EFFECT OF MULTIPLE DENERVATIONS ON THE EXERCISE HYPERPNEA IN AWAKE PONIES

In three previously reported studies, we had documented that the norma l exercise hyperventilation in ponies is accentuated by carotid body d enervation (CBD), not affected by hilar nerve pulmonary vagal denervat ion (HND), and mildly attenuated by spinal cord ablation of the dorsal lateral columns at L(2) (SA). In the present study, we hypothesized t hat if redundancy of control existed in exercising ponies, then multip le denervations of theoretically important pathways in the same animal might attenuate the ventilatory response to exercise in a way not pre dictable by the individual lesion experiments alone. There were three major findings in the various combinations of CBD, HND, and SA in poni es during treadmill exercise. First, the combination of CBD with HND o r SA resulted generally in an accentuation of the hypocapnia during ex ercise that was predictable on the basis of CBD alone. However, in one pony that showed a hypercapnic exercise response after SA alone, CBD subsequently caused a greater exercise hypercapnia. Second, HND in a C BD or SA pony did not affect the exercise arterial PCO2 response, whic h is consistent with previous data showing the lack of an HND effect i n otherwise intact ponies. Third, in ponies with all three denervation s together, the predominant response was an increase, not a decrease, in the exercise hyperventilation; this increase was greater than that predicted from the individual lesions. We conclude that these data do not provide evidence of redundancy in mechanism for the exercise hyper pnea other than instances of carotid chemoreceptor error sensing when hypercapnia occurs during exercise.