Human pancreatic GRF (hpGRF-40; 1 microgram/kg, iv) selectively stimulates GH release in normal men (9). We now report the effects of graded doses of hpGRF-40 on GH release in 12 normal men. Mean peak increments in serum GH after vehicle and the various doses of hpGRF-40 were 1.13, 11.40, 14.60, 17.01, 14.45, and 15.60 ng/ml after vehicle and 0.1, 0.33, 1.0, 3.3, and 10 micrograms/kg hpGRF-40 (iv bolus), respectively. Peak values were observed 30-60 min after hpGRF-40 treatment. There was considerable variability of responsiveness among individual subjects, and no dose-response relationship between the doses and maximal GH values was found. However, the higher doses of 3.3 and 10.0 micrograms/kg resulted in a more prolonged and biphasic pattern of GH release. A side effect of facial flushing of less than 5-min duration occurred in 4 or 6 subjects who received 3.3 micrograms/kg and in all 5 who received 10 micrograms/kg of hpGRF-40. No changes in serum glucose, LH, TSH, PRL, plasma cortisol, or 8 enteropancreatic hormones occurred after hpGRF-40 treatment. There were small increases in serum somatomedin C levels 24 h after the administration of various doses of hpGRF-40 in 11 of 13 studies. Plasma immunoreactive GRF levels measured 5 min after injection were 0.09, 2.0, 4.9, 23.9, and 66.6 ng/ml after 0.1, 0.33, 1.0, 3.3, and 10 micrograms/kg hpGRF-40, respectively. Serum GH responses after insulin-induced hypoglycemia were compared to GH responses after hpGRF-40. Comparable peak GH stimulation occurred with both provocative tests. Mean +/- SEM peak GH was 20.2 +/- 1.0 ng/ml after insulin and 20.9 +/- 3.2 after hpGRF-40 treatment. hpGRF-40 selectively stimulates GH release in normal men over a dose range of 0.1-10 micrograms/kg and is an effective probe to investigate the dynamics of GH release.

The effects of synthetic hpGRF-40 on GH release from continuously perifused male rat anterior pituitary cells were studied. Pulses (2.5 min) of hpGRF-40 stimulated GH release in a log-linear dose response relationship: concentrations of 0.03, 0.1, 0.3, 1, 3, 10, 30 and 100 nM given in a random order elicited a GH response above baseline of 1.2 +/- 0.3, 2.4 +/- 0.4, 2.8 +/- 0.2, 4.3 +/- 0.2, 6.2 +/- 0.7, 7.0 +/- 1.0, 8.7 +/- 1.7, and 10.8 +/- 0.8 micrograms/10(7) cells (mean +/- SEM; n = 3; r = 0.93), respectively. During a 5-h hpGRF-40 infusion, GH stimulation peaked within 5 min and waned to near baseline by the end of the fifth h. The integrated GH responses to 0.03, 0.1 and 0.3 nM hpGRF-40 were 37.6 +/- 7.4, 52.9 +/- 8.5, and 66.15 +/- 8.2 micrograms/10(7) (mean +/- SEM; n = 3; r = 0.72), respectively. The interaction of TRH and hpGRF-40 in the control of GH secretion was studied to investigate the mechanism of the "paradoxical" TRH stimulation of GH release associated with GH excess states in humans. Dispersed cells were perifused with either 100 nM TRH for 0.5 h, 5 nM hpGRF-40 for 4 h, or 5 nM hpGRF-40 for 4 h, to which a 0.5 h pulse of TRH was added at 2 h. GH levels did not change significantly in the presence of TRH alone. When TRH was added to the ongoing hpGRF-40 perifusion, GH release increased from 1.4 +/- 0.06 to 4.0 +/- 1.0 micrograms/min.10(7) cells (n = 4; P = 0.03). Thus, dispersed pituitary cells are highly sensitive to very low concentrations of hpGRF-40 administered as both an acute pulse and as a tonic infusion. When the cells are exposed to a maximal concentration of hpGRF-40 (i.e. 5 nM), TRH becomes a secretagogue at the pituitary level, thus suggesting the site and mechanism of the "paradoxical" GH response to TRH observed in some acromegalics.