CONNECTIVE TISSUE.

Glucocorticoids in excess inhibit fibroblasts, lead to loss of collagen and connective tissue, and thus result in thinning of the skin, easy bruising, and poor wound healing.

BONE.

The physiologic role of glucocorticoids in bone metabolism and calcium homeostasis is not well understood. However, in excess, they directly inhibit bone formation by decreasing cell proliferation and the synthesis of RNA, protein, collagen, and hyaluronate. Glucocorticoids also directly stimulate bone-resorbing cells, leading to osteolysis and increased urinary hydroxyproline excretion. They also potentiate the actions of parathyroid hormone (PTH) and 1,25-dihydroxycholecalciferol (1,25[OH]₂D₃) on bone, and this may further contribute to net bone resorption.

CALCIUM METABOLISM.

Glucocorticoids markedly reduce intestinal calcium absorption, which tends to lower the serum calcium concentration. This effect results in secondary increases in PTH secretion, in order to maintain the serum calcium concentration in the normal range by stimulating bone resorption. In addition, glucocorticoids may directly stimulate PTH release. The mechanism of decreased intestinal calcium absorption is unknown, although studies have demonstrated that it is not decreased synthesis or decreased serum concentrations of the active vitamin D metabolites; in fact, 1,25-vitamin D concentrations are normal or even increased in the presence of glucocorticoid excess. Increased 1,25-vitamin D synthesis may result from decreased serum phosphorus concentrations, increased serum PTH concentrations, and direct stimulation of renal 1α-hydroxylase. Glucocorticoids also increase urinary calcium excretion, and hypercalciuria is a consistent feature of cortisol excess in humans. Reduced tubular resorption of phosphate, leading to phosphaturia and decreased serum phosphorus concentrations, also occurs (Aron et al, 2001).

GROWTH AND DEVELOPMENT.

Glucocorticoids accelerate the development of various systems and organs in fetal and differentiating tissues, although the mechanisms are unclear. As previously discussed, glucocorticoids are generally inhibitory, and stimulatory effects may be due to glucocorticoid interaction with other growth factors. Examples of development-promoting effects include increases in fetal lung surfactant production and accelerated development of hepatic and gastrointestinal enzyme systems.

Excess glucocorticoids inhibit growth in children, puppies, and kittens. This adverse effect could be a major complication of glucocorticoid therapy in immature individuals. Growth inhibition may be a direct effect of glucocorticoids on bone cells, although decreases in growth hormone (GH) secretion and somatomedin (insulin-like growth factor) generation also contribute.

CARDIOVASCULAR FUNCTION.

Glucocorticoids may increase cardiac output and vascular tone by augmenting the effects of other vasoconstrictors, such as the catecholamines. Thus refractory shock may occur when a glucocorticoid-deficient individual is subjected to stress. Excess glucocorticoids may cause hypertension independent of their mineralocorticoid effects.

RENAL FUNCTION.

Steroids affect water and electrolyte balance by actions mediated either by mineralocorticoid receptors (sodium and water retention, hypokalemia, and hypertension) or glucocorticoid receptors. The polydipsia and polyuria associated with glucocorticoid excess are rather unique to dogs and may be due to inhibition of vasopressin (ADH) secretion and/or interference with ADH action in renal tubular cells. Polyuria and polydipsia are not typical effects of glucocorticoid excess in cats or humans. Also of interest is the lack of renal concentrating ability associated with adrenocortical insufficiency in dogs.

CENTRAL NERVOUS SYSTEM (CNS).

Glucocorticoids readily enter the brain, and although their physiologic role in CNS function is unknown, an excess or deficiency may profoundly alter behavior and cognitive function in humans (Tyrrell et al, 1991).

RED BLOOD CELLS.

Glucocorticoids have little effect on erythropoiesis or hemoglobin concentration. Although mild polycythemia and anemia may be observed in dogs with Cushing’s syndrome and Addison’s disease, respectively, these alterations are more likely secondary to changes in androgen metabolism or secretion.

LEUKOCYTES.

Glucocorticoids influence both white cell movement and function. Under the influence of glucocorticoids, white blood cell counts increase due to an increased rate of neutrophil (polymorphonuclear neutrophil [PMN]) release from the bone marrow, an increased circulating half-life of PMNs, and by inhibiting PMN movement out of the vascular space. Glucocorticoids reduce the numbers of circulating lymphocytes, monocytes, and eosinophils by increasing their movement out of the circulation. Glucocorticoids also reduce migration of inflammatory cells (PMNs, monocytes, and lymphocytes) to sites of injury. This is probably the primary mechanism of the antiinflammatory action and increased susceptibility to infection associated with chronic steroid administration (Aron et al, 2001).

