Cardiorespiratory responses to hypoxia in intact and bilaterally vagotomized pigeons

Abstract

Bilateral, cervical vagotomy in birds denervates, among other receptors, the carotid bodies. To test whether such neural section removes sensitivity to hypoxia, we measured respiratory, cardiovascular, and blood gas responses to hypoxia at 84-, 70-, and 49-Torr inspiratory O2 partial pressure (PIO2) in five pigeons with intact vagi and in five bilaterally, cervically vagotomized pigeons. Normoxic respiratory frequency (fresp) and expiratory flow rate (VE) were decreased after vagotomy. Intact pigeons showed large increases in VE in response to hypoxia, effected mostly by increases in fresp. VE also increased greatly in response to hypoxia in vagotomized pigeons, but increases were largely the result of tidal volume. O2 consumption, CO2 production, and respiratory exchange ratio increased slightly in all pigeons during hypoxia. Normoxic heart rate was greater after vagotomy; cardiac output increased in all pigeons in response to hypoxia, but stroke volume increased only in intact pigeons. During normoxia, arterial and mixed venous O2 partial pressure, O2 concentration, and pH were lower and arterial and mixed venous CO2 partial pressure was higher, after vagotomy. In all pigeons during hypoxia, arterial and mixed venous O2 and CO2 partial pressure and O2 concentration decreased and arterial and mixed venous pH increased; changes were roughly parallel in intact and vagotomized pigeons. The arteriovenous O2 concentration differences during normoxia and hypoxia were similar in all pigeons. We conclude that bilateral, cervical vagotomy in the pigeon causes hypoventilation and tachycardia during normoxia, but strong respiratory and cardiovascular responses to hypoxia are still present.