Serum Protein-vasopressin Binding and Its Influence on the Pressor Activity of the Hormone.

The manner in which the posterior pituitary hormones are carried in the plasma has not been clarified but seems to differ from one species to another1). Several evidences have been brought forward to indicate that vasopressin in human plasma is bound with the plasma protein2, 3, 4). Our previous observation showed different vasopressin binding capacity between the plasma samples obtained from different individuals (unpublished data). The present study was first aimed to reinvestigate the variation in the binding capacity using a larger number of freshly obtained human, dog and rat serum and plasma samples. During the course of the investigation on the vasopressin-serum protein binding we have noticed that the pressor activity of vasopressin added to human serum is usually lower than that of vasopressin in physiological saline in the same concentration as in the serum and that the reduction of the pressor activity by the serum varied in extent from one sample to another. The fall of the pressor activity may probably be due to inactivation of the hormone by serum. But it may also be possible that binding itself participates in reducing the activity5) or interferes the degradation of vasopressin in the serum.