Impaired secretion of growth hormone-releasing hormone, growth hormone and IGF-I in elderly men.

The GHRH test and L-dopa test were performed in 12 normal young men (24.1 +/- 1.1 years) and 12 normal elderly men (77.8 +/- 1.4 years) to investigate age-related changes in secretion of GHRH, GH and IGF-I. The basal plasma levels of GHRH and GH were not significantly different in young and elderly men, but the basal plasma level of IGF-I was higher in the young men (159.0 +/- 11.7 vs 86.7 +/- 11.6 micrograms/l). The area under the curve for plasma GH in the GHRH test was less in the elderly group (35.1 +/- 5.9 vs 11.2 +/- 2.1 micrograms.h-1.l-1, p less than 0.001). The AUCs for the plasma GHRH and GH responses in the L-dopa test in young and elderly men were 32.0 +/- 2.7 vs 20.3 +/- 1.8 ng.h-1.l-1 (p less than 0.001), and 21.8 +/- 4.6 vs 5.4 +/- 1.1 micrograms.h-1.l-1 (p less than 0.01), respectively, indicating decreased releases of GHRH and GH in the elderly. Correlations between the AUCs for plasma GHRH and GH responses in L-dopa were found in both groups, but the ratio of the AUCs for GH/GHRH was lower in the elderly group. The elderly group showed a significant correlation between the basal plasma IGF-I level and the AUCs for plasma GH in the GHRH and L-dopa tests. These results suggest that elderly men have a decreased reserve of hypothalamic GHRH, resulting in secondarily impaired GH release, which may lead to a lower level of IGF-I than in young men.