Cholinergic control of membrane conductance and intracellular free Ca2+ inouter hair cells of the guinea pig cochlea

We have studied the action of cholinergic agonists on outer hair cells, bot h in situ and isolated from the cochlea of the guinea pig, combining new fa st CCD technology for Ca2+ imaging and conventional patch-clamp methods. Ca rbachol (1 mM) activated a current with a reversal potential near -70 mV an d a bell-shaped I-V curve, suggesting that it was a Ca2+-activated K+ curre nt. In a few cells, this current was preceded by a transient inward current , probably owing to an influx of Ca2+ and other cations through the acetylc holine (ACh) receptors. The amplitude of the Ca2+ signal was maximal in a c ircumscribed region at the basal pole of the cell and decreased steeply tow ards the apical pole, compatible with Ca2+-influx and/or Ca2+-induced Ca2+- release at the cells base. The time course of the Ca2+-rise was fastest at the base, but it was still slightly slower, and more rounded, than that of the K+ current. In some recordings the K+ current was observed without any measurable change of intracellular Ca2+. The K+ current was potentiated (18 %) by caffeine (5 mM), and decreased (19%) by ryanodine (0.1 mM) in the maj ority of cells tested. The results are discussed in terms of a labile intra cellular Ca2+ store located at the base of the cell, close to the Ca2+-perm eable ACh receptor channels and Ca2+-activated K+ channels, whose contribut ion to the Ca2+ rise occurring in the region of the channels is variable, a nd probably dependent on its ability to refill with Ca2+. (C) 2000 Harcourt Publishers Ltd.