Radioimmunoassay of vasopressin in unextracted plasma

A radioimmunoassay (RIA) for arg8-vasopressin (AVP) in unextracted human plasma was based on a sensitive anti-AVP rabbit antiserum, inhibition of enzymatic damage to [125I]AVP and AVP, and the use of an individual plasma blank, to correct for interference of plasma factors with the RIA. Sensitivity was 0.4 pg of synthetic AVP detected, corresponding to 1.2 pg/ml of AVP in human plasma. Recovery of AVP added to pooled plasma was 94 +/- 9.3% (mean +/- S.D.) in the low range (AVP, 2.8 pg/ml added) and 106 +/- 11.7% in the high range (45.0 pg/ml added). In 26 healthy, ambulatory subjects on ad lib, water intake, plasma AVP concentration was 2.0 +/- 1.22 pg/ml in the supine position and in 28 healthy subjects, 6.2 +/- 4.3 pg/ml in the upright position. Water loading suppressed the plasma AVP concentration. Smoking caused increased plasma AVP in 3 subjects despite water loading.