Steven D. Lidofsky

Regulation of cation-selective channels in liver cells

Abstract. In liver cells, cation-selective channels are permeable to Ca2+ and have been postulated to represent a pathway for receptor-mediated Ca2+ influx. This study examines the mechanisms involved in the regulation of these channels in a model liver cell line. Using patch-clamp recording techniques, it is shown that channel open probability is a saturable function of cytosolic [Ca2+], with half-maximal opening at 660 nm. By contrast, channel opening is not affected by membrane voltage or cytosolic pH. In intact cells, reduction of cytosolic [Cl−], a physiological response to Ca2+-mobilizing hormones and cell swelling, is also associated with an increase in channel opening. Finally, channel opening is inhibited by intracellular ATP through a mechanism that does not involve ATP hydrolysis. These findings suggest that opening of cation-selective channels is coupled to the metabolic state of the cell and provides a positive feedback mechanism for regulation of receptor-mediated Na+ and Ca2+ influx.n

HCO3 --coupled Na+ influx is a major determinant of Na+ turnover and Na+/K+ pump activity in rat hepatocytes

SummaryRecent studies in hepatocytes indicate that Na+-coupled HCO3− transport contributes importantly, to regulation of intracellular pH and membrane HCO3− transport. However, the direction of net coupled Na+ and HCO3− movement and the effect of HCO3− on Na+ turnover and Na+/K+ pump activity are not known. In these studies, the effect of HCO3− on Na+ influx and turnover were measured in primary rat hepatocyte cultures with22Na+, and [Na+]i was measured in single hepatocytes using the Na+-sensitive fluorochrome SBFI. Na+/K+ pump activity was measured in intact perfused rat liver and hepatocyte monolayers as Na+-dependent or ouabain-suppressible86Rb uptake, and was measured in single hepatocytes as the effect of transient pump inhibition by removal of extracellular K+ on membrane potential difference (PD) and [Na+]i. In hepatocyte monolayers, HCO3− increased22Na+ entry and turnover rates by 50–65%, without measurably altering22Na+ pool size or cell volume, and HCO3− also increased Na+/K+ pump activity by 70%. In single cells, exposure to HCO3− produced an abrupt and sustained rise in [Na+]i, from ≈8 to 12mm. Na+/K+ pump activity assessed in single cells by PD excursions during transient K+ removal increased ≃2.5-fold in the presence of HCO3−, and the rise in [Na+]i produced by inhibition of the Na+/K+ pump was similarly increased ≃2.5-fold in the presence of HCO3−. In intact perfused rat liver, HCO3− increased both Na+/K+ pump activity and O2 consumption. These findings indicate that, in hepatocytes, net coupled Na+ and HCO3− movement is inward and represents a major determinant of Na+ influx and Na+/K+ pump activity. About half of hepatic Na+/K+ pump activity appears dedicated to recycling Na+ entering in conjunction with HCO3− to maintain [Na+]i within the physiologic range.