protein from the ribosome to the endoplasmic reticulum and Golgi apparatus, where glycosylation of the protein backbone occurs. The glycosylated polypeptide is directed to the brush-border where it is inserted. It then “flips” across the apical membrane so that the active sites are on the luminal surface, and the polypeptide is anchored in the membrane by an N-terminal hydrophobic chain. The parts of the exteriorized polypeptide containing the sucrase and isomaltase activity are cleaved by pancreatic proteases but remain in close association, and form a dimer with another sucrose–isomaltase molecule.

As enterocytes migrate to the villus tip, enzymes are cleaved from the brush-border by bacterial and pancreatic proteases and are released into the lumen where they comprise solubilized enzyme activities, commonly called digestive juice. However, this liquid, the succus entericus, is not a true intestinal secretion.

Submucosa

Beneath the muscularis mucosa, the submucosa contains a heterogeneous population of cells: lymphocytes, plasma cells, macrophages, eosinophils, fibroblasts, and mast cells within a connective tissue matrix. An intricate network of blood vessels, nerve fibers, ganglia and interstitial cells of Cajal (Meissner plexus), and lymphatics supply the mucosa and muscularis.

Muscularis

Two muscle layers, the outer longitudinal and inner circular layers, encircle the submucosa. The intermuscular plane is a connective tissue layer bearing the myenteric (Auerbach) neural plexus. Ring contractions by the circular muscle and sleeve contractions by the longitudinal muscle may be tonic or rhythmic, and intestinal movements may be standing or migrating, allowing mixing and propulsion. Contraction of the muscle layers is coordinated by the enteric nervous system to produce peristaltic and segmental movements, with interstitial cells of Cajal acting as pacemakers. Chapter 1 details how intestinal motility is integrated with other functions of the SI.

Serosa

This is a single layer of mesothelial cells surrounding the intestine, and forms the visceral peritoneum.

Small Intestinal Function

The basic functions of the SI, that is digestion, absorption, and elimination, occur as a result of complex intercellular interactions between epithelial cells, immune cells, mesenchymal and neuronal cells and with luminal nutrients and microbes.²¹ The SI is also the largest immunologic organ in the body, interacting with the intestinal microbial flora and a diverse range of food antigens (see Chapters 2 and 3, respectively).

Digestion

To be transported across the MVM, major dietary constituents must be hydrolyzed from their initial polymeric structure into monomers. This digestive process is achieved within the SI lumen by mechanical disruption (in conjunction with bile salt emulsification of fats) that allows enzymatic hydrolysis of polysaccharides, proteins, and triglycerides.²²

The SI provides the optimum environment in terms of solute, temperature, pH, and mixing for the actions of bile salts and digestive enzymes, but most enzymes are secreted by the pancreas, and exocrine pancreatic insufficiency (EPI) is a major cause of malabsorption. The brush-border peptidase enterokinase (enteropeptidase) is important in the initial activation of pancreatic trypsin from trypsinogen by cleaving a terminal octapeptide, trypsinogen activation peptide, from the native protein.

Only terminal digestion of oligomers need normally be performed by brush-border enzymes. However, brush-border activities can partially compensate for the lack of secreted proteases and amylase in EPI, with at least 40% of dietary protein still being hydrolyzed, although severe fat maldigestion persists. Even with significant diffuse SI mucosal disease there is usually sufficient reserve capacity to enable adequate digestion of starch. However, early estimates of a 10-fold reserve capacity of digestive and absorptive activity have been refuted. The “reserve capacity” that is called into action after intestinal resection probably represents not only increases of brush-border protein expression, but also compensatory hypertrophy of the remaining tissue, as it is known to take months to reach maximal effect (see “Short Bowel Syndrome” section). Capacity appears to be regulated according to physiologic demand, and probably does not normally exceed twofold, but it is relevant that it can be modified in response to dietary change.

