The adrenal cortex synthesizes a variety of steroids from cholesterol and releases them into the circulation. Those steroids with effects on intermediary metabolism are termed “glucocorticoids” and are produced mainly in the layers of the adrenal gland called the zonae fasciculata and reticularis. The steroids with primarily salt-retaining activity are called mineralocorticoids and are synthesized in the zona glomerulosa. The adrenal gland is also capable of synthesizing steroids with androgenic and estrogenic activity. The major glucocorticoid in most domestic animals is cortisol, and in most mammals, the most important mineralocorticoid is aldosterone. In some species (e.g., the rat), corticosterone is the major glucocorticoid. It is less firmly bound to protein and therefore metabolized more rapidly. Quantitatively, dehydroepiandrosterone (DHEA) is the major androgen, with part of it being sulfated to DHEA-sulfate. Both DHEA and androstenedione are very weak androgens. A small amount of testosterone is secreted by the adrenal gland and may be of greater importance as an androgen. Little is known about the estrogens secreted by the adrenal gland. However, the adrenal androgens such as testosterone and androstenedione can be converted to estrone in small amounts by nonendocrine tissues (Aron and Tyrrell, 1994; Tyrrell et al., 1994).

In plasma, cortisol is over 90% bound to plasma proteins. The remaining 10% free hormone is the active moiety according to the free-hormone hypothesis. Corticosteroid-binding globulin (CBG), an α₂ globulin synthesized by the liver, binds the majority of circulating hormone under normal circumstances. The remainder is free or loosely bound to albumin and is available to exert its effect on target cells. Cortisol is removed from the circulation by the liver, where it is reduced and conjugated to form water-soluble glucuronides and sulfates, which are excreted into the urine (Aron and Tyrrell, 1994; Grote et al., 1993; Hammond, 1990; Tyrrell et al., 1994).

Genomic mechanisms mediate most effects associated with eventual protein synthesis, such as low-dose physiological replacement therapy and antiinflammatory and immunomodulatory effects; however, it is now clear that some effects, particularly the more rapid ones seen at high glucocorticoid doses, cannot be explained through this mechanism (see Section Nongenomic Effects of Glucocortoids). The genomic mechanisms of glucocorticoid action are cGR-mediated by the cytoplasmic glucocorticoid receptor. The unliganded cGR is a 94-kD protein that exists in the cytoplasm as a multiprotein complex including heat-shock proteins (Hsp), such as Hsp90, Hsp70, Hsp56, and Hsp40. There is also an interaction with immunophilins, (chaperones such as p23 and Src) and several kinases in the mitogen-activated protein kinase (MAPK) signaling system. The glucocorticoid receptor itself consists of three domains: an N-terminal domain containing transactivation functions, a DNA-binding domain with a “zinc-finger” motif, and a glucocorticoid-binding domain. Binding to the cGR ultimately induces (also known as transactivation) or inhibits (also known as transrepression) the synthesis of regulatory proteins. The genomic effects are generally not observed prior to 30 minutes because of the time necessary for cGR activation/translocation, transcription, and translation. It is estimated that glucocorticoids influence the transcription of ∼1% of the entire genome either directly or indirectly through interaction with transcription factors and coactivators (Stahn et al., 2007).

Glucocorticoids play a physiological role in the development of pulmonary surfactant in the near-term fetus, allowing an adaptation to air breathing. GCs stimulate lung maturation through the synthesis of surfactant proteins. GR knockout mice do not survive because of lung atelectasis. Prematurity with delayed development of the adrenal axis in foals has been suspected as a cause of neonatal respiratory distress syndrome. Although glucocorticoids stimulate growth hormone (GH) gene transcription in physiological quantities, glucocorticoids in excess inhibit linear skeletal growth, via catabolic effects on connective tissue and muscle and inhibition of IFG-1 effects. GCs also stimulate the adrenal medullary enzyme converting norepinephrine to epinephrine (Aron and Tyrrell, 1994; Tyrrell et al., 1994; Stewart, 2003).

