The effect of clofillium, a K-channel blocker, on the electrogenic K secretion and the afferent discharge at the frog semicircular canal

It has been reported in both the cochlea and the semicircular canal that a voltage-dependent K channel, called IsK, would be involved in the eletrogenic K secretion in endolymph (Sakagami et al., Hear. Res.. 56. 168. 1991: Marcus and Shen. Am. J. Phvsiol.. 267. C857. 1994). The aim of the present study was to investigate the effect of clofiliurn. a K-channel blocker (Arena and Kass. Mol. Pharmacol., 34, 60, 1988) that has been shown to inhibit the endocochlear potential (Mori et al.. Eur. Arch. Otorhinolaryngol., 250. 186. 1993), on the electrogenic K secretion in the isolated frog semicircular canal, on the one hand, and on the rate of miniature excitatorv post synaptic potentials (mEPSPs) and spikes at the cytoneural junction in intact frog labvrinth. on the other hand. Electrogenic K secretion The frog semicircular canal was isolated and placed in a two compartment chamber. both compartments being filled with perilymph-like solution. In such a condition. the semicircular canal secreted K (inward flux to the lumen: 108 ± 6.9 pmol/min/mm2 mean ± SE, n=17) and generated a slight transepithelial potential (+ 1.3 ± 0.33 mV, lumen positive. n=1 7). In the presence of luminal 10—3 M clofilium tosylate, the K secretion was not altered (96 ± 7.4 pmol/minimm2, n=17) whereas the transepíthelial potential rapidly decreased and then slowly increased to reach. at the end of the 30 min period, a lower value than the initial one (-0.3 ±0.17 mV. lumen negative. n=17). mEPSP and spike rates at the cytoneural junction mEPSP and spike frequencies were recorded from the posterior nerve. both at rest and during sinusoidal rotation of the canal (0.1 Hz. peak acceleration 12 deg/s2 ) in absence or presence of 10-3 M clofilium. The electrophysiological recordings were analysed by means of signal processing techniques as previously described (Rossi et al.. J. Gen. Physiol.. 94. 303. 1989: Rossi et al.. J. Physiol.. 478, 17, 1994). Rate of occurrence., amplitudes and time intervals between successive mEPSPs were evaluated. The amplitude distributions of mEPSPs were always continuous, unimodal and well fit by single log-normal distribution functions, and their time intervals were systematically distributed along single exponentials; this suggests that transmitter release retains its quantal and random nature in the presence of the drug. Clofilium induced: (i) a decrease in mEPSP rate (-30% at rest, as well as during mechanical stimulation (-23%); (ii) a decrease in mEPSP peak ampliitude (-24%). Clofilium induced modifications of the elementary mEPSP waveform: the shape of the event became less peaky (9% increase in the “smoothness" parameter) while the event duration, peak amplitude and time-to-peak were reduced by 21%, 19% and 16%, respectively. These effects were fully reversible on washing. Conclusions Clofilium appears to act at multiple sites in the semicircular canal. The decrease of the transepithelial potential is likely to induce a reduction in the hair cell transduction current; this could account for the observed moderate decrease in mEPSP rate. The disappearance of resting and evoked action potentials (unexplained by the wild decrease in transmitter release) and the reduction in mEPSP peak amplitude point to a postsynaptic action of clofilium. Depolarization of the membrane of the afferent fibre at rest (secondary to decreased potassium conductance) might reduce mEPSP peak amplitude and partially inattivate sodiurn channels, thus raising the firing threshold for the spike encoder.