C. Chabbert et al., CALCIUM HOMEOSTASIS IN GUINEA-PIG TYPE-I VESTIBULAR HAIR CELL - POSSIBLE INVOLVEMENT OF AN NA-CA2+ EXCHANGER(), Hearing research, 89(1-2), 1995, pp. 101-108
In type-I vestibular hair cells (VHCs), the mechanisms involved in int
racellular 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 t
he effect of the removal of external sodium on the cytosolic concentra
tion of calcium ions ([Ca2+](i)), sodium ions ([Na+](i)), and protons
(pH(i)). These concentrations were measured in type-I VHCs isolated fr
om guinea pig labyrinth, using Fura-2, sodium benzofuran isophtalate (
SBFI), and 1,4 diacetoxy-2,3 dicyanobenzol (ADB) respectively. Complet
e replacement of Na+ in the superfusion solution with N-methyl-D-gluca
mine (NMDG(+)), reversibly increased [Ca2+](i) by 276 +/- 89% (n = 46)
and decreased [Na+](i), by 23 +/- 6% (n = 14). Both responses were pr
evented by removing external Ca2+ or chelating internal Ca2+. This sug
gests the presence of coupled Ca2+ and Na+ transport. The [Ca2+](i) in
crease evoked by Na+-free solution was reduced by about 55% with the a
pplication of amiloride derivatives and was totally abolished in the p
resence of high [Mg2+](o). No pH(i) variation was detected during [Na](o) reduction. In the absence of external K+, the Na+-free solution f
ailed to induce [Ca2+](i) increase; the readmission of external K+ res
tored the [Ca2+](i) response. These results are consistent with a Na+-
Ca2+ exchanger operating in reverse mode. An K+ dependence of this exc
hange is also suggested.