Glucocorticoids have an antagonistic effect to that of insulin, leading to increased glucose production from amino acids (gluconeogenesis) and reduced incorporation of amino acids into protein. As stated above, glucocorticoids enhance lipolysis; however, glucocorticoid excess (pharmacological amounts) or spontaneous hyperadrenocorticism may result in redistribution of fat because glucocorticoids stimulate appetite, thereby stimulating hyperinsulinemia, which results in lipogenesis. As a result, diabetes mellitus may result from prolonged glucocorticoid use at high dosages in animals with diminished insulin secretory capacity (prediabetics). Muscle wasting and weakness are not uncommon with glucocorticoid excess; although glucocorticoids stimulate protein and RNA synthesis in the liver, they have catabolic effects in lymphoid and connective tissue, muscle, fat, and skin. While not usually a recognizable clinical problem in domestic animals, osteoporosis may result in people with Cushing’s syndrome or on chronic glucocorticoid administration. The problem likely is most significant in areas of healing bone; glucocorticoids directly inhibit bone formation by inhibiting osteoblast proliferation and the synthesis of bone matrix while stimulating osteoclast activity. In addition, glucocorticoids potentiate the action of parathyroid hormone (PTH) and 1,24-dihydroxycholecalciferol (1,25(OH)₂-D₃) and inhibit the gut absorption of calcium, an effect that can be used to advantage in hypercalcemic states. However, in the young animal, the catabolic effects of excessive amounts of glucocorticoid reduce growth. In children, this reduced growth is not prevented by growth hormone (Aron and Tyrrell, 1994; Tyrrell et al., 1994). In adult dogs, 2 mg/kg prednisone for 30 days reduced bone mineral density by 14% as measured by helical computer tomography (Costa et al., 2010).

Glucocorticoid use invariably leads to polyuria and polydipsia via inhibition of ADH release and action, as well as alteration of the animal’s psyche, resulting in increased water intake. No glucocorticoid given in large doses is completely devoid of mineralocorticoid (salt-retaining and K⁺-losing) activity; therefore, excessive use may precipitate or exacerbate hypertension and induce hypokalemia. In part by increasing extracellular fluid volume, glucocorticoids increase the glomerular filtration rate and are required in physiological amounts for maximal dilution of urine (Ferguson, 1985a; Melby, 1974; Nakamoto et al., 1992; Smets et al., 2012).

Often the desired result of a therapeutic application, antiinflammatory effects of glucocorticoids are primarily seen at pharmacological doses. Glucocorticoids result in alterations in the concentration, distribution, and function of peripheral leukocytes and in inhibition of phospholipase A₂ activity in the plasma membranes of these cells. Glucocorticoids act indirectly by inducing lipocortin synthesis, which in turn inhibits arachidonic acid release from membrane-bound stores, and also by inducing transforming growth factor-β (TGF-β) expression, which subsequently blocks cytokine synthesis and T-cell activation. In addition to contributing to maintenance of the microcirculation and cell membrane integrity, glucocorticoids interfere with progressive dissolution and disruption of connective tissue and cells, possibly by stabilizing lysosomal membranes (Aron and Tyrrell, 1994; Aucoin, 1982; Barragry, 1994). Although lysosomal stabilization by glucocorticoids has been demonstrated experimentally, it is hard to know what benefit these effects have in clinical situations. Glucocorticoids also decrease formation of induced histamine (histamine produced locally by cells during injury), the action of which is not effectively blocked by standard doses of antihistamines. They also antagonize toxins and kinins, reducing the resultant inflammation. It is important to realize, however, that most of their effects are nonspecific; that is, they have profound metabolic effects regardless of the initial insult.

Glucocorticoids are used to advantage to suppress both the number of cells and the actions of the immune system. The suppressive effects on cell-mediated immunity predominate over those on humoral immunity. Antibody production is generally unaffected by moderate dosages of glucocorticoids and is inhibited only at high dosages and with long-term therapy. They cause lymphopenia and eosinopenia, an effect secondary to cell redistribution and/or lysis, and lead to increased vascular demargination of neutrophils from the vascular bed to lymphoid tissue. Glucocorticoids inhibit virus-induced interferon synthesis and diminish the functional capacity of monocytes, macrophages, and eosinophils through inhibition of the formation of ILs such as IL-1 (macrophages), IL-2 (lymphocytes), IL-3, and IL-6 and other chemotactic factors. Glucocorticoids can induce apoptosis on normal lymphoid cells and play a key role in the physiology of thymic selection. They inhibit monocyte differentiation into macrophages and macrophage phagocytosis and cytotoxic activity (Barragry, 1994; Ehrich et al., 1992; McDonald and Langston, 1994; Melby, 1974; Tyrrell et al., 1994; Buttgereit et al., 2004).

