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,
MITCHELL S. HARMAN,
GEORGE R. MERRIAM,
MARIE C. GELATO and
MARC R. BLACKMAN
Endocrinology Section Laboratory of Clinical Physiology, Gerontology Research Center, National Institute on Aging, National Institutes of Health Departments of Medicine, Francis Scott Key Medical Center, and Johns Hopkins University School of Medicine Baltimore, Maryland 21224
Developmental Endocrinology Branch, National Institute of Child Health and Human Development, National Institutes of Health Bethesda, Maryland 20205
Address requests for reprints to:Dr.M.R. Blackman, Gerontology Research Center, 4940 Eastern Avenue, Baltimore, Maryland 21224.
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.
* This work was presented in part at the 67th annual Meeting of the Endocrine Society, Baltimore, MD,June 19–21, 1985 (Abstract 233).
Current address: Eugenia P. Pavlov, Department of Medicine, Greater Baltimore Medical Center, Baltimore, MD.
Received August 29, 1985.
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