Kidney function and the role of arginine vasotocin in three agamid lizards from habitats of differing aridity in Western Australia

Western Australian agamid lizards are diverse and inhabit mesic to very arid areas of the state. Although reptilian kidneys are unable to elaborate hyperosmotic urine, we hypothesised that the renal system of lizards inhabiting arid areas would display an enhanced ability to conserve water under the control of the antidiuretic peptide hormone, arginine vasotocin (AVT). To examine this, the renal physiological and endocrine responses to osmotic challenge in three closely-related Australian agamid lizards inhabiting arid, semi-arid, and mesic environments were studied. The species studied were Pogona minor (mesic), Ctenophorus salinarum (semi-arid), and Ctenophorus nuchalis (arid). Circulating AVT was assayed and renal variables such as glomerular filtration rate (GFR), urine flow rate (V  ), and fractional reabsorption of filtrate (FRH2O)(FRH2O) were measured in response to hypernatraemia, water load, and dehydration. Hypernatraemia and dehydration induced antidiuresis in all three species through similar mechanisms involving both glomerular and tubular responses. However, in salt-loaded P. minor   the response was largely glomerular in nature, as FRH2OFRH2O did not increase relative to the hydrated condition. The magnitude of the antidiuretic response was also greater in P. minor  , indicating a greater sensitivity to osmotic challenge. Plasma concentrations of AVT were significantly correlated with FRH2OFRH2O in P. minor (r2 = 0.38, P = 0.025), but with GFR in C. nuchalis (r2 = 0.16, P = 0.041). We found that the control and mechanisms of renal function among these lizards were largely similar, and there was little support for the hypothesis that arid lizards possess physiological adaptations not present in closely-related mesic lizards. Yet, differences remain in their response to hypernatraemia which may reflect the aridity of their different environments, or their varying habits.