1. Inwardly rectifying ionic currents were studied using patch-clamp recording methods in oscillatory-type and spike-type hair cells and supporting cells dissociated from the goldfish sacculus. These cells had different types of inwardly rectifying currents. The biophysical properties of these currents were investigated. 2. A unique potassium current (Isc) was the sole ionic current recognized in supporting cells. Isc was active throughout the membrane potential range between +30 and -170 mV, but showed weak inward rectification and no inactivation. 3. In spike-type hair cells, inwardly rectifying current (Ik1) was selectively permeable to K+ (K+:Na+ permeability ratio, 1:0.0021). Ik1 could underlie the high negative resting potential of these hair cells because it is partially active at this potential. The strong inward rectification of Ik1 contributed to the low negative plateau potential seen in spike-type hair cells. 4. In oscillatory-type hair cells, hyperpolarization-activated potassium-sodium current (Ih), which had properties similar to that in photoreceptor and other neurons, was present instead of inwardly rectifying K+ current. 5. In the cell-attached and inside-out modes with 125 microM external K+ ([K+]o), IK1 channel had a unitary conductance of 27 pS and showed inactivation with increasing hyperpolarization. Putative Ih and Iso single channels had unitary conductances of 7 and 61 pS, respectively, in the cell-attached mode with 125 microM Ko+.
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