Sodium-dependent taurocholate uptake by isolated rat hepatocytes occurs through an electrogenic mechanism

The uptake mechanism for the bile salt, taurocholate, by the liver cell is coupled to sodium but the stoichiometry is controversial. A one-to-one coupling ratio would result in electroneutral transport, whereas cotransport of more than one sodium ion with each taurocholate molecule cause an electrogenic response. To better define the uptake of this bile salt, we measured the effect of taurocholate on the membrane potential and resistance of isolated rat hepatocytes using conventional microelectrode electrophysiology. The addition of 20 microM taurocholate caused transient but significant depolarization accompanied by a significant decrease in membrane resistance. The electrical effect induced by taurocholate mimicked that induced by L-alanine (10 mM), the uptake of which is known to occur through an electrogenic, sodium-coupled mechanism. The sodium dependence of taurocholate-induced depolarization was further confirmed by: (1) replacing Na+ with choline +, and (2) preincubating cells with ouabain (2 mM) or with the Na+-ionophore, gramicidin (25 micrograms/ml); both suppressed the electrogenic response. Further, cholic acid, which inhibits sodium-coupled taurocholate uptake in hepatocytes, inhibited taurocholate evoked depolarization. These results support the hypothesis that sodium-coupled taurocholate uptake by isolated hepatocytes occurs through an electrogenic process which transports more than one Na+ with each taurocholate molecule.