Responses of growth hormone (GH) and somatomedin-C to GH-releasing hormone in healthy aging men

Although controversy exists regarding the effects of aging on GH secretory responses to indirect stimulation, in the only prior study of GH-releasing hormone (GHRH)-mediated GH secretion decreased GH responsivity occurred in healthy men after age 40 yr. We measured serum GH before and up to 180 min after and somatomedin-C (SM-C) levels before and 24 h after single morning bolus iv injections of GHRH-(1-(44)-NH)₂ (1 μ5;g/kg) in 50 healthy fasted men, aged 21-86 yr, from the Baltimore Longitudinal Study of Aging. Only subjects with a body mass index (BMI; kilograms per m²) between 20.0 and 29.0 were studied. Basal serum GH levels were undetectable (<0.7 ng/ml) in all but 2 men. Neither the frequency of GH responses (P > 0.8), the magnitude of response (P > 0.2), nor the timing (P > 0.05) of the peak GH responses to GHRH were significantly altered with age. Although BMI values did not vary significantly with age in our study group, there was a significant negative correlation (r = -0.37; P<0.01) of peak GH with BMI. Regression analysis revealed a slight but significant increase in the level of fasting blood sugar with age, but no significant correlation between fasting blood sugar and peak GH levels. Serum levels of SM-C were significantly lower in older men both before (P < 0.001) and 24 h after (P <0.02) GHRH injection. Repeated measures analysis of variance revealed significant (P< 0.001) responses of SM-C to endogenous GH elevations produced by GHRH at all ages, but no age-dependent alterations in the magnitudes of these responses (P > 0.7). Our findings suggest that increasing age in adult men has little effect on the secretory responsiveness of pituitary somatotropes to GHRH. However, the finding of lower serum levels of SM-C with intact SM-C responsivity to endogenous GH is compatible with prior observations of an age-related decrease in the total daily spontaneous secretion of GH.