Insulin increases near-membrane but not global Ca2+ in isolated skeletal muscle

It has long been debated whether changes in Ca2+ are involved in insulin-st imulated glucose uptake in skeletal muscle. We have now investigated the ef fect of insulin on the global free myoplasmic Ca2+ concentration and the ne ar-membrane free Ca2+ concentration ([Ca2+](mem)) in intact, single skeleta l muscle fibers from mice by using fluorescent Ca2+ indicators. Insulin has no effect on the global free myoplasmic Ca2+ concentration. However, insul in increases [Ca2+](mem) by approximate to 70% and the half-maximal increas e in [Ca2+](mem) occurs at an insulin concentration of 110 microunits per m i. The increase in [Ca2+](mem) by insulin persists when sarcoplasmic reticu lum Ca2+ release is inhibited but is lost by perfusing the fiber with a low Ca2+ medium or by addition of L-type Ca2+ channel inhibitors. Thus, insuli n appears to stimulate Ca2+ entry into muscle cells via L-type Ca2+ channel s, Wortmannin, which inhibits insulin-mediated activation of glucose transp ort in isolated skeletal muscle, also inhibits the insulin-mediated increas e in [Ca2+](mem). These data demonstrate a new facet of insulin signaling a nd indicate that insulin-mediated increases in [Ca2+](mem) in skeletal musc le may underlie important actions of the hormone.