Angiotensin and Converting Enzyme Regulate Extrarenal Salt Excretion in Ducks

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Abstract
Numerous previous studies have proposed a salt-conserving role for the renin-angiotensin system in mammals, but there is little evidence of this putative role in birds. Especially interesting are marine birds, which have a relatively limited ability to regulate the osmolality and ionic composition of their urine but possess extrarenal salt glands capable of excreting a highly concentrated NaCl solution. Hypertonic saline, angiotensin I (ANG I) and captopril were infused i.v. into chronically cannulated ducks to study the neuroendocrine, cardiovascular, renal and extrarenal excretory responses to osmotic stress. Infusion of hypertonic saline elicited nasal salt excretion, which could be stopped completely by coadministration of ANG I. The effective dose of ANG I increased the plasma norepinephrine (NE) concentration, but did not alter heart rate or arterial blood pressure. Captopril enhanced extrarenal salt excretion in the saline-loaded ducks. The converting enzyme inhibitor also blocked the noradrenergic and NaCl-retaining actions of ANG I; conversely, coadministration of captopril and ANG I increased the plasma epinephrine (E) concentration. Apparently, the renin-angiotensin system, in addition to effects on the sympathoadrenal system, regulates NaCl and water metabolism in birds with extrarenal salt glands.