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 cells using a fura-2 fast fluorescent photometry system, during mechanical displacements of the hair bundle. Repetitive displacements of the hair bundle towards the kinocilium (positive stimulation 7 degrees, 300 ms, 2Hz for 10 s), revealed [Ca2+]i variations detectable only in the cuticular plate. [Ca2+]i increased from 105 to 145 nM. Single mechanical displacements of the hair bundle (7 degrees, 200 ms, 0.5 Hz) evoked increases of [Ca2+]i from 50 +/- 23 nM to 139 +/- 79 (n = 12). In the opposite direction, the mechanical stimulations (8 degrees, 400 ms, 0.5 Hz) 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 plate during positive displacements of the hair bundle were reversibly abolished in the presence of 100 microM gentamicin and they could not be evoked in 0.1 mM calcium in the external medium. From these experiments, it has been concluded that the [Ca2+]i variations recorded in the cuticular plate were due to a limited entry of calcium ions through transduction channels localized in the hair bundle. The typical kinetics of variations of [Ca2+]i evoked during positive displacements of the hair bundle should account for the presence of strong calcium regulation systems in the hair bundle and cuticular plate.