Is intracellular Ca2+ involved in insulin stimulation of sugar transport? Fact and prejudice

Insulin stimulates the rate of glucose transport in muscle and fat tissue by incorporation of transporters from internal membranes into the plasma membrane. It is conceivable that cell Ca2+ ions could play a role in transporter translocation. Indeed Ca2+ has been thought to mediate insulin action, but the evidence remains highly controversial. Experiments to this effect include (i) determinations of a requirement for extracellular Ca2+ in the hormonal response, (ii) stimulation of glucose transport by agents thought to elevate cytosolic Ca2+, [Ca2+]i, and (iii) determinations of Ca2+ efflux. Actual measurements of the effect of insulin on [Ca2+]i were missing until recently. Current methods to measure [Ca2+]i include Ca2+-selective intracellular electrodes, metallochromic dyes, and photoproteins. Main drawbacks of these procedures have been the requirement of microinjection for their incorporation, which restricts their use to large cells, and their interaction with cytoplasmic Mg2+ and H+. Recently, a fluorescent Ca2+ chelator, quin-2, has been devised, which circumvents these difficulties. A permeable, nonchelating precursor of quin-2 penetrates cells and once in the cytosol becomes an impermeant, fluorescent Ca2+ chelator. With this technique it was shown that insulin does not change [Ca2+]i while stimulating glucose transport in L6 muscle cells.