Outwardly rectifying currents in guinea pig outer hair cells

Studies of K+ conductances in hair cells report that big-conductance Ca2+-dependent K+ (BK) channels carry parts of the outwardly rectifying currents. Lin et al. (1995) suggested that in guinea pig outer hair cells (OHCs) a portion of these currents is carried via a voltage-dependent and Ca2+-independent K+ channel. The present study tests the hypothesis that there are two separable current components of the outwardly rectifying currents by using patch clamp methods in OHCs to characterize the voltage dependence and sensitivity of the outwardly rectifying currents to channel blockers. Lowering of external Ca2+ caused no change in the currents while charybdotoxin (ChTx; 100 nM), a BK K+ channel blocker, and Cd2+ (200 mu M), an L-type calcium channel blocker, abolished about 50% of the currents. Both ChTx and Cd2+ caused a depolarizing shift in the half-activation voltage paralleled by a decrease in the voltage sensitivity. 4-Aminopyridine (4-AP, 0.01 mM), an A-type and delayed rectifier type channel blocker, abolished about 50% of the currents and caused a hyperpolarizing shift in the half-activation voltage together with an increase in the voltage sensitivity. The outwardly rectifying currents were more sensitive to block by 4-AP at membrane voltages around 40 mV compared to voltages around -20 mV. The differences in the current characteristics may be due to two separate channel types, one of which is similar to the delayed rectifier type channels while the other may be similar to the BK Ca2+-dependent K+ channels. In addition, the largest outwardly rectifying currents were present in long OHCs with the smallest present in short OHCs.