A delayed rectifier conductance in type I hair cells of the mouse utricle

1. Membrane currents of hair cells in acutely excised or cultured mouse utricles were recorded with the whole cell voltage-clamp method at temperatures between 23 and 36 degrees C. 2. Type I and II hair cells both had delayed rectifier conductances that activated positive to -55 mV. 3. Type I, but not type II, hair cells had an additional delayed rectifier conductance (g(K,L)) with an activation range that was unusually negative and variable. At 23-25 degrees C, V-1/2 values ranged from -88 to -62 mV in 57 cells. 4. g(K,L) was very large. At 23-25 degrees C, the average maximum chord conductance was 75 +/- 65 nS (mean +/- SD, n = 57; measured at 54 mV), or similar to 21 nS/pF of cell capacitance. 5. g(K,L) was highly selective for K+ over Na+ (permeability ratio P-Na+/P-K+ : 0.006), but unlike other delayed rectifiers, g(K,L) was significantly permeable to Cs+ (P-Cs+/P-K + : 0.31). g(K,L) was independent of extracellular Ca2+. 6. At -64 mV, Ba2+ and 4-aminopyridine blocked g(K,L) with apparent dissociation constants of 2.0 mM and 43 mu M, respectively. Extracellular Cs+ (5 mM) blocked g(K,L) by 50% at -124 mV. Apamin (100 nM) and dendrotoxin (10 nM) had no effect. 7. The kinetic data of g(K,L) are consistent with a sequential gating model with at least two closed states and one open state. The slow activation kinetics (principal time constants at 23-25 degrees C: 600-200 ms) had a thermal Q(10) of 2.1. Inactivation (Q(10): 2.7) was partial at all temperatures. Deactivation followed a double-exponential time course and had a Q(10) of 2.0. 8. At 23-25 degrees C, g(K,L) was appreciably activated at the mean resting potential of type I hair cells (-77 +/- 3.1 mV, n = 62), so that input conductances were often more than an order of magnitude larger than those of type II cells. If these conditions hold in vivo, type I cells would produce unusually small receptor potentials. Warming the cells to 36 degrees C produced parallel shifts in g(K,L)'s activation range (0.8 +/- 0.3 mV/degrees C, n = 8), and in the resting potential (0.6 +/- 0.3 mV/degrees C, n = 4). Thus the high input conductances were not an artifact of unphysiological temperatures but remained high near body temperature. It remains possible that in vivo g(K,L)'s activation range is less negative and input conductances are lower; the large variance in the voltage range of activation suggests that it may be subject to modulation.