The effects of pituitary and adrenal hormones on renal and postrenal reabsorption of water and electrolytes in the lizard, Varanus gouldii (Gray)

Abstract

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 conjunction with standard measures of renal function. The effect of three experimental treatments was investigated: hydration, 10% dehydration, and chronic sodium loading, and colonic 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). Injections 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 injection of probenecid. Despite differences between the circulating levels of aldosterone and other corticosteroids in salt-loaded and dehydrated individuals, colon function was essentially 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.