Ventilatory responses to acute and chronic hypoxic hypercapnia in the ground squirrel.

Golden-mantled ground squirrels exhibited a strong hypoxic ventilatory response but a blunted hypercapnic ventilatory response and showed no interactive effects when both stimuli were presented together. They exhibited a resting hypoxic ventilatory drive which was eliminated by carotid body denervation. Carotid denervation also shifted the threshold of the hypoxic ventilatory response but had no effect on the slope of either the hypoxic or hypercapnic ventilatory responses. Chronic exposure (2-12 months) to hypoxic-hypercapnic conditions (16% O2, 4% CO2) resulted in a sustained increase in ventilation. Initial increases in both tidal volume (VT) and breathing frequency (fR) were followed by a subsequent further increase in VT and concomitant decrease in fR (acclimation) which had little overall effect on ventilation (VE) but further increased calculated alveolar ventilation (VA). Respiratory sensitivity to hypoxia and hypercapnia were unaltered under these conditions. On acute return to breathing room air, VE remained elevated (approximately 35%) compared to control animals suggesting that deacclimation takes time. Carotid body denervation in these animals had similar effects to those seen in control animals suggesting that acclimation did not involve changes in carotid body input.