Drinking in Freshwater-Adapted Rainbow Trout Fry, Oncorhynchus mykiss (Walbaum), in Response to Angiotensin I, Angiotensin II, Angiotensin-Converting Enzyme Inhibition, and Receptor Blockade

Drinking in freshwater-adapted rainbow trout fry was investigated in response to the administration of angiotensin I, angiotensin II, the angiotensin II antagonist saralasin, and the angiotensin-converting enzyme inhibitor enalapril. Rainbow trout fry (0. 8-1.5 g), as previously shown, drank at a rate of about 0.7-0.9 mL kg⁻¹ h⁻¹ in freshwater, and the intramuscular administration of angiotensin I or angiotensin II increased drinking rates maximally to about 4-5 mL kg⁻¹ h-1', while intraperitoneal injections at a similar dose were less potent. Intramuscular administration of increasing levels of angiotensin II (0.2-2 nmol g⁻¹ of [Asn¹, Val⁵]-angiotensin II) resulted in a dose-dependent increase of drinking rate, and this response was partially but significantly inhibited by the administration of the angiotensin II receptor blocker saralasin ([Sar¹, Val⁵, Ala⁸]-angiotensin II). Drinking rates were also increased (maximally to about 5 mL kg⁻¹ h⁻¹) in response to intramuscular injection of increasing levels of angiotensin I, with maximum responses between 0.26 and 0.35 nmol⁻¹ g, and this response was significantly lowered by the administration of enalapril and by saralasin. The administration of losartan (DuP 753), a selective inhibitor of the receptor subtype AT, in mammals, had an inconsistent effect on drinking in freshwater and seawater-adapted fish. Although the role of drinking in juvenile freshwater fish is not understood, the present results constitute further evidence for the involvement of the renin-angiotensin system in control of drinking in fish.