Figure 2. Extracellular Ca²⁺ controls the sensitivity of ATP-induced [Ca²⁺]_i transients.

A, [Ca²⁺]_i transients recorded from a single cell when stimulated by the same concentration of ATP applied first in standard, 2 mM Ca²⁺-containing extracellular solution, then in Ca²⁺-free solution. Note that ATP concentrations lower than 0.5 μM failed to induce [Ca²⁺]_i responses in Ca²⁺-containing solutions, but when the same concentrations were applied in Ca²⁺-free medium [Ca²⁺]_i transients were evoked. B, normalized amplitudes of ATP-induced [Ca²⁺]_i transients measured in Ca²⁺-containing (•) and Ca²⁺-free (○) conditions are plotted against ATP concentration. The apparent K_Ds are 0.7 and 13 μM and the Hill coefficients are 1.3 and 1.1 for Ca²⁺-free and Ca²⁺-containing solutions, respectively. Data were collected from five different experiments, 117 cells.