Elementary properties of Kir2.1, a strong inwardly rectifying K+ channel expressed by pigeon vestibular type II hair cells

By using the patch-clamp technique in the cell-attached configuration, we have investigated the single-channel properties of an inward rectifier potassium channel (Kir) expressed by pigeon vestibular type II hair cells in situ. In high-K+ external solution with 2 mM Mg2+, Kir inward current showed openings to at least four amplitude levels. The two most frequent open states (L2 and L3) had a mean slope conductance of 13 and 28 pS, respectively. L1 (7 pS) and L4 (36 pS) together accounted for less than 6% of the conductive state. Closed time distributions were fitted well using four exponential functions, of which the slowest time constant (τC4) was clearly voltage-dependent. Open time distributions were fitted well with two or three exponential functions depending on voltage. The mean open probability (PO) decreased with hyperpolarization (0.13 at −50 mV and 0.03 at −120 mV). During pulse-voltage protocols, the Kir current-decay process (inactivation) accelerated and increased in extent with hyperpolarization. This phenomenon was associated with a progressive increase of the relative importance of τC4. Kir inactivation almost disappeared when Mg2+ was omitted from the pipette solution. At the same time, PO increased at all membrane voltages and the relative importance of L4 increased to a mean value of 47%. The relative importance of τC4 decreased for all open states, while L4 only showed a significantly longer open time constant. The present work provides the first detailed quantitative description of the elementary properties of the Kir inward rectifier in pigeon vestibular type II hair cells and specifically describes the Kir gating properties and the molecule's sensitivity to extracellular Mg2+ for all subconductance levels. The present results are consistent with the Kir2.1 protein sustaining a strong inwardly rectifying K+ current in native hair cells, characterized by rapid activation time course and slow partial inactivation. The longest closed state (τC4) appears as the main parameter involved in time- and Mg2+-dependent decay. Finally, in contrast to Kir2.1 results described so far for mammalian cells, external Mg2+ had no effect on channel conductance.