Ventilatory long-term facilitation in unanesthetized rats

We tested the hypothesis that unanesthetized rats exhibit ventilatory long-term facilitation (LTF) after intermittent, but not continuous, hypoxia. Minute ventilation (VE) and carbon dioxide production (VCO(2)) 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 (FI(O(2))) = 0.08--0.09 and inspired CO(2) fraction (FI(CO(2))) = 0.04] or poikilocapnic (FI(O(2)) = 0.11 and FI(CO(2)) = 0.00). Sixty minutes after intermittent hypoxia, VE or VE/VCO(2) was significantly greater than baseline in both isocapnic and poikilocapnic conditions. In contrast, 60 min after continuous hypoxia, VE and VE/VCO(2) 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 CO(2) feedback), time course, and dependence on intermittent hypoxia to phrenic LTF previously observed in anesthetized, vagotomized, paralyzed rats.