Calcium homeostasis in guinea pig type-I vestibular hair cell: possible involvement of an Na +Ca 2+ exchanger

Abstract

In type-I vestibular hair cells (VHCs), the mechanisms involved in intracellular calcium homeostasis have not yet been established. In order to investigate the involvement of an Na +-dependent ionic exchanger in the regulation of cytosolic free calcium concentration, we analyzed the effect of the removal of external sodium on the cytosolic concentration of calcium ions ([Ca 2+] i), sodium ions ([Na +] i), and protons (pH i). These concentrations were measured in type-I VHCs isolated from guinea pig labyrinth, using Fura-2, sodium benzofuran isophtalate (SBFI), and 1,4 diacetoxy-2,3 dicyanobenzol (ADB) respectively. Complete replacement of Na + in the superfusion solution with N-methyl- d-glucamine (NMDG +), reversibly increased [Ca 2+] i by 276 ± 89% ( n = 46) and decreased [Na +] i by 23 ± 6% ( n = 14). Both responses were prevented by removing external Ca 2+ or chelating internal Ca 2+. This suggests the presence of coupled Ca 2+ and Na + transport. The [Ca 2+] i increase evoked by Na +-free solution was reduced by about 55% with the application of amiloride derivatives and was totally abolished in the presence of high [Mg 2+] o. No pH i variation was detected during [Na +] o reduction. In the absence of external K +, the Na +-free solution failed to induce [Ca 2+] i increase; the readmission of external K + restored the [Ca 2+] i response. These results are consistent with a Na +Ca 2+ exchanger operating in reverse mode. An K + dependence of this exchange is also suggested.