Regulation of transmembrane electrical potential gradient in rat hepatocytes in situ

The transmembrane electrical potential gradient (Em) has been measured in hepatocytes from intact anesthetized rats using conventional intracellular microelectrodes under a variety of conditions. Em measurements in control animals were normally distributed around a mean of -35.5 +/- 4.6 mV (SD) with a coefficient of variation (CV) of 13.1% and a range of -26 to -54 mV. In individual livers, however, measurements of Em at a given point in time exhibited little cell-to-cell variation (cv of 4.5%). The Em was noted to fluctuate spontaneously over time and to change consistently in response to a variety of physiological stimuli including fasting (depolarization to -28.5 +/- 3.8 mV) and infusion of glucagon in physiological amounts (hyperpolarization to -45.0 +/- 1.8 mV). Hepatocyte Em abruptly depolarized (2-5 mV) after an intravenous bolus of taurocholate (3 mumol) or alanine (45 mumol), suggesting that both solutes exhibit electrogenic uptake. The Em returned to or below preinfusion values within 5 min. Continued infusion of alanine (10.8 mumol/min), but not taurocholate (810 nmol/min), caused a sustained and unexpected hyperpolarization of Em of 8.2 +/- 3.1 mV that lasted at least 60 min. In separate studies, alanine administration did not alter the biliary excretion of a taurocholate load. Taken together, these observations demonstrate that rat hepatocytes in situ are tightly coupled electrically and that physiological stimuli, including fasting, glucagon, and sodium-coupled solute uptake can change Em considerably over time. The late hyperpolarization of Em caused by alanine appears to offset the rise in intracellular Na+ associated with alanine uptake and preserve the Na+ electrochemical gradient such that Na+-coupled taurocholate transport is maintained.