Drinking rate in juvenile Atlantic salmon,Salmo salar L fry in response to a nitric oxide donor,sodium nitroprusside and an inhibitor of angiotensin converting enzyme,enalapril

Drinking in freshwater juvenile salmon was investigated in response to vasodilation by sodium nitroprusside (SNP), a nitric oxide donor, which significantly increased blood vessel diameter in Atlantic salmon alevins. Atlantic salmon fry (1–3 g), as previously shown, drank at a significant rate in fresh water which doubled to about 1.2 ml kg^–1 h^–1 following injection of SNP (100 mol kg^–1), through dilation of body vasculature and activation of a vasoconstrictive mechanism, the endogenous renin angiotensin system (RAS). This response was 50% inhibited by injection of about 100 mg kg^–1 enalapril. Fry increased drinking in response to SNP administered in the water, though the concentration required for maximal response, 1.6 mmol l^–1, was much greater than for injected SNP; this response was also inhibited by enalapril injection. Possible involvement of the gill vasculature and branchial osmoreceptors or baroreceptors in control of the drinking response is discussed.