Calcium Currents in Solitary Hair Cells Isolated from Frog Crista Ampullaris

Some properties of Ca²⁺ currents in hair cells isolated from frog semicircular canals by enzymatic or mechanical treatment were studied by using the whole-cell configuration of the patch-clamp technique. After blocking the large outward K⁺ currents by substituting Cs⁺ for K⁺ and adding tetraethylammonium to the pipette filling solution, voltage- and time-dependent inward currents were clearly detectable in the presence of 4 mM Ca²⁺ in the extracellular solution. Ca²⁺ current was recruited at test potentials more positive than -60 mV, showed a rapid activation, and exhibited no inactivation during 150-ms depolarizing pulses. The maximal amplitude was attained at about -20 mV, with an average value of about 80 pA. When Ca²⁺ in the extracellular solution was replaced with Ba²⁺, the magnitude of inward currents increased about twofold. Ba²⁺ currents were blocked more effectively by Cd²⁺ than by Ni²⁺, were suppressed by 0.5 μM ω-conotoxin, and were virtually unaffected by amiloride. The dihydropyridine Bay K 8644 caused a marked voltage-dependent increase in inward currents. The present data suggest that hair cells from frog crista ampullaris are endowed with a homogeneous population of Ca²⁺ channels having several properties similar to those described for neuronal L channels. Since these channels are recruited in a range of potentials close to the resting level, it is suggested that they subserve the control of both resting and evoked transmitter release from the basal pole of the hair cells.