INTRACELLULAR CALCIUM VARIATIONS EVOKED BY MECHANICAL STIMULATION OF MAMMALIAN ISOLATED VESTIBULAR TYPE-I HAIR-CELLS

The variations of intracellular free calcium concentration ([Ca2+](i)) were recorded on-line from guinea-pig isolated vestibular sensory cel ls using a fura-2 fast fluorescent photometry system, during mechanica l displacements of the hair bundle. Repetitive displacements of the ha ir bundle towards the kinocilium (posi tive stimulation 7 degrees, 300 ms, 2Hz for 10 s), revealed [Ca2+](i) variations detectable only in th e cuticular plate. [Ca2+](i) increased from 105 to 145 nM. Single mech anical displacements of the hair bundle (7 degrees, 200 ms, 0.5Hz) evo ked increases of [Ca2+](i) from 50 +/- 23 nM to 139 +/- 79 (n = 12). I n the opposite direction, the mechanical stimulations (8 degrees, 400 ms, 0.5Hz) evoked a decrease of [Ca2+](i) from 68 +/- 17 nM to 37 +/- 12 nM (n = 8). The variations of [Ca2+](i) detected in the cuticular p late during positive displacements of the hair bundle were reversibly abolished in the presence of 100 mu M gentamicin and they could not be evoked in 0.1 mM calcium in the external medium. From these experimen ts, it has been concluded that the [Ca2+](i) variations recorded in th e cuticular plate were due to a limited entry of calcium ions through transduction channels localized in the hair bundle. The typical kineti cs of variations of [Ca2+](i) evoked during positive displacements of the hair bundle should account for the presence of strong calcium regu lation systems in the hair bundle and cuticular plate.