Growth hormone responses to continuous infusions of growth hormone-releasing hormone

The pattern of GH secretion during a continuous 4-h iv infusion of 1 microgram/kg.h GH-releasing hormone (1-44)-NH2 (GHRH-44) or saline was examined in 15 adult men. There was prompt release of GH beginning within 20 min of starting the GHRH-44 infusions, reaching peak GH levels of 43 +/- 11 (+/- SE) ng/ml within 60-90 min. This is similar to the peak GH level reached in men after a single 1 microgram/kg GHRH iv bolus dose (34 +/- 8 ng/ml). GH levels then fell progressively, but did not return to baseline during the GHRH infusions. After GHRH infusions, the response (delta) to a 1 microgram/kg GHRH bolus dose was markedly attenuated (delta GH, 2.7 +/- 0.9 ng/ml) compared to the response (delta GH, 23 +/- 3 ng/ml) after saline infusion. Dispersed rat pituicytes perifused with medium containing 10 nM GHRH-44 responded with an initial rapid rise in GH secretion, followed by a progressive decline, and after 150 min of continuous GHRH exposure, the response to pulses of an equal or higher (100 nM) GHRH concentration was blunted. These results indicate that the peak response to GHRH infusions is similar to that of maximally effective bolus doses; during infusions, the GH response is not sustained; and immediately after GHRH infusions, the response to previously effective bolus doses is reduced. These phenomena could reflect either receptor-mediated desensitization, the depletion of rapidly releasable GH stores, or both. A counterregulatory rise in hypothalamic somatostatin secretion is not necessary to produce these effects, since the same phenomenon occurs in vitro and in vivo.