Regulation of hexokinase II expression in human skeletal muscle in vivo

The phosphorylation of glucose to glucose-6-phosphate (G-6-P) is the first committed step in glucose uptake in skeletal muscle. This reaction is catal yzed by hexokinase (HK). Two HK isoforms, HKI and HKII, are expressed in hu man skeletal muscle, but only HKII is regulated by insulin. The present stu dy was undertaken to determine the time course for the regulation of HK act ivity and expression by physiological plasma insulin concentrations in huma n skeletal muscle in vivo. A hyperinsulinemic-euglycemic glucose clamp and percutaneous muscle biopsy were performed in separate groups of healthy sub jects after 60, 120, 180, and 360 minutes of euglycemic hyperinsulinemia. M uscle biopsies were subfractionated into soluble and particulate fractions to determine HKI and HKII activities. RNA was extracted from a separate por tion of the muscle biopsy, and HKI and HKII mRNA content was determined usi ng an RNase protection assay. Glycogen synthase (GS) activity and fractiona l velocity were also determined, HKII mRNA was increased 2-fold by 120 minu tes and remained high versus the basal value for up to 360 minutes. HKI mRN A was unchanged throughout the study. HKII activity increased after 360 min utes of insulin infusion, and this increase was limited to the soluble frac tion. In contrast, insulin induced a 1.5- to 9-fold increase in GS fraction al velocity that was sustained for 360 minutes. The time course of the abil ity of hyperinsulinemia to increase HKII mRNA indicates that insulin is lik ely a physiological regulator of HKII expression in human skeletal muscle i n vivo. Copyright (C) 2000 by W.B. Saunders Company.