Journal of Physiology (1993), 470, pp. 109-126 109 With 8 figures Printed in Great Britain THE EFFECTS OF LOW CALCIUM ON THE VOLTAGE-DEPENDENT CONDUCTANCES INVOLVED IN TUNING OF TURTLE HAIR CELLS BY J. J. ART*, R. FETTIPLACEt AND Y.-C. WU From the Department of Neurophysiology, University of Wisconsin Medical School, Madison, WI 53706, and the *Department of Pharmacological and Physiological Sciences, University of Chicago, Chicago IL 60637, USA (Received 9 November 1992) SUMMARY 1. The voltage-dependent conductances of turtle cochlear hair cells of known resonant frequency were characterized by tight-seal, whole-cell recording during superfusion with solutions containing normal (28 mM) and reduced (01-10 /OM) Ca2+. 2. In 1 /SM Ca2+, the current flowing through the voltage-dependent Ca2+ channels was increased roughly fivefold and had a reversal potential near 0 mV. This observation may be explained by the Ca2+ channels becoming non-selectively permeable to monovalent cations in low-Ca2+ solutions. Lowering the Ca2+ further to 0-1 /IM produced little increase in the current. 3. The size of the non-selective current increased systematically with the resonant frequency of the hair cell over the range from 10 to 320 Hz. This suggests that hair cells tuned to higher frequencies contain more voltage-dependent Ca2+ channels. 4. There was a good correlation between the amplitudes of the non-selective current and the K+ current which underlies electrical tuning of these hair cells. The amplitude of the K+ current also increased systematically with resonant frequency. 5. In cells with resonant frequencies between 120 and 320 Hz, the K+ current was completely abolished in 1 ,UM Ca2 , consistent with prior evidence that this current flows through Ca2+ activated K+ channels. In a majority of cells tuned between 50 and 120 Hz, the K+ current was incompletely blocked in 1 ,UM Ca2+, but was eliminated in 041 /M Ca2 . In all hair cells the K+ current was abolished by 25 mM tetraethylammonium chloride. 6. In cells tuned to 10-20 Hz, the K+ current was not substantially diminished even in 0-1 /LM Ca2 , which argues that it may not be Ca2+ activated. 7. In cells tuned to frequencies above 100 Hz, the K+ current could still be evoked by depolarization during superfusion with 10 /1M Ca2+. However, its half-activation voltage was shifted to more depolarized levels and its maximum amplitude was systematically reduced with increasing resonant frequency. 8. These observations are consistent with the notion that in cells tuned to more than 50 Hz, there is a fixed ratio of the number of voltage-dependent Ca2+ channels t To whom reprint requests should be addressed. MS 1889 ) by guest on September 12, 2011 jp.physoc.org Downloaded from J Physiol (