The effects of low calcium on the voltage‐dependent conductances involved in tuning of turtle hair cells.

Abstract

SUMMARY1. 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 1889J. J. ART, R. FETTIPLACE AND Y.-C. WU to Ca2+-activated K+ channels, the numbers of each increasing in proportion to resonant frequency. The results also provide indirect evidence that the Ca2+-activated K+ channels in cells tuned to higher frequencies may be less sensitive to intracellular Ca2+.

Tonotopic map of potassium currents in chick auditory hair cells using an intact basilar papilla

“…It was already established that there was mechanical tuning in the chick cochlea with von Bëkësy's (1960) observations that the basilar membrane optimally vibrated at di¡erent frequencies along the length of the papilla. The question remained, however, as to whether avian hair cells are capable of sharpening this tuning through electrical resonance as demonstrated in hair cells of reptiles and amphibians (Crawford and Fettiplace, 1981 ;Lewis and Hudspeth, 1983 ;Art and Fettiplace, 1987 ;Pitchford and Ashmore, 1987;. Direct evidence for electrical resonance in the chick cochlea was provided by studies of isolated tall hair cells .…”

“…Although kinetics and pharmacology vary, I KDR can display slow kinetics (Hille, 1992;Johnston and Wu, 1995) and relative insensitivity to TEA (Tasaki and Hagiwara, 1957;Stan¢eld, 1970 ;Wong and Binstock, 1980). A current bearing properties very similar to I KDR has been reported in dissociated chick hair cells as well as turtle cochlear hair cells (Art et al, 1993).…”

“…This initial fast outward current displays kinetics and pharmacology very similar to the calcium-activated potassium current (I KCa ) as described in dissociated chick tall hair cells , isolated chick vestibular hair cells (Ohmori, 1984), turtle auditory (Art et al, 1993) and frog saccular hair cells (Lewis and Hudspeth, 1983).…”

“…Based on previous work Art et al, 1993), it is likely that the TEA sensitive current is I KCa while the remaining current, that is not blocked by TEA and has slower kinetics, is I K . The left panel of Fig.…”

“…This gradient in frequency selectivity is partly achieved through changes in the mechanical properties such as variation in the passive mechanical properties of the basilar membrane (von Bëkësy, 1960) and length of hair cell bundles (Tilney and Saunders, 1983). In addition to mechanical variations along the cochlea, the hair cells of many amphibians (Lewis and Hudspeth, 1983 ;Pitchford and Ashmore, 1987), reptiles (Crawford and Fettiplace, 1981 ;Art and Fettiplace, 1987 ; and birds are thought to be tuned as a result of intrinsic electrical properties which allow cells to act as electrical resonators. When depolarized by current injection, the hair cells of these animals exhibit damped, sinusoidal membrane voltage oscillations that arise from the intrinsic membrane conductances.…”