Carbohydrate

Starch and glycogen are the major carbohydrates in the diet and must be hydrolyzed completely to glucose for absorption (Figure 57-5, A). There is no salivary amylase activity in dogs and cats, and these complex carbohydrates are hydrolyzed by pancreatic α-amylase. Straight-chain starch molecules (amylose) are split to maltose, maltotriose, and some glucose. Branched-chain starch molecules (amylopectin) and glycogen are also hydrolyzed to the same products, except that the branched parts of their molecules remain as α-limit dextrins as their 1,6-glycosidic bond cannot be hydrolyzed by α-amylase. The digestion products of α-amylase are subsequently hydrolyzed, particularly by brush-border maltase (glucoamylase) and isomaltase (α-dextrinase). The brush-border enzyme trehalase hydrolyzes the 1,1 link in the fungal sugar trehalose, but is not expressed in cats.

Figure 57-5 Diagram of the digestion and absorption of (A) carbohydrate, (B) protein, and (C) fat.

BS, bile salts; DG, diglyceride; FA, fatty acids; MG, monoglyceride; PL, phospholipids; TG, triglyceride.

(Adapted from Batt RM: The molecular basis of malabsorption. J Small Anim Pract 21:555, 1980.)

Sucrose is an unusual constituent of canine and feline diets unless semimoist pet foods or human foods are fed. It is hydrolyzed directly at the brush-border to glucose by sucrase, part of the sucrase–isomaltase brush-border enzyme complex. Congenital sucrase–isomaltase deficiency is a rare genetic defect of man, but has not been recorded in small animals. Sucrase activity in cats is lower than dogs (Table 57-1), probably reflecting the average composition of their diets.

Table 57-1 Jejunal Brush-Border Disaccharidase Activities (U/mg Protein) Reported in Dogs and Cats

Lactose is found almost exclusively in dairy products and its hydrolysis to glucose and galactose by brush-border lactase is nutritionally most important in the nursing animal. At weaning, activities of lactase begin to decline, especially in cats, and adult animals may exhibit lactose intolerance if fed excess milk. This mirrors the age-related downregulation of lactase expression seen in certain human races. If animals have underlying SI disease, dairy products should be avoided as marginal lactase activities will be reduced even further. Absolute congenital lactase deficiency, as seen rarely in humans, has not been demonstrated in cats or dogs.

Protein

Digestion of proteins follows a similar overall pattern to carbohydrate digestion (see Figure 57-5, B), and the amounts of pancreatic enzyme secreted and mucosal peptidases expressed are influenced by the protein content of the diet. Digestion is initiated by acidic denaturation and the proteolytic activity of pepsin in the stomach. Luminal digestion under a more neutral pH is continued in the SI by pancreatic proteases (trypsin, chymotrypsin, elastase, and carboxypeptidase), which are initially secreted as inactive proforms, and are subsequently activated by enterokinase and trypsin. Luminal proteolysis results in a mixture of oligo-, tri-, and dipeptides as well as free amino acids. Oligopeptides are subsequently hydrolyzed by brush-border peptidases, which have some selectivity for particular amino acid residues. However, there is considerable overlap in specificity, and a selective deficiency of aminopeptidase N reported in dogs is of no clinical consequence. Furthermore, any tri- and dipeptides can still be absorbed on a brush-border carrier. Theoretically a deficiency of enterokinase could cause protein malabsorption through failure of trypsin activation, but this has never been documented in dogs and cats, and trypsin autoactivation would probably still occur.

Lipid

Fat digestion is completed entirely within the GI lumen by secreted enzymes and bile salts. Partial digestion is begun in the stomach by the action of gastric lipase secreted by gastric epithelial mucus cells. Subsequent mixing of the fat emulsifies it into small droplets. Further mixing with bile and pancreatic juice results in the formation of mixed micelles, which are approximately the size of the smallest fat droplet and solubilize approximately 1000 times more fatty acids. At the surface of mixed micelles, triglyceride is hydrolyzed by pancreatic lipase to di- and monoglycerides and free fatty acids (Figure 57-6). Maximal lipase activity in the gut lumen is dependent on a protein cofactor, colipase, which is secreted by the pancreas as inactive procolipase. There is some reserve capacity for fat digestion if pancreatic function is normal, and a fat-rich diet, especially one rich in unsaturated fatty acids, also stimulates increased pancreatic lipase secretion. However, neuroendocrine mechanisms initiated by the presence of fat in the duodenum and ileum, control the rate of gastric emptying and hence the rate of fat delivery. Thus a fat-rich diet or intestinal fat malabsorption delays gastric emptying.

