Na⁺-and Energy-Dependent Transport of Cadmium into LLC-PK₁ Cells

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Effects of sodium ions (Na⁺) and metabolic inhibitors on cadmium (Cd) uptake were investigated in LLC-PK₁ cells derived from pig kidney under the nontoxic conditions of Cd. The inoculated cells became confluent on day 7 after the logarithmic growth phase. The initial uptake of Cd (1-60 μM) by the confluent cells (day 7) was assayed. The relationship between the Cd uptake at 37°C and Cd concentration was nonlinear, but the relationship at 4°C was linear. Subtraction of the Cd uptake at 4°C from that at 37°C showed a saturable uptake against the Cd concentration. By the Eadie-Hofstee analysis of saturable uptake, K_m and V_max were 24.6 μM and 164 pmol Cd/mg protein/min, respectively. The uptake of Cd by the cells was significantly decreased by ouabain and the metabolic inhibitors, and by the replacement of Na⁺ with potassium or choline ion in the incubation medium. These results suggest that Cd is incorporated into the confluent LLC-PK₁ cells not only by simple diffusion but also by the carriermediated transport involved in Na⁺-and energy-dependent process (es).