Changes in Basal and Stimulated Growth Hormone Secretion in the Aging Rhesus Monkey: A Comparison of Chair Restraint and Tether and Vest Sampling*

Abstract

We studied basal serum GH and GH responses to iv clonidine and insulin-induced hypoglycemia in a group of four young (5-7 yr old) and four older (10-14 yr old) adult male rhesus monkeys under two restraint conditions, chair adaptation and a tether and vest system, to determine what changes in GH secretion occur with aging. The serum GH response to iv administration of GH-releasing hormone (GHRH) was also studied in the groups under tether and vest restraint. Serum samples were collected every 15 min and assayed for GH using a human GH RIA and for cortisol using an enzyme-linked immunosorbant assay. GH and cortisol concentrations in the young and older groups were analyzed with analysis of variance (ANOVA). In the chaired studies the older animals had a lower mean 6-h basal GH concentration than did the younger animals (2.7 +/- 0.8 vs. 3.5 +/- 0.5 micrograms/L; P = 0.0002). Prestimulation GH was lower before clonidine and insulin in the older chaired group (1.1 +/- 0.5 and 2.3 +/- 0.6 micrograms/L, respectively) compared to the younger group (3.6 +/- 0.8 and 3.8 +/- 0.7 micrograms/L, respectively; P less than 0.001). Poststimulation GH was lower after clonidine and insulin in the older chaired group (3.2 +/- 2.4 and 7.1 +/- 2.8 micrograms/L, respectively) compared to the younger chaired group (6.3 +/- 2.2 and 10.3 +/- 3.0 micrograms/L, respectively; P less than 0.05), but the differences in GH increments were not statistically significant. In the tether and vest studies the older animals had a lower mean 6-h basal GH concentration than did the younger animals (1.7 +/- 0.4 vs. 3.5 +/- 1.2 micrograms/L; P less than 0.0001). Prestimulation GH concentrations were also lower in the older tethered animals before clonidine (2.1 +/- 0.3 micrograms/L) and GHRH (1.4 +/- 0.2 micrograms/L) compared to levels in the younger animals (3.1 +/- 0.9 and 3.2 +/- 0.7 micrograms/L; P = 0.0023 and P = 0.0001, respectively). The younger tethered animals had greater poststimulation responses to clonidine (8.7 +/- 3.0 micrograms/L), insulin (8.8 +/- 3.6 micrograms/L), and GHRH (6.0 +/- 2.4 micrograms/L) than the older animals (3.8 +/- 0.9, 3.9 +/- 2.5, and 2.9 +/- 0.7 micrograms/L; P less than 0.0001, P = 0.0025, and P less than 0.03, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)