GENERAL AND COMPARATIVE ENDOCRINOLOGY 44, 82-93 (1981) The Effects of Pituitary and Adrenal Hormones on Renal and Postrenal Reabsorption of Water and Electrolytes in the Lizard, Varanus gouldii (Gray)’ S.D. BRADSHAW AND G.E. RICE Zoology Department, University of Western Australia, Nedlands, Western Australia 6009, Australia Accepted October 21, 1980 An in situ perfusion technique has been employed to estimate transmural fluxes of water and electrolytes across the isolated colon and cloaca of the lizard Varanus gouldii in con- junction with standard measures of renal function. The effect of three experimental treat- ments was investigated: hydration, 10% dehydration, and chronic sodium loading, and col- onic perfusion rates were adjusted to approximate previously measured rates of ureteral urine production. Plasma aldosterone concentrations fell significantly with sodium loading, whereas total corticosteroid levels increased significantly with the same treatment. Adrenal hormone levels did not differ in hydrated and dehydrated individuals. At the level of the kidney, both saline loading and dehydration were accompanied by an antidiuresis, of greater proportions in the former case due to an enhanced glomerular as well as a tubular response and this was associated with elevated circulating levels of arginine vasotocin (AVT). Injec- tions of AVT evoked an antidiuresis due to a fall in the glomerular filtration rate and an increase in tubular water permeability and both effects were abolished by subsequent injec- tion of probenecid. Despite differences between the circulating levels of aldosterone and other corticosteroids in salt-loaded and dehydrated individuals, colon function was essen- tially identical with the two treatments showing an increased rate of water reabsorption and a decreased rate of sodium reabsorption when compared with values measured in hydrated individuals. Aldosterone appears to function as a mineralocorticoid on the kidney of V. gouldii, acting to limit sodium loss and promote potassium excretion, but changes in colon function do not appear to be aldosterone mediated. Adrenal corticosteroids have been impli- cated in the control of osmoregulation in reptiles (Bradshaw, 1972, 1975; Bentley, 1976; Callard and Callard, 1978) but their precise modes and loci of action await elucidation. Suppression of ACTH secre- tion, either by dexamethasone blockade or by hypophysectomy, abolishes the normal renal and postrenal response to salt loading in the lizards Amphibalurus ornatus and Dipsosaurus dorsalis (Bradshaw, 1972; Bradshaw et al., 1972) and led these au- thors to suggest that corticosterone may promote sodium excretion through a re- duction in the fractional tubular reabsorp- ’ A preliminary report of this work was presented at the International Symposium on Steroids and their Mechanism of Action in Non-Mammalian Vertebrates, Capri, October 1979. tion of this ion. Aldosterone, which is also secreted by the reptilian adrenal gland (Sandor, 1972; Vinson et al., 1979), has been shown to exert a potent sodium- retaining effect on the external nasal salt gland ofD. dorsalis (Templeton et al., 1968; Shoemaker et al., 1972) but attempts to re- veal a similar action of this hormone on the reptilian kidney have met with little suc- cess. To date, the only unequivocal demon- stration of an effect of aldosterone on kid- ney function in a reptile has been obtained with the water snake Natrix cyclopion where injections into salt-loaded indivi- duals increased renal sodium and water reabsorption by about 24% (LeBrie and Elizondo , 1969). Recent studies on the water and electro- lyte economy of reptiles have underlined 82 0016-6480/81/050082-12$01.00/O Copyright @ 1981 by Academic Pmss, Inc. All rights of reproduction in any form reserved.