Cytosolic free calcium concentration and glucose transport in isolated cardiac myocytes

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Abstract
The role of cytosolic free Ca2+ concentration, [Cai], in mediating insulin's stimulatory effect on glucose transport was investigated in isolated Ca2+-tolerant rat ventricular cells. Approximately 98% of glucose transport in isolated myocytes was inhibited by phloretin. Insulin-accelerated glucose transport by 50-115% over basal transport rate. Removal of extracellular Ca2+ had no effect on either the basal transport rate or insulin's stimulatory action, indicating that extracellular Ca2+ was not necessary for insulin's effect to be manifest. Addition of A23187 had no effect on glucose transport rate. Under basal conditions, [Cai] was 167 +/- 12 nM as measured by fura-2 fluorescence and 239 +/- 22 nM by null-point titration with arsenazo III. Loading cells with fura-2 did not affect basal glucose transport rates. In addition, the stimulatory effect of insulin on glucose transport was preserved in fura-2 loaded cells. In paired experiments, insulin did not increase [Cai] as measured by fura-2 fluorescence or null-point titration despite acceleration of glucose transport. In contrast, addition of KCl (40 mM) increased [Cai] from 168 +/- 30 to 287 +/- 51 nM and resulted in 50% reduction in glucose transport rate. In other experiments designed to control for the hyperosmolar effects of KCl, NaCl (40 mM) caused no change in [Cai] but also inhibited glucose transport rate by 50%. We conclude that an elevation in [Cai] is unlikely to be the intracellular signal mediating insulin's effect on glucose transport since insulin's stimulatory effect was not reduced by Ca2+ -free media, insulin had no detectable effect on [Cai], and elevation of [Cai] by KCl did not result in stimulation of glucose transport.

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