OUTWARDLY RECTIFYING CHLORIDE CURRENT IN RABBIT OSTEOCLASTS IS ACTIVATED BY HYPOSMOTIC STIMULATION

1. We characterized chloride currents in freshly isolated rabbit osteo clasts using whole-cell and single channel patch-clamp recording confi gurations. Depolarization activated an outwardly rectifying current in 40-50% of cells, distinct from the inwardly rectifying K+ current we have previously reported in osteoclasts. 2. The outwardly rectifying c urrent persisted under conditions where all K+ currents were blocked. Furthermore, the outward current was reversibly inhibited by Cl- trans port blockers acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS); 4,4'-diisothio-cyanostilbene-2,2'-disulphonic acid (DIDS); 4,4 '-dinitrostilbene-2,2'-disulphonic acid (DNDS); and niflumic acid. The blocked current had a reversal potential close to the predicted chlor ide equilibrium potential and was dependent on the chloride concentrat ion gradient. 3. In those osteoclasts in which outwardly rectifying cu rrent was not initially apparent, exposure to hyposmotic extracellular solution resulted in its reversible activation. The induced current w as due to Cl-, based on its reversal close to the chloride equilibrium potential and sensitivity to blockade by Cl- channel inhibitors. The hyposmotically induced current could be activated in Ca2+-free solutio ns containing 0.2 mM EGTA. 4. When studied in the current-clamp config uration, hyposmotic stimulation caused depolarization from -76 +/- 5 t o -5 +/- 6 mV (mean +/- S. D., n = 7). 5. Unitary Cl- currents were re corded in the cell-attached patch configuration at positive potentials . Single channels had a slope conductance of 19 +/- 3 pS (n = 5). Redu ction of the external [Cl-] shifted the current-voltage relationship i n the positive direction, supporting the conclusion that these were Cl - currents. Like the whole-cell currents, single channel Cl- currents were activated by exposure of cells to hyposmotic bathing solution. 6. We conclude that rabbit osteoclasts express an outwardly rectifying C l- current that can be activated by osmotic stress. Cl- channels may p lay a role in cell volume regulation and may also provide conductive p athways for dissipating the potential difference that arises from elec trogenic proton transport during bone resorption.