Evidence for organic cation transporter-mediated metformin transport and 5'-adenosine monophosphate-activated protein kinase activation in rat skeletal muscles

OBJECTIVE

5'-Adenosine monophosphate-activated protein kinase (AMPK) is a key molecule of metabolic enhancement in skeletal muscle. We investigated whether metformin (MET) acts directly on skeletal muscle, is transported into skeletal muscle via organic cation transporters (OCTs), and activates AMPK.

MATERIALS/METHODS

Isolated rat epitrochlearis and soleus muscles were incubated in vitro either in the absence or in the presence of MET. The activation status of AMPK, the intracellular energy status, and glucose and MET transport activity were then evaluated. The effect of cimetidine, which is an OCT inhibitor, on AMPK activation was also examined.

RESULTS

MET (10 mmol/L, ≥60 min) increased the phosphorylation of Thr¹⁷² at the catalytic α subunit of AMPK in both muscles. AMPK activity assays showed that both AMPKα1 and AMPKα2 activity increased significantly. The AMPK activation was associated with energy deprivation, which was estimated from the ATP, phosphocreatine (PCr), and glycogen content, and with increased rates of 3-O-methyl-D-glucose (3MG) transport. MET did not change the basal phosphorylation status of insulin receptor signaling molecules. MET was transported into the cytoplasm in a time-dependent manner, and cimetidine suppressed MET-induced AMPK phosphorylation and 3MG transport.

CONCLUSION

These results suggest that MET is acutely transported into skeletal muscle by OCTs, and stimulates AMPKα1 and α2 activity in both fast- and slow-twitch muscle types, at least in part by reducing the energy state.

Salicylate acutely stimulates 5'-AMP-activated protein kinase and insulin-independent glucose transport in rat skeletal muscles

Salicylate (SAL) has been recently implicated in the antidiabetic effect in humans. We assessed whether 5'-AMP-activated protein kinase (AMPK) in skeletal muscle is involved in the effect of SAL on glucose homeostasis. Rat fast-twitch epitrochlearis and slow-twitch soleus muscles were incubated in buffer containing SAL. Intracellular concentrations of SAL increased rapidly (<5 min) in both skeletal muscles, and the Thr(172) phosphorylation of the α subunit of AMPK increased in a dose- and time-dependent manner. SAL increased both AMPKα1 and AMPKα2 activities. These increases in enzyme activity were accompanied by an increase in the activity of 3-O-methyl-D-glucose transport, and decreases in ATP, phosphocreatine, and glycogen contents. SAL did not change the phosphorylation of insulin receptor signaling including insulin receptor substrate 1, Akt, and p70 ribosomal protein S6 kinase. These results suggest that SAL may be transported into skeletal muscle and may stimulate AMPK and glucose transport via energy deprivation in multiple muscle types. Skeletal muscle AMPK might be part of the mechanism responsible for the metabolic improvement induced by SAL.