Normal Growth Hormone Secretory Reserve in Men with Idiopathic Osteoporosis and Reduced Circulating Levels of Insulin-Like Growth Factor-I

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Abstract
Many,men,with,idiopathic osteoporosis,have,reduced,circulating insulin-like growth,factor-I (IGF-I) levels. The major,source of circu- lating IGF-I is GH-mediated,production,by the liver. The known anabolic,effects of GH on the skeleton,raised the possibility of GH deficiency,in these men. We sought,to test this hypothesis,in this study. Fourteen men (mean age, 52.1 6 3.2 yr, range 31‐ 68) with idiopathic,osteoporosis,were,studied. Mean lumbar,spine bone,min- eral density (BMD) was 0.723 g/cm,, T score, 22.05. Eleven of 14 (79%) had frank reductions in serum,IGF-I levels compared,with,age,and,sex-matched,values (158.5 6 50 SD vs. 180 6 45 SD). GH secretion,was,stimulated,by iv arginine,infusion,(30 g) over 30 min followed 1 h later by oral L-dopa (500 mg). Serum GH was measured at time (t) 52 15, 0, 30, 45, 60, 90, 120, 150, and 180 min. All patients responded to at least one stimulus,with the majority,(n 5 9) responding,to both. Five patients responded,either to arginine,or to L-dopa but not to both. Baseline GH for the entire group,was 0.77 1 0.08 ng/mL (SEM). Peak GH following arginine (t 5 45‐ 60 min) was 14.0 6 2.8 ng/mL, a 17.7 6 2.8-fold rise. Peak GH following L-dopa (t 5 120 ‐180 min) was 5.7 6 1.0 ng/mL, a 9.2 6 2.2-fold rise. No difference in maximal,secretion,was observed between,those with low or normal,IGF-I levels. Neither IGF-I nor IGF binding,protein-3 concentrations,changed,significantly,during,the short period,of GH stimulation. These data,suggest,that men,with osteoporosis,and,reduced,IGF-I levels do not appear,to have,a defi- ciency in the GH axis. Other hormonal,or local factors may,be im-

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