Hair cell-specific splicing of mRNA for the α1D subunit of voltage-gated Ca2+ channels in the chicken's cochlea

The L-type voltage-gated Ca2+ channels that control tonic release of neurotransmitter from hair cells exhibit unusual electrophysiological properties: a low activation threshold, rapid activation and deactivation, and a lack of Ca2+-dependent inactivation. We have inquired whether these characteristics result from cell-specific splicing of the mRNA for the L-type alpha(1D) subunit that predominates in hair cells of the chicken's cochlea. The alpha(1D) subunit in hair cells contains three uncommon exons: one encoding a 26-aa insert in the cytoplasmic loop between repeats I and II, an alternative exon for transmembrane segment IIIS2, and a heretofore undescribed exon specifying a IO-aa insert in the cytoplasmic loop between segments IVS2 and IVS3. We propose that the alternative splicing of the alpha(1D) mRNA contributes to the unusual behavior of the hair cell's voltage-gated Ca2+ channels.