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 ⁴⁵Ca²⁺ uptake. It was found that 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 initial rate of Ca²⁺ influx at different external Ca²⁺ concentrations indicated 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 high affinity, La³⁺ and Gd³⁺ increased both kinetic parameters K_m and V_max of basal Ca²⁺ influx. La³⁺ and Gd³⁺ compete directly with Ca²⁺ for Ca²⁺ binding site for low affinity. The kinetics is competitive.