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

Abstract

It has long been debated whether changes in Ca²⁺ are involved in insulin-stimulated glucose uptake in skeletal muscle. We have now investigated the effect of insulin on the global free myoplasmic Ca²⁺ concentration and the near-membrane free Ca²⁺ concentration ([Ca²⁺]_mem) in intact, single skeletal muscle fibers from mice by using fluorescent Ca²⁺ indicators. Insulin has no effect on the global free myoplasmic Ca²⁺ concentration. However, insulin increases [Ca²⁺]_mem by ≈70% and the half-maximal increase in [Ca²⁺]_mem occurs at an insulin concentration of 110 microunits per ml. The increase in [Ca²⁺]_mem by insulin persists when sarcoplasmic reticulum 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, insulin appears to stimulate Ca²⁺ entry into muscle cells via L-type Ca²⁺ channels. Wortmannin, which inhibits insulin-mediated activation of glucose transport in isolated skeletal muscle, also inhibits the insulin-mediated increase in [Ca²⁺]_mem. These data demonstrate a new facet of insulin signaling and indicate that insulin-mediated increases in [Ca²⁺]_mem in skeletal muscle may underlie important actions of the hormone.