Inward barium current (I-Ba) through voltage-gated calcium channels wa
s recorded from chick cochlear hair cells using the whole-cell clamp t
echnique. I-Ba was sensitive to dihydropyridines and insensitive to th
e peptide toxins omega-agatoxin IVa, omega-conotoxin GVIa, and omega-c
onotoxin MVIIC. Changing the holding potential over a -40 to -80 mV ra
nge had no effect on the time course or magnitude of I-Ba nor did it r
eveal any inactivating inward currents. The activation of I-Ba was mod
eled with Hodgkin-Huxley m(2) kinetics. The time constant of activatio
n, 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 ha
lf activation voltage of -15 mV and a steepness factor of 7.8 mV. Open
ing 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 th
e 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 pre
dominantly, if not exclusively, express noninactivating L-type calcium
channels. These channels are therefore responsible for processes requ
iring voltage-dependent calcium entry through the basolateral cell mem
brane, such as transmitter release and activation of Ca2+-dependent K channels.