Regulation of the Na⁺-Dependent Glycine Transport across the Plasma Membrane by Calcium Ion and Membrane Potential

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Involvement of the Ca²⁺ ion and membrane potential in the regulation of the Na⁺-dependent glycine transport was investigated in the Chang liver cell. Depolarization of the cell membrane was associated with the uptake of glycine, showing a half-maximal change in the potential at 1.7 mM of glycine, a value corresponding to the Km for the glycine transport. Addition of EGTA to the assay medium inhibited the glycine uptake in a dose-dependent manner in accordance with the decrease of the intracellular free Ca²⁺ level and the extent of depolarization of the cell membrane. Trifluoperazine inhibited the glycine uptake with a half-maximal effect at about 16μM, a concentration giv-ing a half-maximum of membrane depolarization. Quinine, a blocker of the Ca²⁺-dependent K⁺ channel in liver cells, inhibited the glycine transport and depolarized the cell membrane, depressing K+ release from the cells. The glycine uptake was stimulated by the addition of a calcium ionophore (A23187) to the assay medium, but the uptake was extremely depressed by addition of quinine to the medium or by pretreatment of cells with trifluoperazine, to an extent as low as half of the control. Concomitantly, the K⁺ release increased by A23187 was depressed in the corresponding ratio by addition of quinine to the medium. These results support the concept that the Na⁺-dependent glycine transport is regulated by the cytosolic free Ca²⁺ concentration through the change of the cell membrane potential which is modulated by the activity of the Ca²⁺-dependent K⁺ channel requiring calmodulin as a regulator.