Adrenergic and cholinergic regulation of bronchial vascular tone.

The autonomic nervous system plays a vital role in the regulation of vasomotor tone. Previous studies of systemic arteries and veins have shown that the classic autonomic neurotransmitters, norepinephrine and acetylcholine, act on smooth muscle, adrenergic nerve endings, and endothelial cells in the blood vessel wall to regulate vasomotor tone. Similar studies with isolated tissues indicate that bronchial arteries are innervated by adrenergic neurons and that norepinephrine activates postjunctional alpha 1-adrenoceptors to cause smooth muscle contraction. Isolated canine bronchial arteries fail to relax in response to beta-adrenoceptor agonists, and they do not contract when exposed to acetylcholine. This lack of responsiveness may be species-specific, however, since isolated bronchial arteries from other species respond to these agonists. Acetylcholine causes endothelium-dependent relaxation of bronchial arteries in vitro; this response is mediated by endothelial M3-muscarinic receptors. The role of the endothelium in mediating responses to adrenergic agonists, as well as the prejunctional effects of norepinephrine and acetylcholine on adrenergic nerve endings, remain to be explored in isolated bronchial arteries.