Chick cochlear hair cell exocytosis mediated by dihydropyridine-sensitive calcium channels.

1. A semi-intact preparation of the chick basilar papilla was developed to study calcium-dependent neurotransmitter release by tall hair cells (avian equivalent of cochlear inner hair cells). 2. Tall hair cell depolarization resulted in changes in cell membrane capacitance (DeltaC(m)) that reflected cell surface area increases following synaptic vesicle exocytosis and provided a surrogate measure of neurotransmitter release. Both calcium current (I(Ca)) and DeltaC(m) were reversibly blocked by cobalt, and exhibited a similar bell-shaped dependency on voltage with a peak response around -10 mV. 3. Pharmacological agents selective for L-type calcium channels were employed to assess the role of this channel type in neurotransmitter exocytosis. Nimodipine, a dihydropyridine (DHP) antagonist, suppressed I(Ca) and blocked DeltaC(m). Conversely, the DHP agonist Bay K 8644 increased both I(Ca) and DeltaC(m) amplitude nearly 3-fold. These findings suggest that chick tall hair cell neurotransmitter release is mediated by calcium influx through L-type calcium channels.