The present study was undertaken to determine whether β-adrenoceptors could be physiologically detected in the lung periphery and whether they were under tonic stimulation in the resting state in anesthetized dogs. A fiberoptic bronchoscope was wedged in a sublobar segment of lung in anesthetized male mongrel dogs for measurement of resistance through the collateral system (Rcs). β-Agents were delivered locally as aerosols through the bronchoscope, and the response was evaluated by changes in Rcs. Distilled water alone produced a mean increase of 8.5 ± 2.43% (SE) in Rcs at 2 min in six dogs, whereas dl-isoproterenol produced a mean decrease of 8.9 ± 2.10% (P < 0.03), thus demonstrating the presence of submaximally stimulated β-receptors. To test whether the β-receptors were under tonic stimulation, we compared the effect of aerosolized d- and dl-propranolol in 5 dogs. d-Propranolol that lacks significant β-blocking activity and dl-propranolol both produced large transient increases in Rcs. However, with d-propranolol, Rcs had returned to base line at 15 min, whereas with dl-propranolol Rcs remained elevated at a mean of 20% above base line for >2 h (P < 0.01). Local timolol aerosol also produced a sustained increase in Rcs. After pretreatment with reserpine or after bilateral adrenalectomy, both d- and dl-propranolol still produced large transient increases in Rcs, but dl-propranolol no longer produced a sustained increase. Neither isoproterenol nor atropine affected Rcs in the presence of dl-propranolol, nor did pretreatment with atropine affect the response of Rcs to dl-propranolol. We conclude that β-adrenoceptors in the lung periphery are under tonic stimulation in the anesthetized dog from adrenally donated circulating catecholamines, and that the increase in Rcs after propranolol cannot be attributed to cholinergic tone.
ASJC Scopus subject areas
- Physiology (medical)