Calcium dependence of P2X7R deactivation. (A) Repetitive 1-s application of BzATP in cells bathed in 2 mM Ca²⁺ + 1 mM Mg²⁺ (six applications, left) and Ca²⁺-deficient medium + 1-mM Mg²⁺ medium (four applications, right). (B) Superimposed records showing a slowing of receptor deactivation caused by repetitive agonist application (black) and by removal of Ca²⁺ (gray). (C) Comparison of the rates of current deactivation during prolonged (40 s) agonist application in the presence and absence of bath Ca²⁺. 1 and 2 are the first and second agonist applications. (D) Dependence of the rates of receptor deactivation on Ca²⁺ concentrations. Representative traces (gray) and fitted biexponential curves (black). (inset) Bars indicate mean ± SEM values of τ_off2, and asterisks indicate significant difference (P < 0.01) versus 0 mM Ca²⁺. n = 5–10 per data point. (E and F) Example records of change in the rates of current deactivation by replacing 2-mM Ca²⁺–containing medium with Ca²⁺-deficient medium (indicated by horizontal bars) in cells expressing the wild-type (E) and H130A mutant (F) P2X7Rs.