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Turtle auditory hair cells contain multiple isoforms of the pore-forming 
-subunit of the large-conductance Ca2+-activated K+ (KCa) channel due to alternative splicing at two sites. Six splice variants were studied by expression in Xenopus oocytes.
The isoforms possessed differences in apparent Ca2+ sensitivity and kinetics. The lowest Ca2+ sensitivity was observed in a novel variant resulting from a 26 amino acid deletion around one of the splice sites.
Co-expression of a bovine 
-subunit slowed the current relaxation 10-fold compared with channels formed from -subunits alone but preserved the original order of kinetic differences. The -subunit also increased the Ca2+ sensitivity of isoforms to bring them nearer the range of sensitivity of the native KCa channels of the hair cell.
With channels formed from -subunits or + -subunits, the half-activation voltage in a fixed Ca2+ concentration, and the time constant of the current relaxation, varied linearly with the combined size of the insertions/deletions at the splice sites.
Experiments in which the / concentration ratio was varied indicated that the -subunit exerts an all-or-none effect on the Ca2+ sensitivity and kinetics of the channel.
Co-expression of an avian 2-subunit had effects on kinetics and Ca2+ sensitivity of several -isoforms which were qualitatively similar to those produced by the bovine -subunit.
We conclude that differential expression of alternatively spliced -subunit variants and a non-uniform distribution of a -subunit can produce a range of KCa channel properties needed to explain the tonotopic organization of the turtle cochlea.
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