Ventilatory long-term facilitation in unanesthetized rats

We tested the hypothesis that unanesthetized rats exhibit ventilatory longterm facilitation (LTF) after intermittent, but not continuous, hypoxia. Minute ventilation (VE) and carbon dioxide production (V(over dot)CO2) were measured in unanesthetized, unrestrained male Sprague-Dawley rats via barometric plethysmography before, during, and after exposure to continuous or intermittent hypoxia. Hypoxia was either isocapnic [inspired O-2 fraction (FIO2) = 0.08-0.09 and inspired CO2 fraction (FICO2) = 0.04] or poikilocapnic (FIO2 = 0.11 and FICO2. = 0.00). Sixty minutes after intermittent hypoxia, V(over dot)E or V(over dot)E/VCO2 was significantly greater than baseline in both isocapnic and poikilocapnic conditions. In contrast, 60 min after continuous hypoxia, V(over dot)E and V(over dot)E/VCO2 were not significantly different from baseline values. These data demonstrate ventilatory LTF after intermittent hypoxia in unanesthetized rats. Ventilatory LTF appeared similar in its magnitude (after accounting for CO2 feedback), time course, and dependence on intermittent hypoxia to phrenic LTF previously observed in anesthetized, vagotomized, paralyzed rats.