KINETIC-ANALYSIS OF BARIUM CURRENTS IN CHICK COCHLEAR HAIR-CELLS

Inward barium current (I-Ba) through voltage-gated calcium channels was recorded from chick cochlear hair cells using the whole-cell clamp technique. I-Ba was sensitive to dihydropyridines and insensitive to the peptide toxins omega-agatoxin IVa, omega-conotoxin GVIa, and omega-conotoxin MVIIC. Changing the holding potential over a -40 to -80 mV range had no effect on the time course or magnitude of I-Ba nor did it reveal any inactivating inward currents. The activation of I-Ba was modeled with Hodgkin-Huxley m(2) kinetics. The time constant of activation, tau(m), was 550 mu s at -30 mV and gradually decreased to 100 mu s at +50 mV. A boltzmann fit to the activation curve, m(x), yielded a half activation voltage of -15 mV and a steepness factor of 7.8 mV. Opening and closing rate constants, alpha(m) beta(m), were calculated from tau(m) and mx, then fit with modified exponential functions. The H-H model derived by evaluating the exponential functions for alpha(m) and beta(m) not only provided an excellent fit to the time course of I-Ba activation, but was predictive of the time course and magnitude of the I-Ba tail current. No differences in kinetics or voltage dependence of activation of I-Ba were found between tall and short hair cells. We conclude that both tall and short hair cells of the chick cochlea predominantly, if not exclusively, express noninactivating L-type calcium channels. These channels are therefore responsible for processes requiring voltage-dependent calcium entry through the basolateral cell membrane, such as transmitter release and activation of Ca2+-dependent K+ channels.