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ENVIRONMENTAL PHYSIOLOGY

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

¹Department of Biology and Evolution, Section of Physiology and Biophysics, and National Institute of Neuroscience, Ferrara University, Ferrara, Italy; ²Department of Physiology, Pavia University, Pavia, Italy; and ³Centre of Neuroscience and Department of Structural and Functional Biology, Insubria University, Varese, Italy

Submitted 5 December 2008 ; accepted in final form 22 February 2009

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.

labyrinth; hair cells; microgravity; ionic currents