Chronic growth hormone treatment in normal rats reduces post-prandial skeletal muscle plasma membrane GLUT1 content, but not glucose transport or GLUT4 expression and localization.

Abstract

Whether skeletal muscle glucose transport system is impaired in the basal, post-prandial state during chronic growth hormone treatment is unknown. The current study was designed to determine whether 4 weeks of human growth hormone (hGH) treatment (3.5 mg/kg per day) would impair glucose transport and/or the number of glucose transporters in plasma membrane vesicles isolated from hindlimb skeletal muscle of Sprague-Dawley rats under basal, post-prandial conditions. hGH treatment was shown to have no effect on glucose influx (Vmax or K(m)) determined under equilibrium exchange conditions in isolated plasma membrane vesicles. Plasma membrane glucose transporter number (Ro) measured by cytochalasin B binding was also unchanged by hGH treatment. Consequently, glucose transporter turnover number (Vmax/Ro), a measure of average glucose transporter intrinsic activity, was similar in hGH-treated and control rats. hGH did not change GLUT4 protein content in whole muscle or in the plasma membrane, and muscle content of GLUT4 mRNA also was unchanged. In contrast, GLUT1 protein content in the plasma membrane fraction was significantly reduced by hGH treatment. This was associated with a modest, although not significant, decrease in muscle content of GLUT1 mRNA. In conclusion, high-dose hGH treatment for 4 weeks did not alter post-prandial skeletal muscle glucose transport activity. Neither the muscle level nor the intracellular localization of GLUT4 was changed by the hormone treatment. On the contrary, the basal post-prandial level of GLUT1 in the plasma membrane was reduced by hGH. The mRNA data suggest that this reduction might result from a decrease in the synthesis of GLUT1.