The P2X₂ receptor subunit is essential for the normal ventilatory response to hypoxia in mice. A, Respiratory responses to varying levels of hypoxia in conscious P2X₂-deficient mice (P2X₂^-/-), in P2X₃-deficient mice (P2X₃^-/-), in mice deficient in both P2X₂ and P2X₃ receptor subunits (P2X₂/P2X₃^Dbl-/-), and in the respective wild-type control mice (P2X₂^+/+, P2X₃^+/+, and P2X₂/P2X₃^Dbl+/+). B, Respiratory responses to varying levels of hypercapnia in P2X₂/P2X₃^Dbl-/- and P2X₂/P2X₃^Dbl+/+ mice. Data represents the mean ± SE. Numbers in parentheses indicate sample sizes. f_R, Respiratory rate; V_T, tidal volume; V_E, minute ventilation (f_R × V_T). ^*p < 0.05 indicates significant difference (ANOVA; post hoc Scheffe's test).