Voltage-dependent Ca²⁺ channels in the spiral ganglion cells of guinea pig cochlea

Voltage-dependent Ca²⁺ channels in spiral ganglion cells (SGCs) isolated from guinea pig cochlea were investigated using the patch-clamp technique in a whole-cell recording mode. The voltage-dependent Na⁺ and K⁺ currents were blocked by adding tetrodotoxin, 4-aminopyridine, and tetraethylammonium to the external solution and by using choline or Cs⁺ in the external and internal solutions instead of Na⁺ or K⁺, respectively. The depolarizing voltage steps evoked inward currents with slow current decay. The maximum amplitude of the inward current increased in a hyperbolic manner with increasing extracellular Ca²⁺ concentration, indicating that the inward current was a voltage-dependent Ca²⁺ current (I_Ca). In 5 mM Ca²⁺ external solution, the I_Ca activated from a membrane potential around -60 mV and reached full activation at about -10 mV. The I_Ca inactivated from about -60 mV and became fully inactivated at about 0 mV, consistent with the high-voltage-activated Ca²⁺ channel subtype. Ionic selectivities for Ca²⁺ channels in SGCs were as follows: Ca²⁺>Ba²⁺>Sr²⁺. Effects of both inorganic and inorganic Ca²⁺ antagonists also were examined. The inhibitory strength was as follows: La³⁺>Cd²⁺>Ni²⁺≥Co²⁺ for inorganic Ca²⁺ antagonists, and flunarizine>nicardipine>methoxyverapamil >dilitiazem for organic ones.