Inhibition by prednisone of growth hormone (GH) response to GH-releasing hormone in normal men

Glucocorticoids increase GHRH-stimulated GH secretion when added in vitro to cultured monkey, rat, and human pituitary cells and when injected in vivo into anesthetized rats. Yet, in man glucocorticoids inhibit linear growth and GH secretion. To clarify this apparent disparity and to determine if glucocorticoid stimulation can augment GH release in man after direct pituitary stimulation with GHRH, we administered 1 microgram/kg GHRH dosage to seven normal men before and after a 4-day course of prednisone (20 mg, orally, three times daily). The second GHRH test was done 12 h after the last dose of prednisone was given. Prednisone significantly inhibited the mean maximal increase in serum GH after GHRH treatment [20.7 +/- 4.5 (+/- SE) vs. 6.3 +/- 2.4 micrograms/L; P less than 0.01] as well as the GH value obtained by summing and averaging the individual means of the 15, 30, 45, 60, 75, and 90 min serum GH concentrations (11.1 +/- 1.2 vs. 4.3 +/- 0.9 micrograms/L; P less than 0.05). The mean serum insulin-like growth factor I and plasma glucose concentrations were not significantly altered by prednisone administration. These results together with previous in vitro findings imply that glucocorticoid-induced inhibition of GH secretion in man does not occur at the level of the pituitary gland, but, rather, at the hypothalamus or above.