Characteristics of K⁺Outward Currents in the Cochlear Outer Hair Cells of Circling Mice within the First Postnatal Week

Characteristics of K⁺Outward Currents in the Cochlear Outer Hair Cells of Circling Mice within the First Postnatal Week

K⁺ outward currents in the outer hair cells (OHCs) of circling mice (homozygous (<i>cir</i>/<i>cir</i>) mice), an animal model for human deafness (DFNB6 type), were investigated using a whole cell patch clamp technique. Littermate heterozygous (+/<i>cir</i>) mice of the same age (postnatal day (P) 0 &#8722;P6) were used as controls. Similar slow rising K⁺ currents were observed in both genotypes, but their biophysical and pharmacological properties were quite different. The values of V_half for activation were significantly different in the heterozygous (+/<i>cir</i>) and homozygous (<i>cir</i>/<i>cir</i>) mice (－8.1±2.2 mV, heterozygous (+/<i>cir</i>) mice (n=7) and －17.2±4.2 mV, homozygous (cir/cir) mice (n=5)). The inactivation curve was expressed by a single first order Boltzmann equation in the homozygous (cir/cir) mice, while it was expressed by a sum of two first order Boltzmann equations in the heterozygous (+/<i>cir</i>) mice. The K⁺ current of homozygous (<i>cir</i>/<i>cir</i>) mice was more sensitive to TEA in the 1 to 10 mM range, while the 4-AP sensitivities were not different between the two genotypes. Removal of external Ca²⁺ did not affect the K⁺ currents in either genotype, indicating that the higher sensitivity of K⁺ current to TEA in the homozygous (<i>cir</i>/<i>cir</i>) mice was not due to an early expression of Ca²⁺ activated K⁺ channels. Our results suggest that the K⁺ outward current of developing homozygous (<i>cir</i>/<i>cir</i>) mice OHCs is different in both biophysical and pharmacological aspects than that of heterozygous (+/<i>cir</i>) mice.