L-dopa stimulates release of hypothalamic growth hormone-releasing hormone in humans

A sensitive RIA for human GH-releasing hormone-(1-44)-NH2 [hGHRH-(1-44)-NH2] was developed which allows its measurement in human plasma extracts. The assay did not detect hGHRH-(1-37)-OH or hGHRH-(1-40)-OH. A method to extract hGHRH from plasma was developed using silicic acid and acid-acetone, by which recovery of synthetic hGHRH-(1-44)-NH2 from plasma averaged 74.3%. Serial dilutions of plasma extracts gave an inhibition curve parallel with that of synthetic hGHRH-(1-44)-NH2 in the RIA system. On Sephadex G-50 columns, hGHRH-like immunoreactivity (hGHRH-LI) in plasma extracts eluted as a single peak corresponding to hGHRH-(1-44)-NH2. This hGHRH-LI peak, when subjected to reverse phase HPLC, emerged at the position where hGHRH-(1-44)-NH2 was eluted. hGHRH-LI was detectable in the peripherally circulating plasma of all subjects tested. The mean basal level of plasma hGHRH-LI in normal subjects was 9.4 +/- 0.7 (+/- SE) pg/ml (n = 22; range, 2.8-18.1 pg/ml), comparable to the basal plasma hGHRH-LI concentration in patients with hypothalamic lesions (11.3 +/- 1.1 pg/ml; n = 7). Oral administration of L-dopa (0.5 g) caused a significant increase in both plasma hGHRH-LI and GH levels in normal subjects, and the plasma hGHRH-LI peak slightly preceded or coincided with that of plasma GH in individual subjects. There was also a significant correlation between plasma hGHRH-LI and the GH rises after L-dopa administration when their net increments were compared. All of the patients with hypothalamic lesions had significant increases in plasma GH after hGHRH-(1-44)-NH2 injection (1 microgram/kg BW, iv), indicating the presence of functioning somatotrophs in their pituitaries. When L-dopa was orally administered to these patients, neither plasma hGHRH-LI nor GH concentration changed throughout a 120-min observation period. These findings suggest that 1) hGHRH, immunologically and chromatographically indistinguishable from synthetic hGHRH-(1-44)-NH2, is detectable in peripheral plasma in humans; 2) L-dopa stimulates the release of hypothalamic hGHRH, alterations of which are reflected in changes in peripheral levels; and 3) the source of circulating hGHRH is not restricted to the hypothalamus, since hGHRH-LI is present in the peripheral plasma of patients with hypothalamic lesions in amounts similar to those found in normal subjects.