CHELATING PROPERTIES OF THE CA2-CA+K EXCHANGER - EVIDENCE FOR A COMMON CA2+ AND NA+ BINDING-SITE( TRANSPORT SITE OF THE RETINAL ROD NA)

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 Ca-45 uptake via reverse Na-Ca+K exchan ge or via Ca-Ca exchange and (2) as a Ca2+-induced rise in cytosolic f ree Ca2+ concentration via reverse Na-Ca+K exchange measured with the fluorescent Ca2+-indicating dye fluo-3. Na+ transport was inferred fro m 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 in creased from 6.0 to 8.8) pH dependency when measured at low occupancy of the external transport site with Na+ or Ca2+, respectively. High oc cupancy of the transport site with either Na+ or Mg2+ caused a dramati c decrease in the pH dependence of either Ca2+ or Na+ influx. These ob servations corroborate the proposed presence of a common Na+ and Ca2transport site of the ROS Na-Ca+K exchanger.