Effects of La3+ and Gd3+ on Ca2+ influx in rat hepatoma H-35 cells

Effects of La3+ and Gd3+ on Ca2+ influx were investigated in rat hepatoma H -35 cells by measuring the initial rate of Ca-45(2+) uptake. It was found t hat the maximum initial rate of Ca2+ uptake was increased six- to ten-fold at low concentrations of La3+ and Gd3+. Kinetic analyses by measuring the i nitial rate of Ca2+ influx at different external Ca2+ concentrations indica ted the existence of two intracellular exchangeable components in the basal Ca2+ system, with low and high affinities for Ca2+, and only one class of Ca2+ binding sites was observed in the La3+- or Gd3+-treated cells. For hig h affinity, La3+ and Gc(3+) increased both kinetic parameters K-m and V-max of basal Ca2+ influx. La3+ and Gd3+ compete directly with Ca2+ for Ca2+ bi nding site for low affinity. The kinetics is competitive.