A myosin II ATPase inhibitor reduces force production, glucose transport, and phosphorylation of AMPK and TBC1D1 in electrically stimulated rat skeletal muscle

Abstract

Contraction-stimulated glucose transport by skeletal muscle appears to be caused by the cumulative effects of multiple inputs [potentially including AMP-activated protein kinase (AMPK), Ca2+flux, and force production], making it challenging to isolate the roles of these putative regulatory factors. To distinguish the effects of force production from the direct consequences of Ca2+flux, the predominantly type II rat epitrochlearis muscle was incubated without (vehicle) or with N-benzyl- p-toluenesulfonamide (BTS), a highly specific myosin II ATPase inhibitor that prevents force production by electrically stimulated (ES) type II fibers without altering cytosolic Ca2+. In ES muscles, BTS vs. vehicle had an 84% reduction in force production and a 57% decrement in contraction-stimulated 3- O-methylglucose transport (3MGT). BTS did not alter the ES increase in phosphorylation of CaMKII (indicative of cytosolic Ca2+) or the amount of glycogen depletion. ES caused significant reductions in ATP (48%) and phosphocreatine (67%) concentrations for vehicle-treated muscles. For BTS-treated muscles, ES did not reduce ATP and caused only a 42% decrease in phosphocreatine. There was an ES increase in phosphorylation of AMPK, acetyl-CoA carboxylase (an AMPK substrate), and TBC1D1 for vehicle-treated muscles but not for BTS-treated muscles. These results point toward an essential role for tension-related events, including AMPK activation, in the 57% contraction-stimulated increase in 3MGT that was inhibited by BTS and further suggest a possible role for TBC1D1 phosphorylation. Non-tension-related events (e.g., increased cytosolic Ca2+rather than increased AMPK and TBC1D1 phosphorylation) are implicated in the contraction-stimulated increase in 3MGT that persisted in the presence of BTS.