Figure 57-6 Mechanisms of diffusion.

Pancreatic phospholipase A₂ is secreted in an inactive form, and once activated in the intestinal lumen hydrolyzes phospholipid to lysophospholipids. Finally, pancreatic cholesterol esterase deesterifies cholesterol. After fat absorption, the bile salts may form further mixed micelles, but ultimately are reabsorbed by a specific sodium-linked cotransporter in the ileum and recycled from the portal blood back into bile.

Nucleotides

Little is known of the digestion of dietary nucleotides and hydrolysis of nucleic acids released from exfoliated enterocytes. Ribonuclease and deoxyribonuclease are present in pancreatic secretions, and there is a common sodium-dependent brush-border carrier for the uptake of purines and pyrimidines.

Absorption

Digested Nutrients

Simple sugars, amino acids and oligopeptides, and fatty acids and other lipids are delivered to the body across the mucosal barrier and then via the lymphatics or bloodstream.^23,²⁴ Uptake occurs by passive diffusion or by active or facilitated carrier-mediated transport mechanisms (see Figure 57-6). Endocytosis of small, antigenic peptides is of no nutritional significance, but is involved in the neonatal absorption of colostral antibodies, and is crucial to the mucosal immune response.

The products of fat digestion are absorbed by passive diffusion from mixed micelles across the MVM and ultimately lipoproteins are passed into lacteals. The limiting factors, assuming normal pancreatic function, are the intestinal surface area and lymphatic functionality, and so villus atrophy and lymphangiectasia are likely to cause malabsorption of fat.

Mechanisms of Absorption

Passive Diffusion

Lipid-soluble products of digestion do not need a specific carrier mechanism to be absorbed across the mucosal barrier, and can bypass passive diffusion by “dissolving” in the brush-border membrane. This absorptive process obeys the Graham Law of Diffusion, being proportional to the concentration gradient across the membrane, nonsaturable, and limited only by the surface area of the membrane. Diffusion back across the brush-border is prevented by reesterification of free fatty acids within the enterocyte.

Carrier-Mediated Transport

Some small, nonpolar, water-soluble molecules (and perhaps water molecules) also appear to be absorbed by passive diffusion through “pores” in the mucosal membrane. The structure of these hypothetical pores is likened to that of ion channels in other membranes (i.e., small fixed channels through the membrane that function without a conformational change). Molecules with a molecular radius greater than 0.4 nm appear to be excluded, but larger, although numerically fewer pores may traverse tight junctions. Debate remains as to whether the predominant route is via an intracellular or paracellular route through tight junctions.

Water-soluble nutrient molecules that are too large to diffuse via the “pore route” and are insoluble in the lipid MVM must cross the brush-border on specific carrier proteins that bridge the membrane. Conformational changes in the carriers are thought to shuttle the substrate molecule across. The products of carbohydrate and protein digestion enter by this process. Transport proteins are usually highly specific and may only carry the L or D isomer of a molecule, but competitive inhibition with related solutes may occur. The number of carrier molecules in the mucosa is finite, so that the process is saturable, and although the expression of the carriers may be inducible, dietary overload is likely to cause transient intolerance and diarrhea.

Active transport of substrates across the MVM into the enterocyte is usually against a concentration gradient and energy must be expended to drive the process (see Figure 57-6). Usually the uptake of the nutrient is linked to the entry of sodium down its electrochemical gradient, with energy expenditure by a N⁺-K⁺-ATPase on the basolateral membrane of the enterocyte pumping sodium back out of the cell.

