Lagnado L, Cervetto L, McNaughton P A
Physiological Laboratory, Cambridge, United Kingdom.
Proc Natl Acad Sci U S A. 1988 Jun;85(12):4548-52. doi: 10.1073/pnas.85.12.4548.
The inward membrane current generated by the coupled exchange of external sodium for internal calcium has been investigated in isolated rod outer segments. The exchange rate is sensitive to voltage, with a reduction by a factor of e occurring for a 70-mV depolarization in normal Ringer's solution. The voltage sensitivity is not a constant property of the exchange, as it is reduced by an increase in external Na+ or by the removal of external Ca2+, Mg2+, or K+. Changes in membrane potential do not appear to affect the affinity of the exchange mechanism for internal Ca2+, but hyperpolarization increases the affinity for external Na+. When the external Na+ concentration is raised sufficiently to saturate the exchange mechanism, the voltage sensitivity is no longer apparent. We propose that the voltage dependence of the exchange is due to the external Na+-binding site being sensitive to membrane potential, perhaps because it is located within the membrane electric field.
在分离的视杆细胞外段中，对由外部钠离子与内部钙离子的耦合交换所产生的内向膜电流进行了研究。交换速率对电压敏感，在正常林格氏溶液中，70毫伏的去极化会使其降低至原来的1/e。电压敏感性并非交换的恒定特性，因为外部钠离子浓度增加或去除外部钙离子、镁离子或钾离子都会使其降低。膜电位的变化似乎并不影响交换机制对内部钙离子的亲和力，但超极化会增加对外部钠离子的亲和力。当外部钠离子浓度升高到足以使交换机制饱和时，电压敏感性就不再明显。我们认为，交换的电压依赖性是由于外部钠离子结合位点对膜电位敏感，这可能是因为它位于膜电场内。
