Interaction of Regulation of State of Man Osmotic and Nonosmotic Stimuli in Vasopressin Secretion in Hypoosmolar

Vasopressin (AVP) secretion is principally under osmotic regulation, which is altered by nonosmotic stimuli. It is known that the manner of osmotic regulation of AVP secretion in hypoosmolar state of man consists of four types. The types have (A) random changes in plasma AVP without relation of plasma osmolality; (B) plasma AVP secretion correlated closely to plasma osmolality with a low osmotic threshold for AVP release; (C) nonsuppressible AVP secretion with normal osmotic release of AVP; (D) no abnormalities in AVP secretion. In this study, we found an entirely different type of AVP secretion from the above types in six patients with hyponatremia resulting from various causes during infusion of 5% hypertonic saline. To clarify the mechanism underlying the AVP secretion, we analyzed the interaction between osmotic and nonosmotic stimuli of AVP secretion in these patients. Despite hyponatremia, plasma AVP levels in all patients were not suppressed, which was attributed at least in part to the presence of nonosmotic stimuli for AVP release. These stimuli include nausea, hypotension, blood volume contraction, glucocorticoid deficiency or their combinations. Hypertonic saline infusion increased both serum sodium concentrations and plasma osmolality, although to subnormal levels, and concomitantly, alleviated some of the nonosmotic stimuli for AVP release formerly present in these patients. However, plasma AVP concentrations decreased rapidly during the infusion and reached the nadir in all patients. This phenomenon may be due to alleviation of nonosmotic stimuli for AVP release. Thus, the findings indicate that the potentiating effect of nonosmotic stimuli for AVP secretion may modify the osmotic regulation of AVP secretion in hypoosmolar state, resulting in the type of AVP secretion in this study.