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1 University of Michigan
* To whom correspondence should be addressed. E-mail: gcartee{at}umich.edu.
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 force production's effects 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 adenosine triphosphate (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% of 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 for the contraction-stimulated increase in 3MGT that persisted in the presence of BTS.
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