Plasma disappearance half-time and metabolic clearance rate of exogenous human growth hormone-releasing hormone-(1-44)-NH2 in normal subjects.

By means of human growth hormone-releasing hormone (hGHRH)-RIA using an antiserum directed toward the C-terminal region of hGHRH-(1-44)-NH2, the plasma disappearance half-time and metabolic clearance rate (MCR) of immuoreactive hGHRH (IR-hGHRH) were examined in normal subjects after an iv bolus injection of synthetic hGHRH-(1-44)-NH2 (1 microgram/kg BW). The disappearance of IR-hGHRH from plasma was characterized by a biexponential decay curve, with initial distribution and subsequent metabolic t1/2 value of 5.0 +/- 1.0 and 29.6 +/- 5.4 min (mean +/- SE), respectively. The MCR of IR-hGHRH was 6.1 +/- 1.2 ml/min X kg. The volume of distribution of IR-hGHRH was 3.3 +/- 0.4 liters. The molecular size of the plasma IR-hGHRH was not different from that of hGHRH-(1-44)-NH2 for at least 90 min after the injection of hGHRH-(1-44)-NH2 when examined by gel-filtration chromatography. This prolonged clearance of hGHRH from human plasma relative to that of other hypothalamic hormones may in part explain the sustained plasma GH rises after hGHRH injection in man.