Distribution, blood transport, and degradation of antidiuretic hormone in man

The distribution, blood transport, and metabolic clearance of physiological concentrations of antidiuretic hormone were studied in 10 hydrated normal subjects with radioiodinated arginine vasopressin (125I-AVP). At 37 degrees C no binding of 125I-AVP to plasma proteins could be demonstrated, but some metabolites were associated with plasma proteins. 125I-AVP was rapidly distributed into a space approximating the extracellular fluid volume. Metabolic breakdown products became demonstrable within minutes after injection. The mean metabolic clearance rate of 125I-AVP was 4.1 ml/min/kg and the mean plasma half-life 24.1 min. Renal clearance had a mean value of 80 ml/min and accounted for 27% of the total metabolic clearance. It is concluded that in man antidiuretic hormone circulates as a free (non-protein bound) peptide, diffuses readily into the extracellular fluid space, and is metabolized within minutes. A plasma half-life of 24 min is consistent with the duration of antidiuresis after hormone administration or release.