Thus the MT current in the middle region of the gerbil cochlea (CF = 2.5 kHz) is very similar to that near the apex of the rat cochlea (CF = 4 kHz), both being measured at the same holding Galunisertib potential (−84 mV). Overall there was about a 3-fold increase in MT current as the CF increased from 0.35 to 10 kHz. An increase in the size of the MT current along the tonotopic axis has also been reported in the gerbil hemi cochlea (He et al., 2004). Despite the change in current amplitude, the fraction activated at rest (the resting Popen) in low Ca2+ was invariant with CF and had a mean of 0.46 ± 0.03 (Figure 2D). As a consequence the silent current increased in parallel with the maximum

MT current. The fraction of MT current on at rest depended not only on extracellular Ca2+ around the hair bundle but also the nature and concentration of the mobile intracellular Ca2+ buffer (Ricci et al., 1998 and Beurg et al., 2010). BAPTA (1 mM) had been used selleck inhibitor so far because its properties theoretically match those of the endogenous Ca2+ buffer (Beurg et al., 2010), which in OHCs consists of 2 mM oncomodulin plus 0.25 mM calbindin-28K with no significant apex to base gradient (Hackney et al.,

2005). To provide experimental support for this, perforated patch recordings were performed on apical OHCs of rats, P9–P11, at which age the oncomodulin concentration is similar to that in the adult (Yang et al., 2004 and Hackney et al., 2005). With whole cell recording using 1 mM EGTA (Figures 3A and 3C), exposure to the low Ca2+ endolymph increased the MT current amplitude, as with 1 mM BAPTA, but produced only a small change in the fraction of current turned on at rest (mean = 0.12 ± 0.02, n = 5). Under perforated-patch conditions (Figures 3B and 3D), where mobile proteins such as the Ca2+ buffers are not washed out, the mean MT channel

open resting probability in five OHCs increased from 0.04 ± 0.02 in 1.5 mM Ca2+ to 0.42 ± 0.03 in 0.02 mM Ca2+. The values obtained with perforated patch did not differ significantly from those obtained in low Ca2+ using whole-cell with 1mM intracellular BAPTA in response to either fluid jet (0.43 ± 0.03; Figure 2D) or step stimuli (0.40 ± 0.08) (Beurg et al., 2010). The latter method and of hair bundle stimulation also allowed estimates of the adaptation time constant that, as reported previously (Beurg et al., 2010), were slowed in the low Ca2+ endolymph and were 0.6 ± 0.03 ms (EGTA), 0.5 ± 0.05 ms (BAPTA), and 1.4 ± 0.4 ms (perforated patch). The slower time constant in perforated patch may largely reflect a greater series resistance (see Experimental Procedures). In order to measure the effects of low endolymphatic Ca2+ on membrane time constant and resting potential, current clamp experiments were performed at body temperature on OHCs from isolated gerbil cochleas at around the onset of hearing (P11–P13).