IATROGENIC ADRENAL SUPPRESSION.

The hypothalamic-pituitary-adrenal (HPA) axis of healthy dogs can be suppressed for an average of 32 hours after one intravenous injection of dexamethasone at a dosage of 0.1 mg/kg. Smaller doses result in a shorter mean duration of HPA suppression, which demonstrates that this suppressive effect is dose dependent (Kemppainen and Sartin, 1984). Cortisol typically suppresses the HPA axis for approximately 12 to 24 hours, whereas prednisolone is suppressive for 12 to 36 hours (Table 10-1). Adrenal suppression can follow treatment with almost any form of glucocorticoid. Ocular and topical skin formulations are examples of steroids that can be administered for several days or longer, can be absorbed systemically, and subsequently can suppress the HPA axis for weeks (Roberts et al 1984; Zenoble and Kemppainen, 1987).

TABLE 10-1 DURATION OF ACTION OF NATURAL AND SYNTHETIC GLUCOCORTICOIDS FOLLOWING ORAL OR IV ADMINISTRATION

Glucocorticoids given for days to weeks rarely have any prolonged significant clinical effects, but more chronic administration of any corticosteroid could cause adrenocortical atrophy (iatrogenic hypoadrenocorticism). However, the degree of individual variation in response to glucocorticoid administration among dogs and cats is so pronounced that each dog and cat must be evaluated independently (Brockus et al, 1999).

Recovery of adrenocortical function occurs relatively quickly in dogs and cats after discontinuation of chronic steroid administration. Adrenocorticotropic hormone (ACTH) response test results usually return to normal within weeks. We almost never “taper” dogs or cats off steroids when administration of these drugs has been demonstrated to be unnecessary. Rather, the steroids are simply stopped. If there is concern about clinical signs caused by iatrogenic hypoadrenocorticism, steroids can be given at doses slowly tapered over a period of weeks, but this approach is almost never used. (Further discussion on suppression of the HPA axis is provided under Adverse Reactions, page 477.)

THYROID FUNCTION.

Although basal thyroid-stimulating hormone (TSH) concentrations in humans are usually normal, TSH responsiveness to thyrotropin-releasing hormone (TRH) is frequently subnormal after chronic glucocorticoid therapy. Serum total thyroxine (T₄) concentrations are usually low-normal because of a decrease in thyroxine-binding globulin, but free T₄ concentrations are normal. Both the total and free T₃ concentrations may be low, because glucocorticoid excess decreases conversion of T₄ to T₃ and increases conversion to reverse T₃. Despite these alterations, manifestations of hypothyroidism are not apparent (Aron et al, 2001).

The serum total and free T₄ concentrations may be low normal or decreased both in dogs treated with glucocorticoids and in those with naturally occurring hyperadrenocorticism. Serum T₄ response to TSH may parallel but remain below normal reference limits (Fig. 10-4). The clinical and biochemical changes associated with chronic glucocorticoid excess in dogs are similar to those observed in hypothyroidism (lethargy, weakness, dermatologic alterations, hypercholesterolemia). However, none of these require direct therapy, because correcting the cortisol excess resolves these problems (Woltz et al, 1983).

FIGURE 10-4 Thyroid-stimulating hormone (TSH) response test results in normal dogs and in dogs with spontaneous hyperadrenocorticism.

GONADAL FUNCTION.

Glucocorticoids affect gonadotropin and gonadal function. In males, glucocorticoids inhibit gonadotropin secretion, as demonstrated by decreased circulating testosterone concentrations and testicular atrophy. In females, hypothalamic-pituitary suppression is also suggested by the interruption of estrus cycle activity.

OVERVIEW.

The capacity of glucocorticoids to prevent or suppress inflammatory reactions not only is grossly visible by limiting heat, redness, swelling, and tenderness, but also can be documented microscopically. Glucocorticoids inhibit the early inflammatory phenomena of edema, fibrin deposition, capillary dilation, migration of leukocytes, and phagocytic activity. Glucocorticoids also limit later manifestations of inflammation such as capillary proliferation, fibroblast proliferation, deposition of collagen and, still later, cicatrization (Gilman et al, 1980; Aron et al, 2001).

