CALCIUM-DEPENDENT REGULATION OF CATION-TRANSPORT IN CULTURED HUMAN NONPIGMENTED CILIARY EPITHELIAL-CELLS

We performed Rb-86 flux studies to examine Na-K-adenosinetriphosphatase (ATPase), Na-K-2Cl cotransporter, and potassium channel activity in an established cell line derived from human nonpigmented ciliary epithelium (ODM2). The elevation of intracellular calcium by A23187 (3 muM) or thapsigargin (200 nM) increased both ouabain-sensitive potassium (Rb-86) uptake (Na-K-ATPase mediated) and ouabain-insensitive potassium (Rb-86) uptake. The ouabain-insensitive component could be inhibited substantially by bumetanide (0.1 mM), suggesting the involvement of a Na-K-2Cl cotransporter. The increase of potassium (Rb-86) uptake caused by thapsigargin could be prevented by the intracellular calcium buffer 1,2-bis(2-amino-phenoxy) ethane N,N,N',N'-tetraacetic acetoxymethyl ester (BAPTA/AM); in BAPTA/AM-treated cells, the thapsigargin stimulation of the bumetanide-sensitive portion of Rb-86 uptake was abolished. After A23187 (5 muM), the Rb-86 efflux rate was significantly increased; the increase could be blocked partially by quinidine (0.1 mM) and partially by bumetanide, suggesting that potassium channels and the Na-K-2Cl cotransporter contribute to the effect. We propose that the cell potassium loss after activation of quinidine-sensitive potassium channels is involved in the calcium-induced activation of Na-K-ATPase because 0.1 mM quinidine and 100 mM external potassium both markedly inhibited the A23187-induced increases of the ouabain-sensitive component of potassium (Rb-86) uptake. Calcium-induced stimulation of the Na-K-2Cl cotransporter may not be linked to channel activation.