IMPAIRMENT OF OSMOTICALLY STIMULATED AVP RELEASE IN PATIENTS WITH PRIMARY POLYDIPSIA

The secretion of arginine vasopressin (AVP) from the posterior pituita ry is primarily and finely regulated by the osmolality of plasma. Even though a number of factors alter osmolality-induced release of AVP, t here are no published data in humans that have addressed the role of c hronic overhydration on this phenomenon. To address this problem we ha ve identified eight patients with primary polydipsia using criteria no t involving measurement of AVP, and have subjected them to standardize d infusions of hypertonic saline. These patients had less AVP in both plasma and urine in relation to plasma osmolality than was found in no rmal subjects. In addition, their rate of rise of plasma and urine AVP was less than in normal subjects. Their osmotic threshold for AVP rel ease may have been higher than normal. These data demonstrate that chr onic overhydration in humans downregulates the release of AVP in respo nse to hypertonicity. This phenomenon may explain the impairment of ur ine concentration in patients with primary polydipsia and emphasizes t he basis of the difficulty that may occur clinically in differentiatin g between patients with primary polydipsia and partial central diabete s insipidus.