Changes in basal and stimulated growth hormone secretion in the aging rhesus monkey: A comparison of chair restraint and tether and vest sampling

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 μg/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 μg/L, respectively) compared to the younger group (3.6 ± 0.8 and 3.8 ± 0.7 μg/L, respectively; P < 0.001). Poststimulation GH was lower after clonidine and insulin in the older chaired group (3.2 ± 2.4 and 7.1 ± 2.8 μg/L, respectively) compared to the younger chaired group (6.3 ± 2.2 and 10.3 ± 3.0 μg/L, respectively; P < 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 μg/L; P < 0.0001). Prestimulation GH concentrations were also lower in the older tethered animals before clonidine (2.1 ± 0.3 μg/L) and GHRH (1.4 ± 0.2 μg/L) compared to levels in the younger animals (3.1 ± 0.9 and 3.2 ± 0.7 μg/L; P = 0.0023 and P = 0.0001, respectively). The younger tethered animals had greater poststimulation responses to clonidine (8.7 ± 3.0 μg/L), insulin (8.8 ± 3.6 μg/L), and GHRH (6.0 ± 2.4 μg/L) than the older animals (3.8 ± 0.9, 3.9 ± 2.5, and 2.9 ± 0.7 μg/L; P < 0.0001, P = 0.0025, and P < 0.03, respectively). The younger tethered animals also had greater incremental GH responses to clonidine (5.4 ± 3.5 μg/L) and insulin (8.2 ± 3.6 μg/L) than the older animals (1.8 ± 0.8 and 3.2 ± 2.6 μg/L; P = 0.0043 and P = 0.022, respectively). Cortisol levels were consistently higher with chair restraint than with the tether and vest system, but were not consistently different with age in either the chair or tether studies. These data suggest a clear decrease in basal and stimulated GH secretion with advancing age in the adult male rhesus monkey and indicate that the tether and vest restraint system is a superior method for the study of nonhuman primates.