Supplisson S, Kado R T, Bergman C
Laboratoire de Neurobiologie, Ecole Normale Supérieure, Paris, France.
Dev Biol. 1991 Jun;145(2):231-40. doi: 10.1016/0012-1606(91)90122-j.
In manually dissected Xenopus oocytes, we found that the replacement of external sodium by Tris, choline, or lithium induced a large membrane depolarization and, in voltage clamp, a large inward current. This current appears to be due to activation of a calcium-dependent chloride conductance since it is reversed near ECl, increased by the removal of external chloride, and can be abolished by an injection of BAPTA or by the removal of external Ca2+. Using the Ca-dependent Cl current as a monitor of Ca concentration at the inner surface of the oocyte membrane, we are led to propose that the removal of external Na+ induces an increase in internal Ca2+ via the activation of a Na/Ca exchanger operating in the reverse mode. This interpretation is supported by the finding that the chloride current is diminished in either 3',4'-dichlorobenzamyl (DCB) or high external [Mg2+]o, both of which are known to block the Na/Ca exchanger, whereas it is increased when Li+, rather than Tris or choline, is used as the substitute for Na. The effect of zero [Na+]o was not obtained in oocytes from which follicular cells were removed by enzymatic treatment. This observation led us to test the possibility that the Na/Ca exchanger was present in the follicle cells and not in the oocyte membrane, assuming that entering Ca2+ could pass into the oocyte through gap junctions. Octanol, which blocks gap junctions, or a high [Ca2+]o both considerably reduced the inward current. While octanol probably blocked the gap junctions directly, we propose that the block by high [Ca2+] was due to an excessive rise of [Ca2+]i in the follicular cells. These results, taken together, indirectly suggest the presence of a Na/Ca exchanger in the follicular cells. These results, taken together, indirectly suggest the presence of a Na/Ca exchanger in the follicle cells of Xenopus oocyte which could contribute to the regulation of the internal Ca concentration of the oocyte before fertilization.
在手动解剖的非洲爪蟾卵母细胞中，我们发现用 Tris、胆碱或锂替代外部钠会引起大幅度的膜去极化，并且在电压钳实验中会产生大幅度的内向电流。这种电流似乎是由于钙依赖性氯电导的激活所致，因为它在 ECl 附近反转，通过去除外部氯而增加，并且可以通过注射 BAPTA 或去除外部 Ca2+ 来消除。利用钙依赖性氯电流作为卵母细胞膜内表面钙浓度的监测指标，我们进而提出，去除外部 Na+ 会通过激活以反向模式运行的 Na/Ca 交换器来诱导内部 Ca2+ 增加。这一解释得到以下发现的支持：在 3',4'-二氯苯甲酰（DCB）或高外部 [Mg2+]o 中，氯电流都会减小，这两者都已知会阻断 Na/Ca 交换器，而当用 Li+ 而非 Tris 或胆碱作为 Na 的替代物时，氯电流会增加。在用酶处理去除卵泡细胞的卵母细胞中未观察到零 [Na+]o 的效应。这一观察结果促使我们测试 Na/Ca 交换器存在于卵泡细胞而非卵母细胞膜中的可能性，假定进入的 Ca2+ 可以通过缝隙连接进入卵母细胞。阻断缝隙连接的辛醇或高 [Ca2+]o 都会显著降低内向电流。虽然辛醇可能直接阻断了缝隙连接，但我们认为高 [Ca2+] 的阻断作用是由于卵泡细胞中 [Ca2+]i 的过度升高所致。综合这些结果，间接表明非洲爪蟾卵母细胞的卵泡细胞中存在 Na/Ca交换器。综合这些结果，间接表明非洲爪蟾卵母细胞的卵泡细胞中存在 Na/Ca 交换器，这可能有助于在受精前调节卵母细胞的内部钙浓度。
