Passive calcium influx by plasma membrane vesicles isolated from rat liver

Passive Ca2+ influx independent of ATP addition to the incubation medium, took place in plasma membrane vesicles isolated from rat liver. The rate of Ca2+ influx was found to depend on the concentration of added Ca2+, and on the incubation temperature, and was inhibited by La3+, Hg2+ and by p-chloromercuribenzoate. Influx was not blocked by calcium channel blockers, or affected by a range of uncouplers. Addition of the Ca2+ ionophore A23187 to vesicles that had taken up the ion induced a rapid efflux of Ca2+ especially when EGTA also was added to the incubation medium. A number of divalent cations inhibited Ca2+ influx. The vesicles could be frozen and stored overnight with little loss in activity. The kinetics of Ca2+ influx could be related to that which occurs in the unstimulated perfused rat liver. The data suggest that the plasma membrane vesicle preparation may be useful for further studies on the basal liver cell Ca2+ influx system in vitro.