Insulin increases near-membrane but not global Ca(2+) in isolated skeletal muscle

It has long been debated whether changes in Ca(2+) are involved in insulin-stimulated glucose uptake in skeletal muscle. We have now investigated the effect of insulin on the global free myoplasmic Ca(2+) concentration and the near-membrane free Ca(2+) concentration ([Ca(2+)](mem)) in intact, single skeletal muscle fibers from mice by using fluorescent Ca(2+) indicators. Insulin has no effect on the global free myoplasmic Ca(2+) concentration. However, insulin increases [Ca(2+)](mem) by ≈70% and the half-maximal increase in [Ca(2+)](mem) occurs at an insulin concentration of 110 microunits per ml. The increase in [Ca(2+)](mem) by insulin persists when sarcoplasmic reticulum Ca(2+) release is inhibited but is lost by perfusing the fiber with a low Ca(2+) medium or by addition of L-type Ca(2+) channel inhibitors. Thus, insulin appears to stimulate Ca(2+) entry into muscle cells via L-type Ca(2+) channels. Wortmannin, which inhibits insulin-mediated activation of glucose transport in isolated skeletal muscle, also inhibits the insulin-mediated increase in [Ca(2+)](mem). These data demonstrate a new facet of insulin signaling and indicate that insulin-mediated increases in [Ca(2+)](mem) in skeletal muscle may underlie important actions of the hormone.