ABNORMAL REGULATION OF HEXOKINASE IN INSULIN-RESISTANT SKELETAL-MUSCLE

The present study has assessed the potential involvement of hexokinase in the control of insulin-mediated glucose metabolism in insulin-sens itive and -resistant skeletal muscle. Soleus muscle strips fi om lean (insulin-sensitive) and obese (insulin-resistant) Zucker rats were inc ubated with 10 or 10,000 mu U insulin . ml(-1) and then homogenized us ing a protocol to maintain the location of hexokinase in. situ. Hexoki nase is inhibited by glucose 6-phosphate, a metabolic intermediate whi ch may have a central role in the regulation of glycogen synthesis. Tw o separate measurements of hexokinase activity were made on each muscl e homogenate: the total hexokinase activity (glucose 6-phosphate was m etabolized immediately by glucose 6-phosphate dehydrogenase) and the f ractional hexokinase activity (glucose 6-phosphate accumulated so as t o regulate the enzyme as in vivo). The total hexokinase activity was e qual in insulin-sensitive and -resistant muscle and was unaffected by the extracellular insulin concentration The fractional hexokinase acti vity was significantly increased by insulin (10,000 mu U . ml(-1)) in all muscles (lean 82%; obese, 52%; P < 0.05) although the stimulated f ractional hexokinase activity was lower in the muscle from obese Zucke r rats compared to lean (P < 0.05). These results provide evidence tha t insulin decreases the inhibition of hexokinase by glucose 6-phosphat e in insulin-sensitive but not in insulin-resistant muscle. This study has revealed shortterm regulation of hexokinase by insulin which is d efective in insulin-resistant skeletal muscle. Thus, the study has ide ntified hexokinase as a potential regulatory site of insulin action th at is abnormal in insulin resistance. The altered regulation of hexoki nase may be a major contributing factor to the reduced insulin-mediate d glucose fluxes in insulin-resistant skeletal muscle. (C) 1996 Academ ic Press, Inc.