Development of a cytochemical assay for plasma vasopressin: application to studies on water loading normal man.

A cytochemical assay has been developed to measure human plasma arginine vasopressin. It is based on the stimulation of Na+-K+, ATPase activity located in the outer medulla of the rat kidney, and is capable of detecting very low plasma arginine vasopressin concentrations, limit of detection 0.01 pmol/l. Specificity for vasopressin stimulation of the enzyme is conferred on the assay by the use of specific vasopressin antiserum. Index of precision of the assay is 0.21. Degradation of arginine vasopressin in plasma in inhibited by phenanthroline. Samples may be stored up to 8 weeks at -70 degrees C. Intra- and inter-assay coefficients of variation were 22% (n = 8) and 104% (n = 12), respectively. A sustained water load in eight healthy male adults caused a fall in plasma osmolality from a basal of 286.5 +/- 2.0 (mean +/- SEM) to 279.2 +/- 2.4 mmol/kg after the load (P less than 0.001), which was associated with a reduction in urine osmolality from 867 +/- 54 to 69 +/- 3 mmol/kg. Plasma immunoreactive arginine vasopressin fell from 1.3 +/- 0.3 pmol/l to become undetectable (less than 0.3 pmol/l), but plasma cytochemical arginine vasopressin decreased from 0.96 +/- 0.14 to 0.07 +/- 0.02 pmol/l. There was a curvilinear relationship between plasma osmolality and plasma cytochemical arginine vasopressin, which militated against the concept of an osmotic threshold for vasopressin release.