Pulsatile Growth Hormone Secretion in Normal Man during a Continuous 24-Hour Infusion of Human Growth Hormone Releasing Factor (1-40) Evidence for Intermittent Somatostatin Secretion

Growth hormone (GH) secretory patterns were studied in a patient with ectopic growth hormone releasing factor (GRF)secretion and in normal men given continuous infusions ofhuman growth hormone releasing factor (1-40)-OH (hGRF-40). In the patient with ectopic GRF secretion, GH secretionwas pulsatile despite continuously elevated immunoreactiveGRF levels. To determine if pulsatile GH secretion is maintainedin normal subjects, we administered to six healthy young menvehicle or hGRF-40, 2 ng/kg per min, for 24 h and gave asupramaximal intravenous bolus dose of hGRF-40, 3.3 Aig/kg,after 23.5 h of infusion. hGRF-40 infusion resulted in greaterGH secretion than did vehicle infusion and pulsatile GHsecretion was maintained throughout hGRF40 infusion. Duringthe 23.5 h of vehicle infusion, total GH secretion (microgram;mean±SEM) was 634±151 compared with 1,576±284 duringhGRF-40 infusion (P = 0.042). The GH response to the intravenousbolus of hGRF-40 was greater after vehicle infusionthan after hGRF-40 infusion; 877±170 and 386±125 ,ug ofGH was secreted after the bolus on vehicle and hGRF-40days, respectively (P = 0.015). The total amount of GH secretedduring the 25.5 h of the two study days was not different;1,504±260 and 1,952±383 1g were secreted during vehicleand hGRF40 days, respectively (P = 0.36). Not only waspulsatile GH secretion maintained during hGRF40 infusion,but there was augmentation of naturally occurring GH pulses,which is in contrast to the effect of gonadotropin-releasinghormone on gonadotropin secretion. We suggest that GHpulses are a result of GRF secretion that is associated with adiminution or withdrawal of somatostatin secretion.