Role of hilar nerve afferents in hyperpnea of exercise.

The objective of this study was to determine the role of hilar nerve (lung vagal) afferents in the hyperpnea of exercise. Ten ponies were studied before and 2-4 wk and 3-12 mo after sectioning only the hilar branches of the vagus nerves (HND). After HND, lung volume feedback to the medullary centers was attenuated as indicated in the anesthetized state by 1) attenuation or absence of the Hering-Breuer inflation reflex (P less than 0.01) and 2) attenuation of the lengthened inspiratory time (TI) when the airway was occluded at end expiration (P less than 0.01). Moreover, after HND in the awake state, there was an increase in the ratio of TI to total cycle time (P less than 0.01). These changes verify a compromise in lung innervation comparable to cervical vagotomy. Resting arterial PCO2, PO2, and pH were not altered following HND (P greater than 0.10). Moreover, at three levels of mild and moderate treadmill exercise, no difference in either the temporal pattern or the absolute levels of arterial blood gases and arterial pH was found between pre- and post-HND studies (P greater than 0.10). In addition, minute ventilation (VE) at rest and during exercise was not altered by HND (P greater than 0.10). However, 2-4 wk after HND the increase in breathing frequency (f) during exercise was less, whereas the increase in tidal volume during exercise was greater than pre-HND (P less than 0.05). The reduced f was due to an increase in TI with no change in expiratory time. We conclude that lung afferents via the hilar nerves influence the pattern of breathing at rest and during exercise in ponies.(ABSTRACT TRUNCATED AT 250 WORDS)