Facilitated transport is the carriage of substrates by a transport protein across the MVM, down a concentration gradient without energy expenditure (see Figure 57-6). Some sugars, oligopeptides, and folate are absorbed by this process. The number of carriers is finite, and the process saturable and subject to competitive inhibition.

Endocytosis

Small antigenic peptides may be engulfed nonspecifically within endocytotic vesicles of epithelial cells. The amounts absorbed by this route are negligible from a nutritional standpoint, but this sampling of luminal contents is crucial to the mucosal immune response (see Chapter 3). Receptor-mediated endocytosis enables the uptake of small amounts of a specific intact nutrient and is the mechanism of cobalamin absorption.

Nutrient Absorption

Carbohydrate

The main product of carbohydrate digestion, glucose, is absorbed by active transport on a stereo-specific carrier that recognizes a D-pyranose structure with a C-2 hydroxyl group. Glucose is cotransported with sodium; the energy required for the coupling is provided through the entry of sodium down a concentration gradient but with the basolateral N⁺-K⁺-ATPase reexporting sodium against the concentration gradient. The carrier molecule in the brush-border has been identified in many species, including dogs and cats, as the sodium-glucose cotransporter protein (SGLT1; Figure 57-7). This molecule has the highest affinity for glucose, but it is also the carrier for galactose. Indirect evidence for this is shown by the inability to absorb either sugar in people with glucose–galactose malabsorption in whom a single amino acid mutation (Asp_₂₈ → Asn_₂₈) in the SGLT1 protein has been identified. Glucose and galactose thus may exhibit competitive inhibition, but glucose is the major substrate. There is circumstantial evidence for another aldohexose carrier in cats.

Figure 57-7 Diagram of the absorption of monosaccharides by enterocytes. GLUT, Glucose transporter; SGLT, sodium-glucose co-transporter protein.

Facilitated transport of glucose across mammalian cell membranes is performed by a family of facilitated glucose transporters (GLUTs) with different isoforms found in different tissues. One member of this family, GLUT2, is found on the basolateral membrane of enterocytes, where it shuttles glucose, galactose, and fructose out of the enterocyte by facilitated diffusion (see Figure 57-7). GLUT2 is absent from the brush-border, and so a mechanism exists for active transport of glucose across the MVM into the enterocyte by SGLT1 and facilitated transport into the body by GLUT2. Most of the glucose is not used within the enterocyte, because of expression of a phosphofructokinase isomer that directs metabolism away from glycolysis.

Another member of the facilitated glucose transporter family, GLUT5, is found on the brush-border. It shares homology with other family members but actually allows facilitated diffusion of fructose; it is not even competitively inhibited by glucose. In humans, GLUT5 is also probably the site of D-xylose absorption, as both fructose and xylose absorption are unaffected in glucose–galactose malabsorption. However, the mechanism of D-xylose uptake is species dependent, and evidence for facilitated diffusion in dogs and cats has largely been extrapolated from humans. Nevertheless, fructose uptake in cats is low, and D-xylose absorption is equally low. This may be one reason why the xylose absorption test is unhelpful in cats. However, potential fructose malabsorption has little clinical relevance in cats as the feline diet likely contains little fructose.

Although dietary carbohydrates must be hydrolyzed to monosaccharides to be absorbed and be nutritionally useful, a small but measurable amount of disaccharide can cross the brush-border, probably through leaky tight junctions. This is of no nutritional significance, but increased uptake and subsequent urinary excretion of disaccharides can be a marker of increased intestinal permeability when damaged.

Protein

The products of protein digestion are absorbed on carriers that are stereo-specific for L-amino acids (Figure 57-8; also see Figure 57-5, B).²⁵ Sodium-linked active transport is responsible for free amino acid uptake via one of four different carriers that have a variable degree of selectivity for neutral (Gly, Ala), acidic (Asp, Glu), basic (Arg, Lys), and imino (Pro, HO-Pro) amino acids. The cat has the highest rate of uptake of basic amino acids perhaps because it has an essential requirement for arginine.

