A decrease in nasal CO 2 stimulates breathing in the tegu lizard

Abstract

Tegu lizards decrease ventilatory frequency (f) when constant CO 2, as low as 0.4%, is delivered to the nasal cavities. In contrast, CO 2, as high as 6%, pulsed into the nasal cavities during the expiratory phase of the breathing cycle does not alter f. The purpose of the present study was to investigate further the effect of nasal CO 2 pattern on f in tegu lizards. Specifically, we tested: (1) whether f was affected by CO 2 delivered to the nasal cavities during the inspiratory phase of the breathing cycle, and (2) whether pulsed decreases in nasal CO 2 from 4% to 2% and from 4% to 0% would remove the f inhibition caused by constant nasal CO 2. Ventilation was measured using a pneumotachograph and pressure transducer in-line with an endotracheal T-tube inserted through the glottis. CO 2 was delivered to the nasal cavities through small tubes inserted into the external nares. Ventilatory frequency was not significantly altered when 4% CO 2 was pulsed into the nasal cavities during inspiration. Dropping the CO 2 in the nasal cavities from 4% to 0% at either 15 cycles/min (0.25 Hz) or for one cycle stimulated breathing. There was no significant difference between the f response to a drop in CO 2 from 4% to 0% and that to a drop in CO 2 from 4% to 2%. The failure to link the phasic CO 2 ventilatory response to a phase in the respiratory cycle indicates that the nasal CO 2 receptors do not participate in the breath-by-breath regulation of breathing in these lizards. The observation that small decreases in nasal CO 2 abolished the f inhibition caused by constant nasal CO 2 provides further evidence for the ability of the nasal CO 2 receptors to distinguish between pulsed and constant CO 2.