Mechanisms of metformin action in non-insulin-dependent diabetes mellitus

Depolarization of the liver cell membrane by metformin

Metformin (1,1-dimethylbiguanide; MET) is used in the treatment of type 2 diabetes mellitus. MET's antihyperglycemic action depends at least in part on its inhibitory effect on hepatic gluconeogenesis. As to gluconeogenesis from amino acids (e.g. L-alanine), this is associated with an inhibition of L-alanine uptake into hepatocytes. Since this uptake is mediated by an electrogenic transport mechanism, the aim of the present study was to investigate whether MET has an influence on the liver cell membrane potential which might explain its inhibitory effect on L-alanine uptake. The experiments were performed in vivo in anesthetized rats and in vitro using superfused mouse liver slices with the conventional microelectrode technique. In vivo, MET (160 mg/kg intraperitoneally (i.p.)) significantly depolarized (dV) the liver cell membrane by 6 mV. MET (1 mmol/l) also depolarized the liver cell membrane in vitro (e.g. 15 min after start of superfusion: dV=8 mV). MET's effect was at least partly reversible. Glucagon (10(-7) mol/l), which hyperpolarized the liver cell membrane, abolished MET's effect. Further, the MET-induced depolarization was completely absent during superfusion with low Cl(-) ([Cl(-)]=27 mmol/l) medium, and significantly attenuated by the Cl(-) channel blocker NPPB (25 micromol/l). While MET's effect was only somewhat attenuated by blockade of the Na(+)/K(+)/2Cl(-) cotransporter or by superfusion with (HCO(-)(3)-free) HEPES buffer, the carboanhydrase blocker acetazolamide (1 mmol/l) or blockade of the HCO(-)(3)/Cl(-) exchanger by DIDS (100 micromol/l), which, however, also blocks Cl(-) channels, abolished its effect. The depolarization of the liver cell membrane by MET was unaffected by a blockade of K(+) channels with Ba(2+), a blockade of the Na(+)/K(+) pump or superfusion with low Na(+) medium ([Na(+)]=26 mmol/l). According to these results, the MET-induced depolarization of the liver cell membrane could be due to an activation of the Cl(-)/HCO(-)(3) exchanger and thus depend on intracellular HCO(-)(3) formation. This activation could then lead to a disturbance of the equilibrium between intra- and extracellular Cl(-) and therefore to an enhanced Cl(-) efflux via Cl(-) channels. It is plausible that the depolarizing effect induced by MET is associated with its inhibitory effect on gluconeogenesis by inhibiting uptake of L-alanine and other amino acids into hepatocytes.