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The hypothalamic-pituitary-adrenocortical (HPA) axis was studied in 8 healthy cats after administration of supraphysiologic doses of methylprednisolone (MP). Ovine corticotropin-releasing hormone (oCRH) administration increased cortisol and adrenocorticotropic hormone (ACTH) concentrations. Significant (P < 0.05) suppression of cortisol and a trend toward suppression of ACTH was observed after 1 week of MP administration. The HPA axis quickly recovered from suppressive effects of MP 1 week after administration of the steroid was discontinued. Side effects of oCRH administration were minimal in 7 cats; however, 1 cat had a severe hypotensive reaction. Clinical abnormalities were not associated with MP administration. The HPA axis was suppressed by supraphysiologic doses of MP in all treated cats that lacked clinical signs consistent with iatrogenic HPA axis suppression. Despite the relatively active pars intermedia in cats, compared with human beings and dogs, feedback of MP on the HPA axis resulted in similar trends in oCRH-stimulated ACTH and cortisol concentrations as observed in human beings and dogs. Lack of consistent correlation between ACTH and cortisol concentrations was observed in 3 cats and possibly was related to the active pars intermedia in the cat.

Platelet function, antithrombin and plasminogen activities, and fibrinolytic capabilities in 11 cats with acquired heart disease were compared with results in 4 healthy cats. Of 11 cats with heart disease, 9 had hyperthyroidism with secondary cardiac dysfunction. One cat with hyperthyroidism had renal disease and heart failure, and of 2 cats with idiopathic hypertrophic cardiomyopathy, 1 also had renal disease. At the time of testing, 3 cats had thromboembolic events associated with the disease. Compared with healthy cats, cats with acquired heart disease had increased activity of antithrombin III, a protein that behaves as an acute-phase reactant. Plasminogen activity was decreased, although not significantly, in cats with acquired heart disease, compared with results in healthy cats. In cats with left ventricular dysfunction, clot retraction was decreased (marginal significance, P = 0.058) and might be attributed, in some cases, to the medications received by the cats. Dilute whole blood clots from all cats failed to lyse in vitro. This observation, at present, lacks adequate explanation. Platelets from cats with acquired heart disease, compared with platelets from healthy cats, had decreased responsiveness (aggregation and [14C]serotonin release) to adenosine diphosphate and increased responsiveness to collagen. Hyperthyroid cats were receiving various drugs (propranolol, atenolol, or diltiazem) to empirically treat clinical signs of disease attributable to cardiac dysfunction. Although numbers of cats in each group were small, definite trends were observed in the results of tests. Platelets from cats receiving atenolol had decreased responsiveness to adenosine diphosphate and unaltered responsiveness to collagen, compared with platelets from healthy cats, and may have decreased risk of thrombus formation.(ABSTRACT TRUNCATED AT 250 WORDS)