Figure 57-8 Diagram of the absorption of di- and tripeptides by enterocytes. Pept 1, a peptide carrier.

Traditionally, peptide uptake has been considered to be facilitated diffusion, with the concentration gradient being maintained by intracellular peptide hydrolysis, and only free amino acids being exported from enterocytes into the portal blood (see Figure 57-8). A single carrier for di- and tripeptides with no selectivity for their amino acid content has been demonstrated. However, in people this peptide carrier, Pept-1, is involved in the active influx of peptides, being linked to the influx of H⁺ down an electrochemical gradient. The protons are exchanged across the MVM with sodium, which is pumped out by the basolateral N⁺, K⁺ ATPase. A mixture of peptides and free amino acids is exported to the blood, but it appears that peptides are absorbed more readily than free amino acids. This has clinical significance as the inclusion of dipeptides in elemental diets has a theoretical advantage over simple amino acid solutions. This transport protein is also the carrier for peptidomimetic drugs such as β-lactams and angiotensin-converting enzyme inhibitors.

Lipid

The products of fat digestion are absorbed by passive diffusion from mixed micelles into lacteals (see Figure 57-5, C). The limiting factors, assuming normal pancreatic function, are the intestinal surface area and lymphatic functionality, and so villus atrophy and lymphangiectasia are likely to cause malabsorption of fat.

Generally, the products of fat digestion are reassembled within enterocytes to prevent rediffusion back out of the enterocyte. They are combined with synthesized lipoproteins for passage into the lymphatics as chylomicrons. However, medium-chain triglycerides (length = 8-12 carbon atoms) can be absorbed directly into the portal blood, and provide an alternative route for fat uptake when lymphatic flow is impaired, as in lymphangiectasia. However, evidence exists that a proportion of medium-chain triglycerides are absorbed into the lacteals as they can be found within the thoracic duct.

Fat-Soluble Vitamins

Dietary fat-soluble vitamins A, D, E, and K are solubilized in mixed micelles before passive diffusion across the brush-border. Fat malabsorption associated with inadequate amounts of bile salts (e.g., bile duct obstruction), lymphangiectasia, or severe villus atrophy is also likely to result in vitamin deficiency. This is clinically most relevant for vitamin K as its body stores are not large and, particularly in cats, can lead to vitamin K–dependent coagulation factor deficiencies.

Vitamin A (retinol) is ingested either as a dimer (beta-carotene) or as an ester that must be hydrolyzed by pancreatic esterases. Beta-carotene is absorbed directly from micelles, but retinol is insoluble and must be anchored by a binding protein before absorption. Subnormal serum vitamin A concentrations have been observed in dogs with EPI but no associated signs of deficiency have been reported. However, vitamin A supplementation has been recommended following surgery in animals subsequently treated with corticosteroids to aid wound healing, and would be particularly relevant in animals that have had surgical intestinal biopsies.

Vitamin D is absorbed from mixed micelles. It is important for calcium homeostasis, controlling calcium absorption from the gut, as well as renal excretion. Vitamin D malabsorption may be (partly) responsible for the reductions in serum ionized calcium and magnesium that are reported in protein-losing enteropathies, which cannot be due to reduced protein binding because of hypoalbuminemia.

Vitamin E (α-tocopherol) is absorbed from mixed micelles by passive diffusion, and passes into the lymphatics unchanged. Vitamin E deficiency has been reported in Beagles with severe malabsorption, and is associated with EPI and bacterial overgrowth in German Shepherd dogs.

Vitamin K is derived from dietary sources (K₁) and synthesis by the enteric flora (K₂). Vitamin K₂ is probably absorbed in the ileum and colon. As well as bile salt deficiency, prolonged antibiotic usage may result in vitamin K deficiency.

