Evidence for an Na1-K1-Cl2 cotransporter in mammalian type I vestibular hair cells

Rennie, K. J., J. F. Ashmore, and M. J. Correia. Evidence for an Na1-K1-Cl2 cotransporter in mammalian type I vestibular hair cells. Am. J. Physiol. 273 (Cell Physiol. 42): C1972–C1980, 1997.—In amniotes, there are two types of hair cells, designated I and II, that differ in their morphology, innervation pattern, and ionic membrane properties. Type I cells are unique among hair cells in that their basolateral surfaces are almost completely enclosed by an afferent calyceal nerve terminal. Recently, several lines of evidence have ascribed a motile function to type I hair cells. To investigate this, elevated external K1, which had been used previously to induce hair cell shortening, was used to induce shape changes in dissociated mammalian type I vestibular hair cells. Morphologically identified type I cells shortened and widened when the external K1 concentration was raised isotonically from 2 to 125 mM. The shortening did not require external Ca21 but was abolished when external Cl2 was replaced with gluconate or sulfate and when external Na1 was replaced with N-methyl-D-glucamine. Bumetanide (10–100 μM), a specific blocker of the Na1-K1-Cl2 cotransporter, significantly reduced K1-induced shortening. Hyposmotic solution resulted in type I cell shape changes similar to those seen with high K1, i.e., shortening and widening. Type I cells became more spherical in hyposmotic solution, presumably as a result of a volume increase due to water influx. In hypertonic solution, cells became narrower and increased in length. These results suggest that shape changes in type I hair cells induced by high K1 are due, at least in part, to ion and solute entry via an Na1-K1-Cl2 cotransporter, which results in cell swelling. A scheme is proposed whereby the type I hair cell depolarizes and K1 leaves the cell via voltage-dependent K1 channels and accumulates in the synaptic space between the type I hair cell and calyx. Excess K1 could then be removed from the intercellular space by uptake via the cotransporter.