Arteriovenous difference of plasma vasopressin in normal man and effect of posture

In 13 normotensive 50-year-old men arterial plasma vasopressin (11.3 +/- 2.1 ng/l, mean +/- SE) was significantly increased over venous (7.8 +/- 1.4 ng/l) in the supine position with an arteriovenous difference of 3.5 +/- 1.2 ng/l (p less than 0.05). After 30 min in the upright position, an average increment of 45% to 11.3 +/- 1.8 ng/l was observed for venous vasopressin. Since a similar increase was not found for arterial vasopressin, the arteriovenous difference decreased with 29% to 2.5 +/- 2.1 ng/l and was no longer statistically significant. The correlation between supine and standing vasopressin was statistically significant both for arterial (p less than 0.001) and venous plasma (p less than 0.05). These data indicate a substantial removal of plasma vasopressin by receptors even in the peripheral vascular beds (forearm) and not only in the liver and the kidneys as previous literature claims. The arteriovenous difference decreases in the upright position, most likely because of reduced plasma vasopressin clearance.