Role of blood osmolality in the regulation of vasopressin secretion in man: application of a new radioimmunoassay method for vasopressin.

A radioimmunoassay for plasma arginine vasopressin (AVP) has been developed based on R2 antibody of Thomas and Lee, synthetic standard (Ferring) and extraction on Sep-Pak column. High recovery of AVP (approximately 79%) was achieved with a detection limit of 0.25 pg/ml. By improving the technique of measurement of plasma osmolality an intraassay coefficient of variation less than 1% was obtained. Physiological studies performed with this method demonstrated that AVP becomes undetectable after water loading 20 ml per Kg of water po; (N = 6) and increases in response to hypertonic saline infusion (0.05 ml/kg/min; N = 15) with a linear relationship between plasma osmolality and AVP in individual subjects; this relationship is maintained when the test is repeated in the same subjects. However when pooling all data together, the relationship between plasma osmolality and AVP is best expressed by an exponential relationship. This implies that after AVP release is initiated, the concentration of the hormone increases more rapidly than plasma osmolality and the release is continuous possibly due to recruitment of increasing number of neuronal units whose osmotic threshold varies from individual to individual.