The relationship between bath calcium and P2X7R agonists. (A) Concentration-dependent effects of ATP and BzATP on the peak amplitude of current in cells bathed in Ca²⁺-deficient (open circles) and 2-mM Ca²⁺–containing (closed circles) medium. The amplitude of current was measured after 40 s of agonist application. Data shown are mean ± SEM values from at least four responses per dose during initial agonist applications. Representative traces are shown in Fig. 1. Arrows indicate leftward shifts in EC₅₀ values, reflecting an increase in sensitivity to agonist stimulation in Ca²⁺-deficient medium. (B) Concentration-dependent effects of ATP and BzATP on free Ca²⁺ concentration. The background buffer contained 147 mM NaCl, 3 mM KCl, 10 mM glucose, and 10 mM HEPES supplemented with 0.95 mM Ca²⁺. Free Ca²⁺ concentration was measured with a calcium ion selective electrode as described in Materials and methods. Data shown are means from three experiments, and SEM values are within the symbols. The calculated free calcium concentrations were obtained using the Maxc program.