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°, 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°, 200 ms, 0.5Hz) evoked increases of [Ca²⁺]_ifrom 50±23 nM to 139±79 (n=12). In the opposite direction, the mechanical stimulations (8°, 400ms, 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 plate during positive displacements of the hair bundle were reversibly abolished in the presence of 100 M 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.