The hypertonicity-induced Na+ conductance of rat hepatocytes: Physiological significance and molecular correlate

The initial event in the regulatory volume increase (RVI) of rat hepatocyte s is an uptake of extracellular Na+ that is then exchanged for K+ via stimu lation of Na+/K+-ATPase. While it was generally assumed that this Na+ uptak e is mediated by the activation of Na+/H+ antiport and Na+-K+-2Cl(-) sympor t it could be shown recently that, in addition to these transporters, hyper tonic stress also stimulates conductive Na+ entry. In a quantitative study, it was found that the relative contribution of Na+ conductance, Na+/H+ ant iport, and Na+-K+-2Cl(-) symport to the initial Na+ import as well as to th e RVI process (at 300 --> 400 mosmol/l) is approximately 4 : 1 : 1. When th e osmotic sensitivity of these Na+ importers was tested (at 300 mosmol/l -- > 327, 360, 400, 450 mosmol/l) it became clear that Na+ conductance is the prominent mechanism of RVI from 360 mosmol/l upwards whereas Na+/H+ antipor t is the most sensitive transporter with 65 % of its maximal activation at 327 mosmol/l already. Concerning the intracellular regulation of the Na+ im porters involved in RVI it was found that Na+ concuctance as well as Na+-K-2Cl(-) symport - but not Na+/H+ antiport - are activated via PKC. With res pect to the molecular correlate of the volume activated Na+ conductance it could be shown that it exhibits a rather low affinity to amiloride (IC50 = 6.0 mu mol/l) and an overall sensitivity profile of EIPA > amiloride > benz amil = phenamil that, at first sight, would not speak in favor of a typical epithelial type of Na+ channel (ENaC). Western-blot analysis and RT-PCR te chniques, however, revealed that alpha-, beta-, as well as gamma -ENaC are, in fact, expressed in rat hepatocytes. Moreover, by use of an antisense-DN A based approach it could be shown that at least alpha -ENaC is part of the hypertonicity induced Na+ conductance. Copyright (C) 2000 S. Karger AG Bas el.