Significance of sympathetic innervation of pulmonary vessels in response to acute hypoxia.

The role of the autonomic nervous system on the pulmonary vascular response to hypoxia was studied in anesthetized open-chest dogs. Acute hypoxia elicited reversible decreases in the slope of the instantaneous diameter-pressure relationship of the main pulmonary artery. These decrements in vascular compliance during hypoxia were prevented by either bilateral thoracic sympathectomy or by section of the cervical vagus and glossopharyngeal nerves and were reproduced by bolus injections of lobeline (0.1 mg) at the root of the aorta. Using separation of the airways with a tracheal divider, the effects of systemic and of alveolar hypoxia on vascular resistance of the left caudal lobe were compared. Systemic hypoxia in the absence of alveolar hypoxia had no effect on pulmonary vascular resistance. Alveolar hypoxia elicited reversible increases in vascular resistance that were not prevented by sympathectomy. It is concluded that at least two vascular sites of action and two mechanisms are involved in the pulmonary vascular response to hypoxia.