Calcium balance and mechanotransduction in rat cochlear hair cells.
Auditory transduction occurs by opening of Ca(2+)-permeable mechanotransducer (MT) channels in hair cell stereociliary bundles. Ca(2+) clearance from bundles was followed in rat outer hair cells (OHCs) using fast imaging of fluorescent indicators. Bundle deflection caused a rapid rise in Ca(2+) that decayed after the stimulus, with a time constant of about 50 ms. The time constant was increased by blocking Ca(2+) uptake into the subcuticular plate mitochondria or by inhibiting the hair bundle plasma membrane Ca(2+) ATPase (PMCA) pump. Such manipulations raised intracellular Ca(2+) and desensitized the MT channels. Measurement of the electrogenic PMCA pump current, which saturated at 18 pA with increasing Ca(2+) loads, indicated a maximum Ca(2+) extrusion rate of 3.7 fmol x s(-1). The amplitude of the Ca(2+) transient decreased in proportion to the Ca(2+) concentration bathing the bundle and in artificial endolymph (160 mM K(+), 20 microM Ca(2+)), Ca(2+) carried 0.2% of the MT current. Nevertheless, MT currents in endolymph displayed fast adaptation with a submillisecond time constant. In endolymph, roughly 40% of the MT current was activated at rest when using 1 mM intracellular BAPTA compared with 12% with 1 mM EGTA, which enabled estimation of the in vivo Ca(2+) load as 3 pA at rest. The results were reproduced by a model of hair bundle Ca(2+) diffusion, showing that the measured PMCA pump density could handle Ca(2+) loads incurred from resting and maximal MT currents in endolymph. The model also indicated the endogenous mobile buffer was equivalent to 1 mM BAPTA.
Beurg
Maryline
M
Institut National de la Santé et de la Recherche Médicale U587, Université Victor Segalen Bordeaux 2, Hôpital Pellegrin, Bordeaux, France.
Nam
Jong-Hoon
JH
Chen
Qingguo
Q
Fettiplace
Robert
R
D000465:Algorithms; D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology; D020013:Calcium Signaling / Q000502:physiology; D000252:Calcium-Transporting ATPases / Q000502:physiology; D002462:Cell Membrane / Q000201:enzymology; D003051:Cochlea / Q000166:cytology / Q000502:physiology*; D004710:Endolymph / Q000502:physiology; D005455:Fluorescent Antibody Technique; D005456:Fluorescent Dyes; D006198:Hair Cells, Auditory / Q000502:physiology*; D006706:Homeostasis / Q000502:physiology; D007091:Image Processing, Computer-Assisted; D066298:In Vitro Techniques; D040542:Mechanotransduction, Cellular / Q000502:physiology*; D008839:Microelectrodes; D008928:Mitochondria / Q000378:metabolism / Q000502:physiology; D009925:Organ of Corti / Q000502:physiology; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley
D015220:Calcium Channels; D005456:Fluorescent Dyes; D000252:Calcium-Transporting ATPases; D002118:Calcium