Submitted on December 5, 2008
Revised on February 9, 2009
Accepted on February 22, 2009
Ionic currents in hair cells dissociated from frog semicircular canals after preconditioning under micro-gravity conditions
Marta Martini1, Rita Canella1, Alessandro Leparulo1, Ivo Prigioni2, Riccardo G Fesce3, and Maria Lisa Rossi1*
1 University of Ferrara
2 University of Pavia
3 Insubria University
* To whom correspondence should be addressed. E-mail: rsm{at}unife.it.
The effects of micro-gravity 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, anaesthetized 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 IA kinetic mechanism. The amplitude of ICa was significantly reduced in micro-gravity conditioned cells. The delayed current, IKD (a complex of IKV and IKCa), was drastically reduced, mostly in its IKCa component. Micro-gravity also affected IKD kinetics by shifting the steady-state inactivation curve towards negative potentials and increasing the sensitivity of inactivation removal to voltage. As concerns the fast, transient potassium current, IA, the I-V and steady-state inactivation curves were indistinguishable under normo- or micro-gravity conditions; conversely, IA decay systematically displayed a two-exponential time course and longer time constants in micro-gravity, thus potentially providing a larger K+ charge; furthermore, IA inactivation removal at -70 mV was slowed down. Stimulation in the RPM machine under normo-gravity conditions resulted in minor effects on IKD and, occasionally, incomplete IA 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 micro-gravity conditioning.
