Influence of Ca2+ on the voltage dependent mechanosensitivity of the hair cells in the lateral line organs of Xenopus laevis

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Abstract
The influence of the transepithelial voltage (Vt) on the degree of synchronization (DOS) of afferent spikes from lateral line organs to mechanical stimulation was examined in the isolated voltage clamped skin of X. laevis. With 1 mM CaCl2, 5 mM MOPS[morpholinopropane sulfonic acid]-KOH (pH = 7.1) in the apical solution and Ringer''s solution on the serosal side of the skin, changes in V1 from -60 to 65 mV (inside relative to outside) lead to a nonlinear reduction in DOS to a minimum value, where the afferent spikes were no longer time-locked. Beyond this value of Vt, DOS increased until at Vt = 100-120 mV DOS was again reduced. The reduction of DOS to the minimum value was associated with a monotonic increase in the mean phase of DOS, which abruptly changed .apprx. 180.degree. as the minimum was passed. Change in the apical Ca2+ concentration from 0.1 to 10 mM increased the Vt at which the minimum value was attained, by 39 mV. The voltage dependency of DOS is in agreement with a model where the driving force for the receptor current across the apical membrane of the hair cells controls the generation of time-locked afferent spikes to mechanical stimulation. The Vt, at which the minimum value of DOS was attained, is then equivalent to the reversal potential across the apical membrane. The influence of the apical Ca2+ concentration on this Vt may be caused by the change in the gradient of Ca2+ or change in the ionic selectivity of the transduction channels.