Role of calcium in organic osmolyte efflux when MDCK cells are shifted from hypertonic to isotonic medium.

Madin-Darby canine kidney (MDCK) cells accumulate the nonperturbing osmolytes myo-inositol and betaine when grown in hypertonic medium. When returned to isotonic conditions, there is a transient basolateral efflux of these osmolytes, contributing to regulatory volume decrease. Using fura-2 fluorescence, we estimated intracellular calcium concentrations after switching MDCK cells from 500 to 300 mosM medium. Cell calcium increased 565 +/- 93 nM within 5 min. Lowering extracellular calcium inhibited the increase in cell calcium and osmolyte efflux when cells were shifted from 500 to 300 mosM medium. The calcium channel blockers lanthanum and nifedipine also inhibited osmolyte efflux after the shift from 500 to 300 mosM. In the absence of change in medium tonicity, increasing cell calcium by exposure to 1 microM ionomycin did not alter osmolyte efflux. As in PAP-HT25 cells, the cytochrome P-450 inhibitors ketoconazole and SKF-525A inhibited the efflux of both osmolytes caused by a reduction in osmolarity. Thus an early rise in cell calcium that is dependent on extra-cellular calcium and a pathway blocked by inhibitors of cytochrome P-450 oxidase are critical in regulation of osmolyte efflux when MDCK cells are shifted from hypertonic to isotonic medium.