Tidal volume in CO2 regulation: peripheral denervations and ablation of area postrema

Abstract

Experiments were performed on unanesthetized cats that had been decerebrated midcollicularly under halothane anesthesia. Respiratory excursions were recorded by means of a body plethysmograph that allowed head and forelimbs to be exteriorized. End-expiratory percent CO2 was monitored continuously with an infrared analyzer. CO2 reactivity was studied over the range of alveolar concentration from 2 to 10%. Mixtures of CO2 in air or in oxygen were given by inhalation to attain steady-state concentrations above the resting level, and positive-pressure hyperventilation was used to achieve hypocapnic levels. A straight-line function was found over the entire physiological range from the apneic threshold to the peak respiratory response when tidal volume VT was plotted against log[CO2]. This strict relationship was maintained independently of alterations in respiratory frequency. Denervation of the carotid bifurcation resulted in a parallel shift of the VT-log[CO2] line to the right, whereas vagotomy caused a shift to the left regardless of the operative sequence employed. Chronic ablation of the area postrema in the medulla oblongata, confirmed by emetic refractoriness to deslanoside, produced no detectable effect on CO2 reactivity as judged by ventilatory measurements made under the aforementioned conditions before and after peripheral denervation .