Ventilatory CO2 drive in the tortoise Testudo horsfieldi

1. Ventilation was recorded by pneumotachography, before and after bilateral vagotomy, in conscious tortoises (Testudo horsfieldi) (breathing 0, 2, 3 or 4% CO2 in air or oxygen). 2. Each breath consists of expiratory and inspiratory phases and an apneic plateau (absence of air flow). Inhalation of hypercapnic mixtures led to increased ventilatory flow, augmentation of tidal volume, and an increase in respiratory frequency through the shortening of the apneic plateau. 3. Intact tortoises breathing hypercapnic-hyperoxic mixtures hyperventilated less than with hypercapnic-normoxic mixtures. 4. In bivagotomized animals, the respiratory frequency decreased, the expiratory and inspiratory durations lengthened, and the apneic plateau was prolonged. The tidal volume was increased, but ventilation, nevertheless, decreased slightly. 5. Bivagotomized animals breathing hypercapnic-normoxic or hypercapnic-hyperoxic mixtures hyperventilated, but less than intact animals under the same conditions. 6. It is concluded that in tortoises there are: (1) peripheral chemoreceptors which are innervated by branches of the vagus nerves, and are sensitive to CO2; and (2) an extrathoracic, probably central, ventilatory CO2 drive.