Plasma growth hormone and somatomedin-C responses to continuous growth hormone-releasing factor infusion in normal adult men.

The pituitary growth hormone (GH) responses during a 20-hour iv infusion of saline or human GH-releasing factor (hGRF-44) at 40 micrograms/h, followed by an iv bolus injection of hGRF at 2 micrograms/kg body weight, were studied in four normal adult men. During saline infusion only one or two pulses of plasma GH were observed. However, during hGRF infusion up to eight or ten pulses of GH were measured with an amplitude not different from that obtained during saline infusion. The mean +/- SEM integrated amount of GH secreted was 107 +/- 38.2 ng/ml.h in response to hGRF infusion, which was greater than the value of 25.4 +/- 3.5 ng/ml.h obtained during saline infusion. Plasma somatomedin-C also increased after hGRF infusion, but not after saline. After saline or hGRF infusion most of the subjects still responded to an iv bolus injection of the peptide (2 micrograms/kg). These results indicate that hGRF infusion augments GH secretion by increasing the number, but not the amplitude of GH pulses and that the infusion does not cause the pituitary somatotrophs to lose their capacity and ability to respond to hGRF subsequently.