FIG. 6.

Calcium antagonism on the P2X7R is allosteric. (A) Concentration–response curves to ATP and BzATP in P2X7R-expressing HEK293 cells. The experiments were performed in the absence (open circles) or in the presence (closed circles) of 2 mM extracellular Ca²⁺. (B) Concentration-dependent effects of ATP and BzATP on free Ca²⁺ concentration measured with a calcium ion selective electrode. The calculated free Ca²⁺ concentrations were obtained using the web Maxc program. (C) A comparable free BzATP concentration of 1.6 μM was achieved with variable total BzATP and variable Ca²⁺ concentrations. (D) Current amplitudes of the different media shown in (C), containing identical free BzATP concentrations (1.6 μM) but increasing Ca²⁺ concentrations, demonstrating that Ca²⁺ itself and not its effects on free BzATP are responsible for current inhibition.