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.
Lutz
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Institute of Veterinary Physiology, University of Zurich, Winterthurerstrasse 260, 8057 Zurich, Switzerland. tomlutz@vetphys.unizh.ch
Wild
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Boutellier
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Sutter
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Volkert
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Scharrer
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D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology*; D004594:Electrophysiology; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D005632:Fructose / Q000494:pharmacology; D005943:Gluconeogenesis; D019344:Lactic Acid / Q000494:pharmacology*; D008099:Liver / Q000648:ultrastructure*; D051379:Mice; D008813:Mice, Inbred ICR; D010042:Ouabain / Q000494:pharmacology; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000502:physiology*; D019289:Pyruvic Acid / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D019789:Tetraethylammonium / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology
D004791:Enzyme Inhibitors; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D005632:Fructose; D019344:Lactic Acid; D010042:Ouabain; D019789:Tetraethylammonium; D019284:Thapsigargin; D000584:Amiloride; D019289:Pyruvic Acid; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium