CALCIUM HOMEOSTASIS IN GUINEA-PIG TYPE-I VESTIBULAR HAIR CELL - POSSIBLE INVOLVEMENT OF AN NA+-CA2+ EXCHANGER

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 ([Ca2+](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 [Ca2+](i) by 276 +/- 89% (n = 46) and decreased [Na+](i), by 23 +/- 6% (n = 14). Both responses were prevented by removing external Ca2+ or chelating internal Ca2+. This suggests the presence of coupled Ca2+ and Na+ transport. The [Ca2+](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 [Mg2+](o). No pH(i) variation was detected during [Na+](o) reduction. In the absence of external K+, the Na+-free solution failed to induce [Ca2+](i) increase; the readmission of external K+ restored the [Ca2+](i) response. These results are consistent with a Na+-Ca2+ exchanger operating in reverse mode. An K+ dependence of this exchange is also suggested.