Ionic currents in hair cells dissociated from frog semicircular canals after preconditioning under microgravity conditions

Abstract

The effects of microgravity on the biophysical properties of frog labyrinthine hair cells have been examined by analyzing calcium and potassium currents in isolated cells by the patch-clamp technique. The entire, anesthetized frog was exposed to vector-free gravity in a random positioning machine (RPM) and the functional modification induced on single hair cells, dissected from the crista ampullaris, were subsequently studied in vitro. The major targets of microgravity exposure were the calcium/potassium current system and the kinetic mechanism of the fast transient potassium current, I(A). The amplitude of I(Ca) was significantly reduced in microgravity-conditioned cells. The delayed current, I(KD) (a complex of I(KV) and I(KCa)), was drastically reduced, mostly in its I(KCa) component. Microgravity also affected I(KD) kinetics by shifting the steady-state inactivation curve toward negative potentials and increasing the sensitivity of inactivation removal to voltage. As concerns the I(A), the I-V and steady-state inactivation curves were indistinguishable under normogravity or microgravity conditions; conversely, I(A) decay systematically displayed a two-exponential time course and longer time constants in microgravity, thus potentially providing a larger K(+) charge; furthermore, I(A) inactivation removal at -70 mV was slowed down. Stimulation in the RPM machine under normogravity conditions resulted in minor effects on I(KD) and, occasionally, incomplete I(A) inactivation at -40 mV. Reduced calcium influx and increased K(+) repolarizing charge, to variable extents depending on the history of membrane potential, constitute a likely cause for the failure in the afferent mEPSP discharge at the cytoneural junction observed in the intact labyrinth after microgravity conditioning.

Exposure to reduced gravity impairs junctional transmission at the semicircular canal in the frog labyrinth

“…8, are presumably generated by the dynamical changes brought about by the slow continuous protocol, which produces a higher intracellular Ca 2ϩ load. MicroG-conditioning does not affect either the threshold or the peak in standard ICa I-V curves (10). During sinusoidal voltage migration the threshold is unaffected by microG, but the peak in the dynamic I-V relation appears to be shifted (Fig.…”

“…Functional aspects have been little investigated. Changes of the biophysical properties of K ϩ and Ca 2ϩ currents in the vestibular hair cell have been reported in the frog, after conditioning the surviving animal in an artificial microG environment (10). Still, no morphofunctional correlations have been attempted so far, and the effects on the peripheral vestibular organs remain poorly understood.…”

“…8, which presumably reflect the physiological excitatory-inhibitory bending of hair cell cilia (9). The IA kinetic changes result in a prolonged duration of this current and in an overall increase of the hair cell repolarizing power (10). As concerns the Ca 2ϩ currents, they have been reported to be generally reduced by the microG treatment (10), whereas the shape of the related I-V curves, and therefore the voltage dependent gating kinetics, apparently were not modified (10).…”

“…The IA kinetic changes result in a prolonged duration of this current and in an overall increase of the hair cell repolarizing power (10). As concerns the Ca 2ϩ currents, they have been reported to be generally reduced by the microG treatment (10), whereas the shape of the related I-V curves, and therefore the voltage dependent gating kinetics, apparently were not modified (10). Here, we present a comparative study of the time course of Ca 2ϩ currents in normal and microG-treated cells during membrane potential oscillations imposed to mimic the effects of rotational stimulation on the receptor: in addition to the overall decrease of Ca 2ϩ currents, the dynamics of calcium fluxes appear to be altered during such sinusoidal activation pattern.…”

“…11), behavioral (2), and developmental (3) modifications have been reported to occur in several cellular systems following exposure to conditions of altered gravity. Functional data are scarce: an increased response sensitivity of toadfish utricular afferents has been described shortly after a space flight (1) and, more recently, alterations have been reported in the biophysical properties of hair cell K ϩ and Ca 2ϩ currents at the frog labyrinth, after conditioning the surviving animal in a microG environment (10).…”