Calcium currents were characterized in tall hair cells isolated from the chick's cochlea to determine what types of Ca2+ channels existed and if these varied in cells with differing voltage responses to current injection. 2. Whole-cell, tight-seal recordings showed that the current-voltage relation of cochlear hair cells of the chick was dominated by K+ current. However, when outward K+ current was blocked it was found that all hair cells had a smaller, maintained inward current. 3. This inward current was a Ca2+ current since it required Ca2+ in the external medium, could also be carried by Ba2+, and was blocked reversibly by 5 mM-Co2+ and by Ni2+ and Cd2+ at micromolar concentrations. The Ca2+ channels were opened at membrane potentials positive to -50 mV, and the current was maximal near 0 mV. 4. The dihydropyridine BayK8644 (0.5 microM) produced a voltage-dependent increase of inward current. Ten micromolar nifedipine partially blocked the inward current. The outward Ca2(+)-activated K+ current was also reduced in the presence of 10 microM-nifedipine. These effects of dihydropyridines were completely reversible. 5. The Ca2+ current had rapid activation kinetics, reaching steady-state levels within 1 ms. If all outward currents were completely blocked the Ca2+ current showed no inactivation during depolarization lasting 200 ms. 6. No differences in voltage activation range, pharmacology, or kinetics of the Ca2+ current were found in tall hair cells from apical and basal regions of the cochlea. This is in contrast to the marked differences in K+ currents amongst cells from these two widely separated regions of the cochlea.
