IMMUNOREACTIVE GROWTH HORMONE-RELEASING HORMONE (IR-GHRH) IN THE FETOPLACENTAL CIRCULATION AND DIFFERENTIAL-EFFECTS OF L-DOPA, L-ARGININE AND SOMATOSTATIN-14 ON THE PLASMA-LEVELS OF IR-GHRH IN NORMAL ADULTS

The relation of the physiological releases of growth hormone-releasing hormone (GHRH) and growth hormone (GH) into the circulation in variou s conditions was investigated using a sensitive and specific radioimmu noassay for plasma GHRH. The mean fasting plasma level of immunoreacti ve (IR)-GHRH in 72 normal adults was 10.3 +/- 0.5 (mean +/- SEM) pg/ml and there was no significant sex difference in the level. The concent rations of IR-GHRH in plasma from the umbilical artery and umbilical v ein were 107.3 +/- 20.5 pg/ml and 33.6 +/- 3.8 pg/ml, respectively, an d a marked arterio-venous gradient was observed in all 12 individuals examined. The plasma level of IR-GHRH in the maternal vein was signifi cantly lower than that in the cord blood, but was similar to that in n on-pregnant women. In normal adults, although there was no apparent fl uctuation in the level of plasma IR-GHRH or of plasma GH during bed re st, a significant increase of plasma IR-GHRH was detected followed by, or synchronized with the surge of plasma GH after oral administration of L-dopa. In contrast, on L-arginine infusion, no proportional eleva tion of plasma IR-GHRH with increase in plasma GH was observed. During and after intravenous infusion of somatostatin, the circulating IR-GH RH level did not increase, but on stopping the infusion there was an i mmediate and marked rebound surge of GH. We conclude that 1) the eleva ted IR-GHRH in the cord blood plasma originates from the fetus and may have a primary role in enhancing secretion of GH which promotes growt h in early human life, and 2) the participations of GHRH in the mechan isms of GH secretion seen after administrations of L-dopa, L-arginine and somatostatin are different.