Glyburide-Stimulated Glucose Transport in Cultured Muscle Cells via Protein Kinase C–Mediated Pathway Requiring New Protein Synthesis

To study the mechanism of action of sulfonylurea agents on peripheral tissues without the potentially confounding influences of insulin, the direct effect of glyburide (i.e., in the absence of insulin) was evaluated in the L6 cultured myogenic cell line. Glyburide approximately doubled the incorporation of [¹⁴C]-glucose into glycogen. The rate-determining enzymes of glycogen metabolism, glycogen synthase and glycogen phosphorylase, were unaffected by the drug. Glucose transport (2-deoxyglucose uptake) was also ∼ doubled. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) also doubled glucose transport and showed the same lag period (4–6 h) as glyburide before an effect occurred. Blockade of protein kinase C activity by either 1-(5-isoquinolinesulfonyl)-2 methyl piperazine (H7) or chronic exposure to TPA completely abolished the stimulation by glyburide. Cycloheximide, a protein synthesis inhibitor, also completely eliminated the effect of glyburide. The presence of ATP-sensitive K⁺ channels was assessed by measuring ⁸⁶Rb efflux in ATP-depleted L6 muscle cells and RINm5F cells (which served as a positive control). Such channels were present and responded appropriately to glyburide and diazoxide in pancreatic β-cells but were not present in muscle cells. Glyburide stimulation of glucose transport was completely eliminated by both Quin 2, an intracellular chelator of Ca²⁺, and verapamil, a Ca²⁺ channel blocker. However, glyburide did not raise intracellular Ca²⁺ levels. We conclude that glyburide stimulates glucose transport in cultured L6 muscle cells by a protein kinase C–mediated pathway that requires new protein synthesis. Although intracellular Ca²⁺ metabolism may also be involved, the initial step in the mechanism of action is probably different between pancreatic β-cells and muscle cells.