Chelating properties of the Ca2+ transport site of the retinal rod Na-Ca+K exchanger: evidence for a common Ca2+ and Na+ binding site.

pH-dependent Ca2+ and Na+ transport via Na-Ca+K exchange was measured in isolated and purified bovine retinal rod outer segments (ROS). Ca2+ transport was measured (1) as 45Ca uptake via reverse Na-Ca+K exchange or via Ca-Ca exchange and (2) as a Ca(2+)-induced rise in cytosolic free Ca2+ concentration via reverse Na-Ca+K exchange measured with the fluorescent Ca(2+)-indicating dye fluo-3. Na+ transport was inferred from a Na(+)-induced lowering of cytosolic free [Ca2+] measured with fluo-3. Both Na+ and Ca2+ influx into ROS mediated by the Na-Ca+K exchanger showed a steep (40-100-fold increase in influx rate when the pH was increased from 6.0 to 8.8) pH dependency when measured at low occupancy of the external transport site with Na+ or Ca2+, respectively. High occupancy of the transport site with either Na+ or Mg2+ caused a dramatic decrease in the pH dependence of either Ca2+ or Na+ influx. These observations corroborate the proposed presence of a common Na+ and Ca2+ transport site of the ROS Na-Ca+K exchanger.