Low level CO₂ supplementation maintains isocapnia and reveals ventilatory long-term facilitation in rats

Acute, intermittent hypoxia (AIH) induces ventilatory long-term facilitation (vLTF) in awake, freely behaving rats under poikilocapnic and isocapnic experimental conditions. Establishing pre-clinical methods for vLTF induction that more closely align with successful protocols in humans and anesthetized rats would minimize dissonance in experimental findings and improve translational aspects of vLTF. Here, we tested several levels of low-dose CO₂ supplementation during and after AIH to determine 1) the lowest amount of inspired CO₂ that would maintain isocapnia in rats during a vLTF protocol, and 2) the net impact of supplemental CO₂ on vLTF expression. Rats received one of four levels of inspired CO₂ (0%, 0.5%, 1% or 2%) administered during AIH and for the 60 min following AIH to quantify vLTF. Our findings indicated that 2% inspired CO₂ was sufficient to maintain isocapnia across the AIH protocol and reveal significant vLTF. These findings provide evidence-based support for using 2% supplemental CO₂ during and after AIH when assessing vLTF in rats.