Potassium current properties in apical and basal inner hair cells from guinea-pig cochlea
Introduction
In the mammalian cochlea, there are two types of hair cells that subserve distinct functions and receive characteristic patterns of innervation. The single row of inner hair cells (IHCs) that receive nearly all the afferent innervation and are primary acoustic transducers. The three rows of outer hair cells (OHCs) receive efferent axons from neurons located in the superior olivary complex of the brainstem and form part of the feedback loop that regulates frequency selectivity and sensitivity (Guinan and Stankovic, 1996, Ulfendahl and Flock, 1998). Variations in the expression of OHC potassium conductances along the tonotopic gradient of the adult mammalian cochlea have been investigated by Housley and Ashmore, 1992, Mammano and Ashmore, 1996, Raybould and Housley, 1997. They demonstrated a position-dependent increase in the basolateral membrane conductance from the apex to the base along the length of the cochlea. However, variation in the expression of IHC potassium conductance along the tonotopic axis of the cochlea is unknown.
The two IHC potassium currents are distinguishable by their pharmacology and their activation kinetics (Kros and Crawford, 1990). The fast activating current, Ik,f, was blocked by tetraethylammonium (TEA) but was resistant to 4-aminopyridine (4-AP). This current has recently been implicated in developmental changes in IHCs during the postnatal days just preceding functional maturation of hearing in mice (Kros et al., 1998). Another potassium current, Ik,s, was activated more slowly on depolarization and was blocked by 4-AP but not by TEA. The physiological role of this current has not yet been elucidated.
In this study, we obtained separately isolated IHCs from the apical turn and basal turn and measured the potassium currents by the whole-cell voltage-clamp technique. This study investigated and compared the two types of potassium currents in the apex and basal IHCs by their pharmacology and activation kinetics.
Section snippets
Preparation of isolated IHCs
Adult albino guinea-pig (200–350 g) were killed by rapid cervical dislocation and both bullae were removed and the cochlea was exposed. The cochlea, fused to the bulla, was placed in a Ca2+-free external solution (142 mM NaCl, 4 mM KCl, 3 mM MgCl2, 2 mM NaH2PO4, 8 mM Na2HPO4, adjusted to pH 7.4 with NaOH). The otic capsule was opened, allowing removal of the organ of Corti attached to the modiolus. IHCs were isolated by micro-dissecting a selected turn of the organ of Corti, from turn 1–2 and
Membrane currents under voltage-clamp in apical and basal IHCs
Currents in response to depolarizing voltage steps from a holding potential of −80 mV were recorded from IHCs in apical and basal turns. Typical current records are shown in Fig. 1A. Both apical and basal IHCs had outwardly rectifying currents in response to depolarizing voltage pulses, with only a slight inward current when hyperpolarized. Fig. 1B represents the steady-state current–voltage relation (I–V) measured at the end of each 40 ms command step for all seven cells from apical turn and
Discussion
The potassium current in IHCs flows through at least two types of membrane conductance: a fast (Ik,f), TEA-sensitive conductance and a slow (Ik,s), TEA-resistant conductance. Although membrane conductance demonstrated no differences between apical IHCs and basal IHCs (Fig. 1), TEA-sensitive Ik,f is larger in basal IHCs than in apical IHCs (Fig. 4, Fig. 5). The fast and slow time constants in the rising phase of control currents were consistent with those of TEA-sensitive Ik,f and TEA-resistant I
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research 13671789 from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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