Water-Soluble Vitamins

Water-soluble vitamins B and C are absorbed by a mixture of passive diffusion (e.g., pyridoxine [B₆] and C), saturable facilitated transport (e.g., riboflavin [B₂]), or active and facilitated transport (e.g., thiamine [B₁]) in other species, but the mechanisms in cats and dogs are uncertain. The absorptive mechanisms for folic acid and vitamin B₁₂ are more complex and important clinically as they may be helpful in determining the site and nature of intestinal disease.

Folic acid is present in adequate amounts in most commercial foods, but is also produced by the enteric flora. It is usually conjugated in a poorly absorbable polyglutamate form and must be hydrolyzed by folate deconjugase, a brush-border enzyme, before absorption. Folate (pteroyl monoglutamate) is absorbed by a carrier-mediated process at low luminal concentrations and by passive diffusion at high concentrations (Figure 57-9). After absorption folate is methylated in the cell to form methyltetrahydrofolate.

Figure 57-9 Diagram of the absorption of folate and cobalamin.

Folate is absorbed in the proximal SI by means of carrier-mediated diffusion. Dietary cobalamin is initially protected from digestion by R proteins, and is then absorbed in the ileum through receptor-mediated endocytosis bound to intrinsic factor (IF).

(From Ettinger SJ, Feldman EC, editors: Textbook of Veterinary Internal Medicine, ed 7, Philadelphia, 2010, Saunders, p 1528, Figure 270-1A.)

Vitamin B₁₂ (cobalamin) is absorbed by receptor-mediated endocytosis in the ileum (see Figure 57-9), but the process is complex so that intact cobalamin is absorbed and potentially harmful analogues are excluded. Following ingestion, cobalamin is released from food in the stomach and then bound by R proteins (haptocorrins), which are nonspecific binding proteins of salivary and gastric origin. At acidic pH, cobalamin has high affinity for R proteins, but on entering the more alkaline environment of the SI, R proteins bind cobalamin less avidly and undergo proteolysis. Thus cobalamin is transferred to another binding protein, intrinsic factor, which promotes cobalamin absorption in the ileum. The source of intrinsic factor is the stomach and pancreas in dogs and solely the pancreas in cats. Intrinsic factor–bound cobalamin complexes pass to the ileum until they bind specific receptors and are endocytosed. Cobalamin is passed into the portal blood where it is bound to a protein, transcobalamin 2, enabling it to enter tissues and to be reexcreted in bile. Inherited abnormalities of the cobalamin–intrinsic factor receptor in breeds such as the Giant Schnauzer and Border Collie cause selective cobalamin deficiency.

Minerals

Zinc and copper are absorbed via divalent cation transporters. There is competition for binding, while intracellular binding of copper by metallothionein is also part of the normal homeostatic mechanism, as the copper is trapped within effete enterocytes and shed.

Iron is absorbed by the duodenum and proximal jejunum both as heme and nonheme iron. Heme, found largely in meat, is better absorbed as it is unaffected by other dietary constituents or intraluminal factors. Gastric acid and chelation with mucopolysaccharide, ascorbate, and citrate help maintain iron in solution for absorption, and ferrous forms of nonheme iron are better absorbed than ferric forms. Ferrous iron is absorbed by an energy-dependent carrier mechanism, and is carried out of the enterocyte bound to transferrin. However, absorption is regulated, and if the body is iron-replete, crypt cells synthesize apoferritin, which traps iron in enterocytes as ferritin. When the enterocyte is exfoliated at the villus tip, the trapped iron is excreted back into the lumen.

Calcium absorption is complex and is modulated by systemic control mechanisms. In particular, vitamin D stimulates the activity of calcium-binding protein within enterocytes. However, uptake of calcium at the brush-border is an active process and is markedly influenced by the intraluminal pH and other substances such as organic and inorganic phosphates.