In recent years, greater understanding of the mechanisms of GC inhibition of cell-mediated immunity has developed, and the GC-induced leucine zipper (GILZ) gene has emerged as a critical mediator. In response to GCs, GILZ up-regulates and inhibits T cells through inhibition of anti-CD3-induced activation and apoptosis. GILZ also interacts with and inhibits NF-κB. Other GILZ targets have been identified, including AP-1, Raf-1, and Ras, and each has been implicated in the effects of GCs. GILZ silencing diminishes the antiproliferative activity of dexamethasone and reduces inhibition of cytokine-induced COX-2 expression (Ayroldi and Riccardi, 2009; Ayroldi et al., 2014). In addition to the GILZ-mediated effects, GCs appear to up-regulate repair-phase cytokines like TGF-β and platelet-derived growth factor (PDGF), possibly explaining the suppressive effect on healing and fibrosis (Pallardy and Biola, 1998; Buttgereit et al., 2004).

In the clinical setting, GCs are used for their ability to induce apoptosis of malignant lymphoid cells. The mechanisms of apoptosis induced by glucocorticoids fall roughly in two categories, depending on the type of lymphocytes: induction of “death genes” such as IκB and c-jun or repression of survival factors such as AP-1 and c-myc. By inhibiting the production of Th1 cytokines, glucocorticoids may enhance Th2 cell activity and generate a long-lasting state of tolerance.

In addition to indirect effects on electrolyte metabolism, glucocorticoids have direct positive chronotropic and inotropic actions on the heart. They appear to block the increased permeability of capillaries induced by acute inflammation, reducing transport of protein into damaged areas and maintaining microcirculation. Because glucocorticoids are necessary for maximal catecholamine sensitivity, they contribute to maintenance of vascular tone (Ferguson et al., 1978; Nakamoto et al., 1992). In shock, production of vasoactive products of lipid peroxidation (arachidonic acid cascade), such as the vasoconstrictor thromboxane A₂, may be decreased by glucocorticoids, but probably only in the early stage of cell disruption. Glucocorticoids cause vasoconstriction when applied directly to vessels. They decrease capillary permeability by inhibiting the activity of kinins and bacterial endotoxins and by reducing the amount of histamine released by basophils.

Glucocorticoids may induce hypertension in animals and humans through the following mechanisms: (i) activation of the renin–angiotensin system due to an increase in plasma renin substrate; (ii) reduced activity of the hypotensive kallikrein–kinin system, prostaglandins (PGs), and the endothelium-derived relaxing factor, nitric oxide (NO); and (iii) increased pressor responses to angiotensin II and norepinephrine. Furthermore, the number of angiotensin II type 1 receptors of vascular smooth muscle cells is significantly increased by glucocorticoids (Saruta, 1996).

Glucocorticoids increase the number and affinity of β-adrenergic receptors. Glucocorticoids prevent receptor down-regulation and therefore tachyphylaxis, resulting in potentiation of the effects of β-adrenergic agonists on bronchial smooth muscle, an important effect in the asthmatic patient (Sprung et al., 1984; Tyrrell et al., 1994; Wilcke and Davis, 1982).

Although rarely described in domestic animals, glucocorticoids (or lack of them) have marked effects on the psyche, resulting in a form of mental, as well as physical, dependence (Ferguson, 1985a; Metz et al., 1982). It is known that pretreatment of neonatal rats with dexamethasone provides protection against hypoxic–ischemic brain damage. This effect is likely mediated via glucocorticoid receptors, because glucocorticoid receptor antagonist RU38486 reverses the benefit. The neuroprotection also appears to be related to alterations in cerebral metabolism. Glucose utilization is reduced prior to hypoxia–ischemia and is better maintained following dexamethasone. High-energy phosphates in the brain are higher in dexamethasone-treated animals. Thus, glucocorticoids may provide their protection against hypoxic–ischemic damage by decreasing basal metabolic energy requirements and/or increasing the availability or efficiency of use of energy substrates (Tuor, 1997).

Glucocorticoids, in addition to being diabetogenic, also have marked effects on hypothalamic and pituitary function. ACTH, β-lipotropin, thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), and growth hormone (GH) synthesis and secretion are all suppressed; however, β-endorphin levels are unaffected. Glucocorticoids, even at “physiological” dosages (0.22 mg/kg prednisolone once daily orally in the dog), result in HPAA suppression, as indicated by reduction in the ACTH-stimulated cortisol concentration increment and by reduction of the ratio of the zona fasciculata and reticularis to zona glomerulosa in the adrenal gland. Antiinflammatory dosages of prednisolone (0.5 mg/kg q 12 h orally) resulted in adrenal suppression within 2 weeks of therapy (Chastain and Graham, 1979). In another study in dogs, 1 month of the same dosage of prednisolone orally resulted in profound suppression of endogenous plasma ACTH and cortisol concentrations, CRH-stimulated ACTH release, and ACTH-stimulated cortisol release. However, following withdrawal of the prednisolone, the HPAA returned to normal within 2 weeks (Moore and Hoenig, 1992). A similar ability to recover from exogenous glucocorticoids was seen in the cat. A dosage of 2 mg/kg q 12 h of methylprednisolone given orally for 7 days resulted in suppression of ACTH-stimulated cortisol and CRH-stimulated ACTH, but these changes completely reversed by 7 days after withdrawal of the exogenous glucocorticoid (Crager et al., 1994). Higher dosages and long-acting preparations (dexamethasone, triamcinolone, depot products) may result in more pronounced HPAA suppression (Kemppainen and Sartin, 1984; Kemppainen, 1986; Kemppainen et al., 1982). Even hydrocortisone at 10 mg/kg twice a day for 4 months led to ultrasonographically evident adrenal atrophy that was reversible after 1 month of withdrawal (Pey et al., 2012).

