Muscle cell depolarization induces a gain in surface GLUT4 via reduced endocytosis independently of AMPK

Abstract

Contracting skeletal muscle increases glucose uptake to sustain energy demand. This is achieved through a gain in GLUT4 at the membrane, but the traffic mechanisms and regulatory signals involved are unknown. Muscle contraction is elicited by membrane depolarization followed by a rise in cytosolic Ca²⁺and actomyosin activation, drawing on ATP stores. It is unknown whether one or more of these events triggers the rise in surface GLUT4. Here, we investigate the effect of membrane depolarization on GLUT4 cycling using GLUT4 myc-expressing L6 myotubes devoid of sarcomeres and thus unable to contract. K⁺-induced membrane depolarization elevated surface GLUT4 myc, and this effect was additive to that of insulin, was not prevented by inhibiting phosphatidylinositol 3-kinase (PI3K) or actin polymerization, and did not involve Akt activation. Instead, depolarization elevated cytosolic Ca²⁺, and the surface GLUT4 myc elevation was prevented by dantrolene (an inhibitor of Ca²⁺release from sarcoplasmic reticulum) and by extracellular Ca²⁺chelation. Ca²⁺-calmodulin-dependent protein kinase-II (CaMKII) was not phosphorylated after 10 min of K⁺depolarization, and the CaMK inhibitor KN62 did not prevent the gain in surface GLUT4 myc. Interestingly, although 5′-AMP-activated protein kinase (AMPK) was phosphorylated upon depolarization, lowering AMPKα via siRNA did not alter the surface GLUT4 myc gain. Conversely, the latter response was abolished by the PKC inhibitors bisindolylmaleimide I and calphostin C. Unlike insulin, K⁺depolarization caused only a small increase in GLUT4 myc exocytosis and a major reduction in its endocytosis. We propose that K⁺depolarization reduces GLUT4 internalization through signals and mechanisms distinct from those engaged by insulin. Such a pathway(s) is largely independent of PI3K, Akt, AMPK, and CaMKII but may involve PKC.