Motility

Slow wave, segmental, and peristaltic contractions of the SI are generated by the coordinated contraction of smooth muscle in response to spontaneous electrical activity.²⁶^–³⁴ Interstitial cells of Cajal are considered coordinating/pacemaker cells and smooth muscle contraction is also modulated by coordinated neurohumoral and neurochemical molecule release. Many of these molecules are also involved in the regulation of intestinal secretion and absorption and the mucosal immune response, producing a complex coordinated process for the digestion of food.

Intestinal motility in the fasted state in dogs is characterized by three phases. A cycle comprising a quiescent phase (lasting approximately 1 hour), phase two comprising minor contractile activity (15 to 40 minutes), and then migrating myoelectric (motor) complexes (MMCs) (4 to 8 minutes), and is repeated approximately every 3 hours. The short MMC phase is a period of intense contractile activity that sweeps undigested food, secretions, desquamated cells, and bacteria down the intestine. This process is known as the intestinal housekeeper wave and is induced by motilin secretion. Erythromycin can stimulate motilin receptors and at low doses it can be prokinetic mimicking the MMC; higher doses overstimulate and may cause emesis. The pattern of intestinal motility in cats is somewhat different, but a migrating spike complex correlates with the MMC.

In the fed state, the pattern of motility is most similar to phase two fasting motility. Its duration is determined by the nature of the diet, with fats and fiber prolonging it. The presence of unabsorbed fat in the distal SI reflexly inhibits gastric emptying by neurohormonal mechanisms. This “ileal brake” mechanism may be the cause of the delayed gastric emptying that is typically seen in malabsorption. Feeding a patient with SI disease more than four times a day is unlikely to be helpful as the stomach will be trickle-feeding it anyway.

Segmental contractions slow intestinal transit and ensure mixing and digestion of nutrients, until peristalsis propels the ingesta onwards. Reduced segmental motility may lead to rapid transit, and decreased peristalsis delays transit, conditions manifesting clinically as diarrhea and ileus, respectively.

Secretion and Absorption of Water and Electrolytes

Intestinal secretion, a function of villus crypt cells, is believed to occur by passive flux of water osmotically following active transcellular chloride secretion into the intestinal lumen. Bacterial toxins can cause hypersecretion.

The ability of the intestine to absorb fluid and electrolytes varies according to the site, with water absorption becoming increasingly efficient distally. The net amount of fluid and electrolytes in the GI tract reflects a balance between absorption and secretion, with net absorption in health. However the daily fluxes are massive (approximately 2.7 L/day in a 20-kg dog) and the consequences of net loss is not only diarrhea but also rapid dehydration.

The absorption of water is passive and follows transport of solutes across the GI epithelium by one of three processes: passive absorption, active absorption, or solvent drag. The jejunum absorbs approximately 50%, the ileum approximately 75%, and the colon approximately 90% of the fluid volume presented to it, leaving approximately 2% in feces. This gradient in absorptive ability is a function of enterocyte pore size, membrane potential difference, and the type of transport processes associated with each intestinal segment. The site of the enterocyte on the villus is also important; villus enterocytes absorb, whereas crypt cells secrete.

Colonic absorption is important in SI disease because it helps to compensate for fluid losses and diarrhea will only occur if the colonic reserve capacity is overwhelmed. SI dysfunction may then present with signs of dysfunction of the large intestine because products from the SI, such as hydroxylated fatty acids and deconjugated bile acids, stimulate colonic secretion.

Control of Fluid Balance

Intestinal fluid balance is regulated by the neurocrine systems in the submucosal plexus as a largely autonomous process.^35,³⁶ Acetylcholine and vasoactive intestinal polypeptide are major mediators of secretion, increasing intracellular calcium and cyclic adenosine monophosphate (cAMP), inhibiting neutral sodium and chloride absorption, and facilitating transcellular chloride efflux. Many bacterial agents exert their diarrheagenic effects by increasing cAMP in enterocytes. The principal regulators of absorption—noradrenaline, somatostatin, and opioids—lower intracellular cAMP and calcium concentrations and stimulate neutral NaCl absorption and thereby can have therapeutic antidiarrheal effects.