Effects of the P2 receptor antagonist PPADS on the responses of chemoreceptor afferents to hypoxia. A, PPADS (30 μm) reduces afferent discharge in response to hypoxia reversibly. B, Accumulative dose of PPADS (100 μm) virtually abolishes the afferent response to hypoxia. C, PPADS reduces the peak discharge frequency of single afferent units (n = 12) during hypoxia. ^**p < 0.01 (paired Student's t test).