Predominance of the alpha1D subunit in L-type voltage-gated Ca2+ channels of hair cells in the chicken's cochlea.

The voltage-gated Ca2+ channels that effect tonic release of neurotransmitter from hair cells have unusual pharmacological properties: unlike most presynaptic Ca2+ channels, they are sensitive to dihydropyridines and therefore are L-type. To characterize these Ca2+ channels, we investigated the expression of L-type alpha1 subunits in hair cells of the chicken's cochlea. In PCRs with five different pairs of degenerate primers, we always obtained alpha1D products, but only once an alpha1C product and never an alpha1S product. A full-length alpha1D mRNA sequence was assembled from overlapping PCR products; the predicted amino acid sequence of the alpha1D subunit was about 90% identical to those of the mammalian alpha1D subunits. In situ hybridization confirmed that the alpha1D mRNA is present in hair cells. By using a quantitative PCR assay, we determined that the alpha1D mRNA is 100-500 times more abundant than the alpha1C mRNA. We conclude that most, if not all, voltage-gated Ca2+ channels in hair cells contain an alpha1D subunit. Furthermore, we propose that the alpha1D subunit plays a hitherto undocumented role at tonic synapses.