Phrenic activity during severe hypercapnia in vagotomized rabbits

Abstract

The central control mechanism of respiratory frequency under varied alveolar CO2 pressure (PACO2 20-200 Torr) was investigated in anesthetized, vagotomized, immobilized and artificially ventilated rabbits. Central inspiratory activity indicated by phrenic motor discharge was tolerant of the extensive hypercapnia. Under light anesthesia the respiratory frequency (f) decreased in a hyperbolic fashion with increasing PACO2. Under deeper anesthesia or after mesencephalic decerebration the hyperbolic f response to PACO2 was abolished or changed to a hill type f response (initial increase and subsequent decrease in f) and on the average, the changes in frequency were much less. In the absence of vagal control the respiratory frequency is primarily determined by the periodicity of the bulbopontine inspiratory activity, which is little dependent on PACO2, and a suprapontine acceleratory mechanism, which is depressed by increased PACO2 and highly sensitive to anesthetics. The mechanism of changes in the type of f response to CO2 is discussed.