Glucocorticoids inhibit inflammation whether the inciting agent is radiant, mechanical, chemical, infectious, or immune mediated. Such therapy is palliative in that the underlying cause of a disorder may remain, but its clinical manifestations are suppressed. This ability to suppress inflammation, regardless of its cause, has made glucocorticoids valuable therapeutic agents. However, these encompassing properties also make glucocorticoid therapy dangerous, because it can mask the clinical expression of a disease process. The inflammation that allows the clinician to recognize and monitor any nonsteroidal course of therapy can be completely suppressed, allowing a disease to continue or worsen while preventing this process from being identified. As with any therapy, the benefits of glucocorticoid medication must be weighed against the risks of their use.

Relative to dogs and cats, the rabbit, rat, mouse, and human being are relatively more sensitive to the antiinflammatory and/or immunosuppressive effects of glucocorticoids. Thus the results of research completed in the last four species, on in vitro models, and even in healthy animals may not apply to dogs and cats with naturally occurring diseases (Papich and Davis, 1989).

With these caveats established, several general effects of glucocorticoids have been documented. Polymorphonuclear neutrophils demonstrate decreased migration and egress into inflammatory tissue. The circulation of T-cell lymphocytes is decreased, and lymphocyte activation is suppressed. Glucocorticoids decrease vascular permeability and also the rate of synthesis of prostaglandins, prostacyclin, thromboxane, and leukotriene (Papich and Davis, 1989).

NEUTROPHILS

LYMPHOCYTES.

The lymphocyte population is composed of a circulating pool and a noncirculating pool. The circulating pool freely moves between the intravascular compartment and the extravascular compartment (lymph nodes, spleen, bone marrow, and thoracic duct). After glucocorticoid administration, lymphocytes of the circulating pool redistribute to the extravascular compartment. T-cell lymphocytes are affected more than B cells (T cells constitute approximately 70% of the circulating pool). The discrepancy in the lymphocyte response is not easily explained, but the ability of B cells to metabolize glucocorticoids faster than T cells may be involved (Cupps and Fauci, 1982; Kehrl and Fauci, 1983; Papich and Davis, 1989). Furthermore, helper or inducer lymphocytes (CD4) are more sensitive than the cytotoxic suppressor cells (CD8). Steroids also inhibit the production of interleukin-2, resulting in inhibition of lymphocyte proliferation and cytotoxic function. Glucocorticoids also cause suppression of B-cell growth factors (BCGFs), affecting the proliferation of this cell type (MacDonald, 2000). Decreased numbers of circulating lymphocytes and decreased proliferative response of lymphocytes reduce these cells’ ability to participate in immunologic and/or inflammatory reactions. Additional observations include a decreased response to mitogens, suppressed lymphokine synthesis, and decreased transformation and antigen recognition (Cupps and Fauci, 1982; Meuleman and Katz, 1985; Papich and Davis, 1989).

Glucocorticoids have few direct effects on B cells, but they indirectly alter B-cell function by modulating accessory cells. A complex series of interactions between T cells, macrophages, antigens, and cell mediators is required for full expression of lymphocyte function (Nossal, 1987). Commonly used doses of glucocorticoids do not prevent an animal from mounting a normal immunologic response to vaccinations or other antigens (Meuleman and Katz, 1985), but as doses of corticosteroids increase, concentrations of immunoglobulins IgG, IgA and, to a lesser extent, IgM are decreased. Synthesis of IgE is not affected. Studies in humans suggest that repeated daily doses of long-acting corticosteroids, such as dexamethasone, suppress antibody synthesis but that alternate-day administration of antiinflammatory doses of an intermediate-acting steroid, such as prednisolone, does not (Papich and Davis, 1989). Glucocorticoids induce eosinopenia, which is most likely a consequence of changes in distribution to various tissues. Evidence for a lytic or toxic effect on eosinophils has not yet been proven (MacDonald, 2000).

MACROPHAGES.

Mononuclear cells have several critical functions. They serve as antigen-presenting accessory cells during the induction of cell-mediated and humoral immune responses. They also function as “professional” phagocytes against bacteria, fungi, and viruses. In combination with antibodies, they may have tumor cell–killing capacity. They are also essential in clearing senescent cells, particularly erythrocytes, and in bone remodeling, wound healing, and lung surfactant turnover (MacDonald, 2000).

