Evidence for an Na(+)-K(+)-Cl- cotransporter in mammalian type I vestibular hair cells

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 K+, 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 K+ concentration was raised isotonically from 2 to 125 mM. The shortening did not require external Ca2+ but was abolished when external Cl- was replaced with gluconate or sulfate and when external Na+ was replaced with N-methyl-D-glucamine. Bumetanide (10-100 microM), a specific blocker of the Na(+)-K(+)-Cl- cotransporter, significantly reduced K(+)-induced shortening. Hyposmotic solution resulted in type I cell shape changes similar to those seen with high K+, 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 K+ are due, at least in part, to ion and solute entry via an Na(+)-K(+)-Cl- cotransporter, which results in cell swelling. A scheme is proposed whereby the type I hair cell depolarizes and K+ leaves the cell via voltage-dependent K+ channels and accumulates in the synaptic space between the type I hair cell and calyx. Excess K+ could then be removed from the intercellular space by uptake via the cotransporter.