A Potent Dipsogenic Action of Homologous Angiotensin II Infused at Physiological Doses in Eels

Angiotensin II at pharmacological doses induces drinking in various vertebrate species. However, its role in physiological regulation of drinking remains to be determined. Eel angiotensin II was infused slowly into freshwater eels to examine whether sustained, physiological increases in its plasma level, as observed after transfer to seawater, stimulate drinking. Hourly infusion was initiated with 0.9% NaCl, followed by increasing concentrations of angiotensin II (0.1–10 pmol/kg/min), and ended with only saline again for 2 hr.Angiotensin II infusion at a rate of 0.1 pmol/kg/min significantly induced drinking; neither plasma angiotensin II concentration nor plasma atrial natriuretic peptide (ANP), Na concentrations or arterial pressure were significantly altered. The dipsogenic effect of angiotensin II was dose-dependent, and enhanced drinking disappeared quickly after infusate was switched back to saline. Blood pressure and plasma ANP concentration increased dose-dependently at more than 1 pmol/kg/min, and both parameters declined to their initial levels after return to normal saline infusion. By contrast, plasma Na concentration was decreased by more than 1 pmol/kg/min of angiotensin II.These results show that drinking is induced by angiotensin II within physiological range in water-replete freshwater eels. At higher doses, enhanced drinking occurs despite hypertension, hyponatremia and increased plasma ANP, all of which are inhibitory to elicitation of drinking. Thus, it is likely that angiotensin II could be a physiological stimulus for drinking in the eel.