Monocytes and macrophages (lymphocyte accessory cells) appear to be more sensitive to the effects of glucocorticoids than neutrophils. Therapeutic corticosteroid doses decrease phagocytosis and bactericidal activity. Steroids also diminish macrophage and monocyte ability to process antigens for presentation to B cells. Synthesis of cell mediators, such as interleukin-1 (IL-1), also may be suppressed. Corticosteroids inhibit both lymphokine synthesis and the cellular response to lymphokines (Cupps and Fauci, 1982; Dinarello and Mier, 1987; Papich and Davis, 1989). Glucocorticoids also inhibit production of tumor necrosis factor (TNF), thereby affecting a wide variety of inflammatory processes. The major antiinflammatory effect of glucocorticoids is related also to decreased accumulation of mononuclear phagocytes at inflammatory foci. This may be a consequence of decreased chemotactic factors by other cells, including T lymphocytes outside the mononuclear cell population. Antienzymatic effects of steroids may also be a factor. Inhibition of cytokines such as IL-1, interferon-γ (IFN-γ), and TNF/cachectin also has been documented (Cohn, 1991; Scott et al, 1995).

BASOPHILS AND MAST CELLS.

Glucocorticoids reduce the number of circulating basophils, most likely through reduced production. Glucocorticoids reduce the number of mucosal mast cells, although the subtype of mast cell found in connective tissue is less sensitive. The reduction of mast cells most likely is a result of inhibition of IL-3 or mast cell growth factor. A direct effect may also occur. Migration of basophils is significantly reduced by steroids. Some suggest that steroids directly inhibit production and release of mast cell inflammatory mediators, or that this may be the simple result of a smaller number of mast cells causing degranulation. Topical glucocorticoids decreasing response to antigen may also involve decreased cell numbers or decreased access of antigen to tissue mast cells (Butterfield and Gleich, 1989; Munck and Guyre, 1989).

ARACHIDONIC ACID METABOLISM.

Injury to cell membranes stimulates the prostaglandin-leukotriene cascade, which in turn activates phospholipase A₂ in cell membranes to form arachidonic acid. The enzymes cyclooxygenase and lipooxygenase form prostacyclin, thromboxane, and leukotrienes from arachidonic acid. The antiinflammatory action of glucocorticoids is believed to be mediated by proteins synthesized in target tissues. Part of this action is related to reduced production of proinflammatory metabolites, especially those derived from arachidonic acid. Prostaglandins I₂ and E₂ plus leukotrienes B₄, C₄, D₄, and E₄ may be responsible for the end-inflammatory response modified by decreased metabolism of arachidonic acid.

In general, glucocorticoids induce an antiphospholipase effect by using second-messenger proteins called lipocortins. These proteins inhibit cellular phospholipase A, thus reducing the release of arachidonic acid and all subsequent proinflammatory mediators (prostaglandins, prostacyclines, thromboxanes, and leukotrienes). Lipocortins regulate cellular metabolism of phospholipases as well. Many cells release arachidonic acid when stimulated by neurotransmitters and certain drugs. Lipocortins are naturally digested by proteases. They promote the maturation of suppressor T cells and inhibit proliferation response to T cells by mitogens. β-Lipocortins have a dose-dependent inhibitory effect on the acute phase of cytotoxic T cells. They also inhibit antibody-dependent cytotoxicity by natural killer cells and protect cells from complement-mediated lysis (Cupps, 1989; Flower, 1989; Hirata, 1989).

EFFECTS ON BLOOD VESSELS.

The beneficial effects of glucocorticoids when administered to animals in shock, to animals with CNS edema, and to those with various other inflammatory conditions are related to “stabilization of microvascular integrity.” This phenomenon reduces edema formation and extravasation of cells from the vasculature to tissues. These effects may be attributed to suppressed neutrophil action and to reductions in prostaglandin synthesis. It has also been suggested that corticosteroids antagonize vasoactive substances such as histamine and kinins, thereby providing protection against the toxic effects of free oxygen radicals released during inflammation. Because it is known that macrophages release plasminogen activators as part of the cascade of reactions during inflammation, it is of interest that glucocorticoids likely inhibit this reaction. This is an important effect of glucocorticoids, because activated plasmin digests supporting tissue of vessels and enhances vascular permeability (Papich and Davis, 1989).