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)-NH2 (1 microgram/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 (less than 0.7 ng/ml) in all but 2 men. Neither the frequency of GH responses (P greater than 0.8), the magnitude of response (P greater than 0.2), nor the timing (P greater than 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 less than 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 less than 0.001) and 24 h after (P less than 0.02) GHRH injection. Repeated measures analysis of variance revealed significant (P less than 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 greater than 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.