The role of Ca2+‐activated K+ channel spliced variants in the tonotopic organization of the turtle cochlea

Abstract

1 Turtle auditory hair cells contain multiple isoforms of the pore-forming α-subunit of the large-conductance Ca²⁺-activated K⁺ (K_Ca) channel due to alternative splicing at two sites. Six splice variants were studied by expression in Xenopus oocytes. 2 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. 3 Co-expression of a bovine β-subunit slowed the current relaxation 10-fold compared with channels formed from α-subunits alone but preserved the original order of kinetic differences. The β-subunit also increased the Ca²⁺ sensitivity of isoforms to bring them nearer the range of sensitivity of the native K_Ca channels of the hair cell. 4 With channels formed from α-subunits or α+β-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. 5 Experiments in which the β/α concentration ratio was varied indicated that the β-subunit exerts an all-or-none effect on the Ca²⁺ sensitivity and kinetics of the channel. 6 Co-expression of an avian β2-subunit had effects on kinetics and Ca²⁺ sensitivity of several α-isoforms which were qualitatively similar to those produced by the bovine β-subunit. 7 We conclude that differential expression of alternatively spliced α-subunit variants and a non-uniform distribution of a β-subunit can produce a range of K_Ca channel properties needed to explain the tonotopic organization of the turtle cochlea.