Influences of chronic denervation of the carotid bifurcation regions on panting in the sheep.

The respiratory response of conscious sheep subjected to severe heat stress has been assessed before and up to 17 weeks after bilateral denervation of the carotid bifurcations. The characteristic response of the intact animal comprises an initial phase of rapid, shallow panting, superseded by panting of a slower, deeper form which results in a severe respiratory alkalosis; this was not significantly altered by the denervation. However, during recovery from heat stress, denervated animals developed a marked hypoxaemia and tachycardia. Blood pressure was much more labile in denervated animals. After denervation, resting arterial PCO2 was higher, PO2 was lower, and blood pressure and heart rate were higher; with the lapse of weeks, these parameters returned towards pre-denervation levels although the carotid bodies remained inactive. It is concluded that the carotid bodies do not play a significant role in control of the biphasic pattern of panting during severe heat stress, but that they normally prevent post-hyperventilation hypocapnic hypoxaemia during recovery. Further, the respiratory pattern during heat stress is the results of an overwhelming thermoregulatory drive, whereas the pattern during recovery is the result of a balance between arterial and medullary chemoreceptor activity, Finally, the carotid bodies normally play a significant role in determining the 'set-point' for CO2 regulation, but in their absence respiratory control mechanism adapt over a considerable time period.