Growth hormone reduces glucose transport but not GLUT-1 or GLUT-4 in adult and old rats.

The primary purpose of this study was to investigate the influence of administration of recombinant-derived human growth hormone (rhGH) to adult male rats of several ages (9, 20, and 31 mo) on skeletal muscle glucose transport. Rats were injected with rhGH (0.7 mg/kg) or vehicle twice daily for 10 days. The rhGH treatment led to a doubling of circulating insulin-like growth factor I levels at each age. Skeletal muscle glucose transport activity was evaluated in isolated epitrochlearis muscle with use of 3-O-methylglucose at three insulin concentrations (0, 100, and 20,000 microU/ml). The results indicate that, after 10 days of rhGH administration, 1) an approximately 20-30% reduction in basal glucose transport activity was evident in muscles from every age group, 2) the ability of a submaximally effective insulin concentration (100 microU/ml) to increase glucose transport activity above basal values was not significantly reduced in any age group, 3) maximal insulin-stimulated glucose transport activity (with 20,000 microU/ml) was significantly reduced (approximately 40%) by rhGH treatment only in the oldest rats, and 4) the alterations in glucose transport activity occurred despite no change in skeletal muscle GLUT-1 or GLUT-4 protein levels.