Hyperpolarization of the cell membrane of mouse hepatocytes by lactate, pyruvate, and fructose is due to Ca2+-dependent activation of K+ channels and of the Na+/K+-ATPase.

Using superfused mouse liver slices combined with a conventional microelectrode technique, we investigated: (1) the ionic mechanisms involved in the hyperpolarization of the hepatocyte membrane induced by lactate and other gluconeogenic substrates; (2) whether these mechanisms are similar to those underlying the hyperpolarization induced by cell swelling in hypo-osmotic medium; and (3) whether the hyperpolarizing effect of lactate on the hepatocyte membrane is related to gluconeogenesis. Lactate (5 mmol/l) hyperpolarized the hepatocyte membrane after an exposure of 10-20 min, and the hyperpolarization was still present after 70 min. The hyperpolarization induced by lactate, pyruvate (5 mmol/l) and fructose (10 mmol/l), and by exposure to hypo-osmotic medium (250 mosmol/l) was antagonized by ouabain, tetraethylammonium (TEA), and cetiedil (lactate; hypo-osmotic medium). Hyperpolarization induced by lactate was eliminated or attenuated by agents impairing activation of Ca2+-dependent K+ channels, by amiloride, and by a blockade of non-selective cation channels with flufenamic acid and gadolinium. Thapsigargin, increasing cytosolic Ca2+, mimicked lactate's hyperpolarizing effect. Lactate's effect was dependent on extracellular Ca2+. Finally, lactate's hyperpolarizing effect was reduced by inhibiting gluconeogenesis. These findings suggest that metabolism of lactate hyperpolarizes hepatocytes by mechanisms analogous to those underlying the hyperpolarization induced by cell swelling in hypo-osmotic medium. Gluconeogenesis from lactate may cause cell swelling, subsequent activation of Ca2+-dependent K+ channels and of the Na+/K+-ATPase, and thus hyperpolarize the hepatocyte membrane.