Reduction of the effects of growth hormone release inhibiting factor enhances plasma growth hormone response to GHRH.

Responses of serum growth hormone (hGH) to glucagon (G), growth hormone releasing hormone (GHRH) and G/GHRH were measured in 8 normal adults and 6 patients with growth hormone deficiency (GHD). In normal adults, serum hGH reached its peak value (12.7 +/- 1.6 ng/ml) at 150 +/- 10 min, as blood glucose declined to its minimum after a transitory hyperglycemia in G test. The normal adults were responsive to GHRH test (GH peak 14.7 +/- 2.3 ng/ml at 30 +/- 0 min). In GHD, the responders to both G and GHRH tests showed a strongly positive response in G/GHRH test, with a serum hGH peak value of 34.6 +/- 4.1 ng/ml at 131 +/- 8 min being much higher than that of either single G or GHRH test (P less than 0.01), but without significant difference to the sum of the two single tests (P greater than 0.10). Among GHD patients, only 2 responded to GHRH and G/GHRH tests with hGH peak values 6.8 +/- 0.7 and 6.9 +/- 0.7 ng/ml at 45 +/- 15 and 90 +/- 0 min, respectively, both peak values being essentially similar (P greater than 0.10). We suggest that the mechanism of stimulation of pituitary hGH secretion in G test might involve inhibition of release of hypothalamic GH release inhibiting factor (GHRIF) caused by hypoglycemia after a transitory hyperglycemia following G injection. These results may further confirm our previous postulation (1986) that insulin hypoglycemia may increase hGH release by inhibiting hypothalamic cell secretion of GH release inhibiting factor.