Identification and characterization of an L-type Ca_v1.2 channel in spiral ligament fibrocytes of gerbil inner ear

Intracellular free Ca²⁺ levels are critical to the activity of BK channels in inner ear type I spiral ligament fibrocytes. However, the mechanisms for regulating intracellular Ca²⁺ levels in these cells are currently poorly understood. Using patch-clamp technique, we have identified a voltage-dependent L-type Ca²⁺ channel in type I spiral ligament fibrocytes cultured from gerbil inner ear. With 10 mM Ba²⁺ as the conductive cation, an inwardly rectifying current was elicited with little inactivation by membrane depolarization. The voltage activation threshold and the half-maximal voltage activation were -40 and -6 mV, respectively. This inward whole-cell current reached its peak at around 10 mV of membrane potential. The amplitude of the peak current varied among cells ranging from 50 to 274 pA with an average of 132.4±76.2 pA (n=19); 10^-6 M nifedipine significantly inhibited the inward currents by 90.3±1.2% (n=11). RT-PCR analysis revealed that cultured type I spiral ligament fibrocytes express the α_1C isoform of the L-type Ca²⁺channels encoded by the Ca_v1.2 gene. The expression of this channel in gerbil inner ear was confirmed by RT-PCR analysis using freshly isolated spiral ligament tissues. The Ca_v1.2 channel may function in conjunction with a previously identified intracellular Ca-ATPase (SERCA) to regulate intracellular free Ca²⁺ levels in type I spiral ligament fibrocytes, and thus modulate BK channel activity in these cells.