Release of ATP at the ventral surface of the medulla oblongata during hypoxia in rats that were mechanically hyperventilated to induce central apnea (arterial P_CO2 and end-tidal CO₂ below apneic threshold). a, Representative raw data illustrating changes in the arterial blood pressure, phrenic nerve discharge, and null and ATP sensor currents during systemic hypoxia (10% O₂ in the inspired air). b, An expanded portion of the raw data obtained in a different experiment, illustrating that, during hypoxia, an increase in ATP concentration on the ventral surface of the medulla occurs after the activation of the respiratory network and peaks during the secondary hypoxia-induced slowing of the respiratory rhythm. PNG, Phrenic neurogram; IPNA, integrated phrenic nerve activity (arbitrary units). Arrows indicate the moment at which the concentration of ATP starts to increase above the baseline.