HYPOSMOTIC ACTIVATION HYPERPOLARIZES OUTER HAIR-CELLS OF GUINEA-PIG COCHLEA

The electrophysiological responses of isolated guinea pig outer hair cells (OHCs) to hyposmotic activation were studied using the whole-cell patch-clamp technique. The cell swelling by hyposmotic activation hyperpolarized OHCs by 6.6 +/- 2.3 mV from the resting membrane potential of - 58.5 +/- 5.9 mV (n = 48). This hyperpolarization was associated with an outward current (97.7 +/- 22.2 pA, n = 15). The hyperpolarization was inhibited by 300 muM quinine, 5 mN Ba2+ and increasing the extracellular K+ to 30 mM from 5 mM. In the absence of extracellular Ca2+ (1 mM EGTA), the hyperpolarization during hyposmotic activation was also abolished while the following depolarization was preserved. 50 muM GdCl3, which is known to block stretch-activated non-specific cation channels, inhibited the hyperpolarization reversibly. 50 muM GdCl3 also inhibited [Ca2+]i increase during hyposmotic activation as shown by the calcium-sensitive dye fura-2. Simultaneously, the [Ca2+]i increase and the hyperpolarization during hyposmotic activation could be observed using the combined method of whole-cell patch clamp and fura-2 technique. It is concluded that the cell swelling by hyposmotic activation may activate the stretch-activated non-specific cation channels in the OHCs which allow a Ca2+ influx. In turn, this [Ca2+]i increase leads to an activation of the Ca2+-activated K+ channels at the basolateral membrane of OHCs which results finally in a reversible hyperpolarization of OHCs by K+ efflux.