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, whi ch significantly increased blood vessel diameter in Atlantic salmon al evins. Atlantic salmon fry (1-3 g), as previously shown, drank at a si gnificant rate in fresh water which doubled to about 1.2 ml kg(-1) h(- 1) following injection of SNP (100 mu mol kg(-1)), through dilation of body vasculature and activation of a vasoconstrictive mechanism, the endogenous renin angiotensin system (RAS). This response was 50% inhib ited by injection of about 100 mg kg(-1) enalapril. Fry increased drin king in response to SNP administered in the water, though the concentr ation required for maximal response, 1.6 mmol l(-1), was much greater than for injected SNP; this response was also inhibited by enalapril i njection. Possible involvement of the gill vasculature and branchial o smoreceptors or baroreceptors in control of the drinking response is d iscussed.