Endocrines and osmoregulatory mechanisms in the Nile crocodile, Crocodylus niloticus.

The changes in body fluid economy and endocrine status associated with exposure of the Nile crocodile, Crocodylus niloticus, to hypertonic media have been related to the responses to altered renin-angiotensin system (RAS) activity observed in fresh water (FW) animals. Animals held in hypertonic media for 7 days showed a 18.2% body weight loss and raised plasma and urinary sodium, potassium, chloride, and osmotic concentrations. Within 6 hr of return to FW rapid imbibition had largely restored body weight and produced significant plasma dilution. Although plasma sodium, chloride, and osmotic concentrations remained higher than in FW controls, plasma levels of corticosterone, aldosterone, and arginine vasotocin were not significantly altered. Angiotensin I (AI) administration in FW crocodiles stimulated drinking and raised plasma aldosterone levels by comparison with animals given the converting enzyme inhibitor, Captopril, together with AI. The compensatory drinking behaviour exhibited by the Nile crocodile may thus involve the RAS. The RAS also appears to influence interrenal steroidogenesis and thus may afford an integrative role in crocodile fluid management as it does in homeotherms.