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1 Dept. of Physiological-Pharmacological Sciences, Universita' di Pavia, Pavia, Italy
2 Dept. of Otolaryngology, Physiology & Biophysics and Anatomy & Neurosciences, The University of Texas Medical Branch, Galveston, Texas, USA
3 Centre for Molecular Biology and Neuroscience, and Dept. of Anatomy, Institute of Basic Medical Sciences, University of Oslo, Oslo, Norway
* To whom correspondence should be addressed. E-mail: smasetto{at}unipv.it.
In birds type I and type II hair cells differentiate before birth. Here we describe that chick hair cells, from the semicircular canals, begin expressing a voltage-dependent Na current (INa) from embryonic day 14 (E14), and continue to express the current up to hatching (E21). During this period INa was present in most (31/43) type I hair cells irrespective of their position in the crista, in most type II hair cells located far from the planum semilunatum (48/63), but only occasionally in type II hair cells close to the planum semilunatum (2/35). INa activated close to -60 mV, showed fast time and voltage-dependent activation and inactivation, and was completely, and reversibly, blocked by submicromolar concentrations of tetrodotoxin (Kd = 17 nM). One peculiar property of INa concerns its steady-state inactivation, which is complete at -60 mV (half-inactivating voltage = -96 mV). INa was found in type I and type II hair cells from the adult chicken as well, where it had similar, although possibly not identical, properties and regional distribution. Current-clamp experiments showed that INa could contribute to the voltage response provided that the cell membrane was depolarized from holding potentials more negative than -80 mV. When recruited, INa produced a significant acceleration of the cell membrane depolarization, which occasionally elicited a large rapid depolarization followed by a rapid repolarization (action- potential-like response). Possible physiological roles for INa in the embryo and adult chicken are discussed.
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