Activation of glucose transport by AMP-activated protein kinase via stimulation of nitric oxide synthase.

Glucose transport in skeletal muscle is stimulated by two distinct stimuli, insulin and exercise. The mechanism by which exercise stimulates glucose transport is not known, although it is distinct from the insulin-mediated pathway. Recently, it has been shown that AMP-activated protein kinase (AMPK) is activated by exercise in skeletal muscle, whereas pharmacological activation of AMPK by 5-amino-4-imidazolecarboxamide riboside (AICAR) leads to increased glucose transport. It has been postulated, therefore, that AMPK may be the link between exercise and glucose transport. To address this, we have examined the signaling pathway involved in the stimulation of glucose uptake after activation of AMPK. Here we show that activation of AMPK by AICAR in rat muscle and mouse H-2Kb muscle cells activates glucose transport approximately twofold. AMPK in H-2Kb cells is also activated by hyperosmotic stress and the mitochondrial uncoupling agent, dinitrophenol, both of which lead to increased glucose transport. In contrast, insulin, which activates glucose transport two- to-threefold in both rat muscle and H-2Kb cells, has no effect on AMPK activity. A previous study has shown that AMPK phosphorylates and activates endothelial nitric oxide synthase (NOS). We show here that NOS activity in H-2Kb cells is activated after stimulation of AMPK by AICAR. Treatment of H-2Kb cells or rat muscle with NOS inhibitors completely blocks the increase in glucose transport after activation of AMPK. In addition, an inhibitor of guanylate cyclase also blocks activation of glucose transport by AICAR in H-2Kb cells. These results indicate that activation of AMPK in muscle cells stimulates glucose transport by activation of NOS coupled to downstream signaling components, including cyclic GMP.