Osmotic regulation of plasma vasopressin and oxytocin after sustained hyponatremia.

Endogenous pituitary secretion of vasopressin (AVP) and oxytocin (OT) was studied in rats with induced dilutional hyponatremia that was sustained for 2-5 days. Graded infusions of hypertonic saline produced progressive increases in plasma osmolality, but despite large relative increases in osmolality, AVP and OT secretion was not significantly stimulated until plasma [Na+] reached normal ranges. Regression analysis of plasma AVP and OT levels once secretion was stimulated showed no significant shift of the osmotic threshold for neurohypophyseal secretion of either hormone, as well as equivalent slopes of plasma AVP and OT increases per unit increases in plasma osmolality, in the hyponatremic rats relative to normonatremic controls. After a 20-25% decrease in plasma osmolality, total brain water content of hyponatremic rats increased only 3-6%, whereas larger decreases (11-17%) in total brain electrolytes were found. During subsequent hypertonic saline infusions in the hyponatremic rats total brain water content decreased linearly, resulting in brain dehydration below normal total brain water levels before stimulation of AVP and OT secretion occurred. The stability of the osmotic threshold for neurohypophyseal secretion, despite evidence of brain adaptation to chronic hypotonicity, argues that cell volume regulation via solute loss is not a likely cause of resetting of the osmostat. Additionally, brains of hyponatremic rats were found to manifest significantly greater susceptibility to dehydration after hypertonic saline infusions than brains of normonatremic controls, which may be a consequence of brain solute loss from adaptation to chronic hypotonicity.