SODIUM DEPRIVATION BLUNTS HYPOVOLEMIA-INDUCED PITUITARY SECRETION OF VASOPRESSIN AND OXYTOCIN IN RATS

The present investigations determined the effects of dietary sodium de privation on the neurohypophysial secretion of arginine vasopressin (A VP) and oxytocin (OT) by rats in response to nonhypotensive hypovolemi a induced by subcutaneous injection of 30% polyethylene glycol solutio n. In rats fed either standard sodium-rich laboratory chow or sodium-d eficient diet for 8 days, AVP secretion increased gradually in proport ion to plasma volume deficits up to 22-28% while pituitary secretion o f OT was not stimulated. However, when hypovolemia was more pronounced , secretion of both hormones was marked in rats fed standard chow, whe reas rats fed sodium-deficient diet were significantly less responsive . These effects did not reflect a general insensitivity of the neurohy pophysial system because sodium-deprived and chow-fed rats secreted AV P and OT equivalently in response to intravenous infusion of 1.5 M NaC l solution. Nor did they reflect a general insensitivity to hypovolemi a because sodium-deprived rats drank substantial, above-normal volumes of water after colloid treatment. Instead, the results appear to refl ect a specific inhibition of stimulatory baroreceptor inputs to AVP an d OT neurons during dietary sodium deprivation in rats.