Direct measurements of Ca2+-activated K+ currents in inner hair cells of the guinea-pig cochlea using photolabile Ca2+ chelators

Abstract

Intracellular photorelease of Ca2+ from caged Ca2+ (DM-nitrophen or nitr5) and the patch-clamp technique in the whole-cell configuration were used to investigate Ca(2+)-activated currents in inner hair cells (IHCs) of the mammalian cochlea. Photoliberation of intracellular Ca2+ activated outward currents with a mean amplitude of 260 +/- 110 pA when IHCs were voltage-clamped, near the resting membrane potential, at -50 mV. The photoactivated currents were reversibly blocked by extracellular application of tetraethylammonium (TEA, 10 mM), neomycin (1 mM) and charybdotoxin (1 microM), but not by apamin. The voltage dependence of membrane currents activated by photolysis of DM-nitrophen demonstrated a reversal potential near the K+ equilibrium potential (Ek) and saturation near 0 mV. The presence of Ca(2+)-activated currents was further confirmed by the effects of extracellular adenosine 5'-triphosphate (ATP, 10 microM) and the Ca2+ ionophore ionomycin (10 microM). Both agents raised intracellular Ca2+ and simultaneously activated outward currents when IHCs were voltage-clamped near the resting membrane potential. In experiments where currents were activated by depolarizing voltage steps, nifedipine (50 microM) and Cd2+ (1 mM) reduced significantly (20-50%) the whole-cell outward currents, suggesting the presence of L-type Ca2+ currents activating K+ currents. These results are the first direct evidence for Ca(2+)-activated K+ currents in mammalian IHCs, these currents being potentially important for cell repolarization during sound-induced depolarization and synaptic transmission.

Inhibition of K⁺ Currents in Type I Vestibular Hair Cells by Gentamicin and Neomycin

“…AG are known to block but also permeate mechanotransduction channels in hair cells [Ohmori, 1985;Kroese et al, 1989;Kros et al, 1992;Marcotti et al, 2005]. In hair cells of the cochlea, direct effects of ototoxic AG on ATP receptors [Lin et al, 1993], nicotinic receptors [Blanchet et al, 2000] and K + channels [Dulon et al, 1995;Leitner et al, 2011] been described. OHC electromotility is not sensitive to AG [Wang et al, 2007].…”

“…Sequestration of phosphoinositides has also been implicated in AG-induced hair cell death [Jiang et al, 2006]. Direct effects of AG on ion channels in hair cells of the cochlea have been described [Lin et al, 1993;Dulon et al, 1995;Blanchet et al, 2000]. Recently, AG were shown to rapidly inhibit the basolateral KCNQ4-mediated current ( I K, n ) in OHC, by sequestering the phosphoinositide PIP 2 .…”

A Protein Interaction Network for the Large Conductance Ca2+-activated K+ Channel in the Mouse Cochlea

“…Although the onset of this channel during cochlear development in both mammals and non-mammals coincides with an increase in hearing sensitivity (6,7), its function is less clear in the former where hair cells are not frequency-tuned and studies report either the presence or the absence of hearing with the loss of BK (8,9). The BK channel has been localized to both the outer hair cells (OHC) (10) and inner hair cells (IHC) (7,(11)(12)(13) in mammals. However, unlike non-mammals, the BK channel appears in both synaptic and extrasynaptic sites near the apical end or neck of the IHC (9).…”

Expression of Ca2+‐activated BK channel mRNA and its splice variants in the rat cochlea

“…Mechanically induced inward currents through the transduction channel depolarize the hair cell and activate outwardly rectifying potassium conductances. In inner hair cells (IHCs) of mammalian organs of Corti, the predominant potassium conductance is a voltage-and Ca 2ϩ -activated K ϩ channel, probably of the BK type (g K,f , Kros and Crawford, 1990;Dulon et al, 1995;Appenrodt and Kros, 1997;Kirkwood and Kros, 1997). In particular, the functional maturation of IHCs in mice at the onset of hearing around postnatal day 12 (P12) coincides with the expression of g K,f (Kros et al, 1998).…”