Ca2+ effects on glucose transport and fatty acid oxidation in L6 skeletal muscle cell cultures

We examined the effect of Ca²⁺ on skeletal muscle glucose transport and fatty acid oxidation using L6 cell cultures. Ca²⁺ stimulation of glucose transport is controversial. We found that caffeine (a Ca²⁺ secretagogue) stimulation of glucose transport was only evident in a two-part incubation protocol (“post-incubation”). Caffeine was present in the first incubation, the media removed, and labeled glucose added for the second. Caffeine elicited a rise in Ca²⁺ in the first incubation that was dissipated by the second. This post-incubation procedure was insensitive to glucose concentrations in the first incubation. With a single, direct incubation system (all components present together) caffeine caused a slight inhibition of glucose transport. This was likely due to caffeine induced inhibition of phosphatidylinositol 3-kinase (PI3K), since nanomolar concentrations of wortmannin, a selective PI3K inhibitor, also inhibited glucose transport, and previous investigators have also found this action.We did find a Ca²⁺ stimulation (using either caffeine or ionomycin) of fatty acid oxidation. This was observed in the absence (but not the presence) of added glucose. We conclude that Ca²⁺ stimulates fatty acid oxidation at a mitochondrial site, secondary to malonyl CoA inhibition (represented by the presence of glucose in our experiments). In summary, the experiments resolve a controversy on Ca²⁺ stimulation of glucose transport by skeletal muscle, introduce an important experimental consideration for the measurement of glucose transport, and uncover a new site of action for Ca²⁺ stimulation of fatty acid oxidation.