Responses of Growth Hormone (GH) and Somatomedin-C to GH-Releasing Hormone in Healthy Aging Men*

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Abstract
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 ( 0.8), 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 0.7). 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.