Major potassium conductance in type I hair cells from rat semicircular canals: characterization and modulation by nitric oxide

Mammalian vestibular organs have two types of hair cell, type I and type II , which differ morphologically and electrophysiologically. Type I hair cell s alone express an outwardly rectifying current, I-K,I-L, which activates a t relatively negative voltages. We used whole cell and patch configurations to study I-K,I-L in hair cells isolated from the sensory epithelia of rat semicircular canals. I-K,I-L was potassium selective, blocked by 4-aminopyr idine, and permeable to internal cesium. It activated with sigmoidal kineti cs and was half-maximally activated at -74.5 +/- 1.6 mV (n = 35; range -91 to -50 mV). It was a very large conductance (91 +/- 8 nS at -37 mV; 35 nS/p F for a cell of average size). Patch recordings from type I cells revealed a candidate ion channel with a conductance of 20-30 pS. Because I-K,I-L was activated at the resting potential, the cells had low input resistances (R -m): median 25 M Omega at -67 mV versus 1.3 G Omega for type II cells. Cons equently, injected currents comparable to large transduction currents (300 pA) evoked small (less than or equal to 10 mV) voltage responses. The cells ' small voltage responses and negative resting potentials (V-R = 281.3 +/- 0.2 mV, n = 144) pose a problem for afferent neurotransmission: how does th e receptor potential depolarize the cell into the activation range of Ca2channels (positive to -60 mV) that mediate transmitter release? One possibi lity, suggested by spontaneous positive shifts in the activation range of I -K,I-L during whole cell recording, is that the activation range might be m odulated in vivo. Any factor that reduces the number of I-K,I-L channels op en at V-R will increase R-m and depolarize V-R. Nitric oxide (NO) is an ion channel modulator that is present in vestibular epithelia. Four different NO donors, applied externally, inhibited the I-K,I-L conductance at -67 mV, with mean effects ranging from 33 to 76%. The NO donor sodium nitroprussid e inhibited channel activity in patches when they were cell-attached but no t excised, suggesting an intracellular cascade. Consistent with an NO-cGMP cascade, 8-bromo-cGMP also inhibited whole cell I-K,I-L.Ca2+-dependent NO s ynthase is reported to be in hair cells and nerve terminals in the vestibul ar epithelium. Excitatory input to vestibular organs may lead, through Ca2 influx, to NO production and inhibition of I-K,I-L. The resulting increase in R-m would augment the receptor potential, a form of positive feedback.