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1. Bio-assay techniques have been used to measure plasma levels of neurohypophysial hormones in man, following either a single injection or a continuous infusion.

2. The median half-life of oxytocin after a single injection of 2 u. was 3·2 min (2·0-5·7, 95% confidence limits); this increased significantly (P < 0·01) to 4·8 min (4·4-6·1) when the hormone was infused at a rate of 500 m-u./min. The vasopressins had appreciably longer half-lives. After a single injection of 1 or 1·5 u. 8-lysine vasopressin (LVP), the half-life was 5·7 min (3·6-6·0). Continuous infusions of the hormones at a rate of 120 m-u./min yielded half-lives of 5·5 min (5·0-7·1) for LVP, and 5·6 min (3·9-9·5) for 8-arginine vasopressin (AVP).

3. The apparent volumes of distribution of the hormones were all of the order of two thirds the extracellular volume.

4. In accordance with its shorter half-life, the clearance of oxytocin was greater than that of the vasopressins (1·5 l./min, compared with 1·0 l./min).

5. The antidiuretic potencies of the hormones were studied in over-hydrated subjects, by measuring the rate of urine excretion following an I.V. injection. Duration of antidiuretic action increased in the order: oxytocin, LVP, AVP. A 5:1 mixture of oxytocin and AVP was not as long-lasting as AVP alone. 8·5% (4-22) of an administered dose of AVP was excreted in the urine, and this amount was significantly correlated with urine volume (r = +0·67, P < 0·05).

6. Ultrafiltration of human plasma containing exogenous hormones showed that 30% (13-50) of AVP was bound, the degree of binding being independent of concentration over the range used (50-400 µ-u./ml.) In contrast, oxytocin was completely unbound.

7. Exogenous oxytocin was more stable than exogenous AVP in human plasma. At 4° C there was no significant loss of oxytocin until 7 days, whereas 20% of AVP was inactivated in 2 days. At 37° C a 20% loss of AVP occurred within 4 hr, and a 50% loss within 24 hr; corresponding times for oxytocin were 24 and 48 hr.

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				P. Grant and P. Wiles The role of 8-arginine vasopressin (antidiuretic hormone) in vascular homeostasis Vascular Medicine, March 1, 1991; 2(1): 85 - 94. [PDF]