Figure 3.

Na⁺ and Ca²⁺ compete for binding. (A) Antiport cycle of a prototypical NCX, highlighting the separate Na⁺/Na⁺ and Ca²⁺/Ca²⁺ exchange reactions postulated by the ping-pong model. (B) Ca²⁺/⁴⁵Ca²⁺ exchange in Na⁺-free (orange) or saturating Na⁺ (blue) conditions. The liposomes were preloaded with buffer containing 50 µM ⁴⁵CaCl₂ and either 100 mM NaCl (blue) or 100 mM choline chloride (orange). In the external buffer, ⁴⁵Ca²⁺ was substituted for the same concentration of CaCl₂. Note that buffers for this experiment were valinomycin-free. A triplicate dataset is averaged (error bars represent SEM). This experiment was reproduced on three separate occasions. (C) Antiport cycle of a prototypical NCX, highlighting the process of external competitive binding to the transporter. (D) Initial rates of Na⁺/Ca²⁺ exchange as a function of [Na⁺], as external and internal concentrations were varied simultaneously. The liposomes were preloaded with 100 µM EGTA; either 30 µM (cyan) or 15 µM (blue) ⁴⁵CaCl₂ was applied to the reaction buffer. A triplicate dataset is averaged (error bars represent SEM).