The effect of glucocorticoids on glucose metabolism is time and dose dependent; insulin and glucose concentrations, or glucose tolerance, were not significantly altered by the administration for 28 days of an antiinflammatory dosage of oral prednisone (Moore and Hoenig, 1993). However, daily administration of high dosages of dexamethasone and growth hormone is a reliable model for induction of diabetes mellitus in the cat (Hoenig et al., 2000). There also have been case reports of diabetes mellitus induced in dogs after administration of corticosteroids and methylprednisolone pulse therapy (Jeffers et al., 1991).

Pharmacological doses of glucocorticoids generally reduce serum thyroid hormone concentrations, presumably through suppression of pituitary TSH. These effects have been well documented in the dog, are not as significant in the cat, and are not well studied in other domestic species (Ferguson and Peterson, 1992; Kaptein et al., 1992; Moore et al., 1993). The metabolic consequences of these lowered concentrations of thyroid hormones are not known in the dog; however, a state of hypothyroidism is not believed to be the result (Jennings and Ferguson, 1984).

Physiological glucocorticoid responses contribute to the maintenance of the gastric mucosal integrity following ulcerogenic stimuli. The role of glucocorticoids in gastroprotection becomes especially important where there is deficiency of PGs or NO or desensitization of intestinal sensory neurons (CSN). In rats with normal corticosterone, these insults do not result in gastric mucosal disruption, but do in adrenalectomized rats (Filaretova et al., 2007). These findings may explain the observation of bloody diarrhea in dogs with glucocorticoid deficiency and/or Addison’s disease.

However, pharmacological doses of glucocorticoids stimulate excessive production of acid and pepsin in the stomach and may cause peptic ulcer. In an experimental canine fundic pouch model of acid secretion, the effect of prednisolone (2 mg/kg per day given parenterally for three doses, 2 weeks or 12 weeks) on gastric mucosal H⁺ permeability was studied. Acute administration (<2 weeks) had no effect. Chronic administration (12 weeks) increased slightly the basal H⁺ secretion rate, and increased mucosal permeability by 50% when simultaneously administered with aspirin (Chung et al., 1978). Similarly, 0.25 mg/kg SC q 12 h for 3 days induced endoscopically evident gastric lesions in dogs (Boston et al., 2003). Glucocorticoid lesions are exacerbated by simultaneous administration of nonsteroidal antiinflammatory drugs (NSAIDs) (Dow et al., 1990; Narita et al., 2007). In rats, dexamethasone-induced ulceration has led to alterations in the phospholipid fatty acid profile (increased linoleic acid and decrease in other polyunsaturated fatty acids, PUFAs). PUFA supplementation or the administration of an H₂ receptor antagonist reduced ulcerization with near normalization of changes in the phospholipid fatty acid profile (Manjari and Das, 2000).

Prednisolone and dexamethasone, studied in canine colon cell culture, had no effect on the intestinal cell tight junction barrier function, but did prevent the increase in permeability induced by TNF-α. Specifically, glucocorticoids, acting through nuclear receptors, inhibited the TNF-α–induced increase in myosin light chain kinase protein expression, which was shown to mediate increased intestinal tight junction permeability (Boivin et al., 2007).

Glucocorticoids facilitate fat absorption and appear to antagonize the effect of vitamin D on calcium absorption. Therefore, glucocorticoids are employed in chronic hypercalcemic states in an attempt to inhibit gastrointestinal calcium absorption (Aron and Tyrrell, 1994; Tyrrell et al., 1994). Glucocorticoid therapy may cause calcinosis cutis in dogs, but no specific dose or set of factors has been identified to consistently cause it (Doerr et al., 2013).