Sodium deprivation blunts hypovolemia-induced pituitary secretion of vasopressin and oxytocin in rats

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