Ca effects on glucose transport and fatty acid oxidation in L6 skeletal muscle cell cultures.

We examined the effect of Ca on skeletal muscle glucose transport and fatty acid oxidation using L6 cell cultures. Ca stimulation of glucose transport is controversial. We found that caffeine (a Ca 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 Ca 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 Ca 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 Ca 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 Ca 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 Ca stimulation of fatty acid oxidation.