Adrenalectomy and Steroid Replacement in a Small Macropodid Marsupial, the Quokka (Setonix brachyurus): Metabolic and Renal Effects

Abstract

Contrary to previously accepted observations, the quokka (Setonix brachyurus), a small macropodid marsupial, survived bilateral adrenalectomy indefinitely if injected with appropriate doses or cortisol and aldosterone, either singly or combined. Normal behavior and plasma solute concentrations could be maintained indefinitely by daily i.m. injection of 0.2 mg cortisol acetate and 0.01 mg aldosterone/kg. Plasma glucose concentration was maintained at the control value when the supplement contained cortisol but was significantly reduced with only a mineralocorticoid. Cortisol also caused a dose-related enhancement of food intake. There was no significant effect of supplementation on plasma urea concentration. Deoxycorticosterone, with or without cortisol, caused Na+ retention but aldosterone had this effect only when combined with cortisol. Urinary K+ excretion was not significantly affected by any of these corticosteroids. Withdrawal of hormone supplements caused muscular weakness and cessation of food intake within 3-6 days, accompanied by a moderate fall in Na+ and rise in K+ concentration of blood plasma as well as instability of plasma glucose concentration with hypoglycemic episodes. Renal excretion of both Na+ and K+ exceeded intake only when food intake had declined to negligible levels. Glomerular filtration rate (GFR) and renal plasma flow (RPF) fell when corticoids were withheld, without change in inulin space. However, p-aminohippurate (PAH) space fell, as did the intercompartmental exchange parameters of both inulin and PAH. Cortisol and aldosterone or aldosterone alone restored GFR, but not RPF, to the normal range. The fractional reabsorption of K+, but not Na+, was reduced in the absence of corticoids. It is suggested that, in this species, the circulatory effects of corticoids are essential for normal tissue perfusion and their influence on renal circulation is important in the control of salt and water balance. Metabolic effects are probably mainly permissive, as in other macropodid marsupials.