Longitudinal changes in growth hormone response to growth hormone-releasing hormone in neonatal rhesus monkeys

To determine whether differential response to growth hormone-releasing hormone (GHRH) could cause the developmental changes seen in growth hormone (GH) secretion, we administered 10 micrograms/kg GHRH (1-44 NH2) to a group of four unanesthetized, fasted, rhesus monkeys via acutely placed venous catheters at 1, 7, 14, and 28 d postnatal age. Serum GH was assayed by hGH RIA in sera collected at -60, -30, 0, 15, 30, 45, 60, and 90 min relative to the GHRH bolus. Serum cortisol was measured by ELISA in the 0-, 30-, and 60-min samples. Differences between age groups were analyzed by repeated measures analysis of variance and paired t tests. Mean basal GH levels were higher at 1 d (9.4 +/- 1.2 micrograms/L, mean +/- SEM) than at 7 (5.5 +/- 0.4), 14 (5.6 +/- 0.5), and 28 d (5.3 +/- 0.5) of age. There were no other significant differences in mean basal GH values between the age groups. Mean post-GHRH GH concentrations decreased significantly with each age after 1 d (22.6 +/- 1.6): 7 d (16.4 +/- 1.3); 14 d (11.3 +/- 1.0); and 28 d (7.9 +/- 0.9). Similarly, mean delta-GH values decreased with each increase in age from 1 d (15.0 +/- 1.9): 7 d (10.9 +/- 1.6); 14 d (5.9 +/- 1.1); and 28 d (2.7 +/- 0.8). Serum cortisol was not correlated with serum GH at any age. Our study demonstrates decreasing basal GH concentration and GH responses to GHRH with advancing age from 1 to 28 d in the rhesus monkey.(ABSTRACT TRUNCATED AT 250 WORDS)