Growth Hormone Regulation by Growth Hormone-Releasing Hormone in Infant Rhesus Monkeys

Abstract

To study the relationship of endogenous GHRH to the changes seen in serum GH concentration in neonatal primates, we administered antiserum to GHRH (GHRH-Ab) or GHRH antagonist, (N-Acetyl-Tyr¹, D-Arg²)GHRH-(1-29)-NH₂, to unanesthetized, fasted rhesus monkeys, 1–31 days of age, via an acutely placed venous catheter. The administration of GHRH-Ab resulted in a significant decrease in serum GH concentration at 1–2 and 7–9 days, but not at 28–31 days of age. The administration of GHRH antagonist resulted in a significant decrease in serum GH concentrations at 1–2 days, but not at 7 or 14 days of age. These results imply that basal serum GH is more dependent on endogenous GHRH at 1–2 days than after 7–28 days of age. This suggests developmental changes in the hypothalamic regulation of GH secretion with advancing age in the neonatal period.

Feeding colostrum increases circulating insulin-like growth factor I in newborn pigs independent of endogenous growth hormone secretion.

“…We should note that even though we observed ablation of GH secretion after acute treatment of newborn pigs with the GRF antagonist, neither GH secretion nor circulating IGF-I concentration was affected in 8-d-old neonatal pigs treated with a similar daily dose of the GRF antagonist in the previous 7 d (our unpublished observations). Decreased responsiveness of GH secretion to GRF-receptor blockade with advancing age also has been reported in monkeys and may be related to maturation of the ultra-short loop feedback system for GRF (Wheeler et al, 1991). Despite the significantly lower plasma GH concentration in GRF-A-treated pigs than that in control pigs, the circulating IGF-I and IGFBP-3 concentrations were similar.…”

“…Our primary objective in this study was to establish the relative effects of nutrient intake and GH on circulat-ing IGF-I in colostrum-fed newborn pigs. In order to determine the relationship between circulating GH and IGF-I, we chose to ablate GH secretion using a GRF receptor antagonist approach used previously (Lumpkin et al, 1989;Wheeler et al, 1991;Jaffe et al, 1993) to eliminate pulsatile GH secretion. This approach has advantages compared with hypophysectomy, because it avoids surgical stress and does not affect either the secretion of other pituitary hormones or feed intake.…”