Innervation of airway smooth muscle in the baboon: evidence for a nonadrenergic inhibitory system

The distribution of receptors and the innervation of intrapulmonary and extrapulmonary baboon airways were examined using tissue bath techniques. Acetylcholine and histamine (via H1-receptors) elicited contractions of all airways. Norepinephrine had both an excitatory (via alpha-receptors) and inhibitory (via beta-receptors) effect in trachealis strips, but in intrapulmonary airways, only inhibitions were observed. Transmural electrical stimulation of resting airways produced sustained contractions that were blocked by either 5 x 10(-6) M tetrodotoxin or 10(-6) M atropine. Electrical stimulation also caused relaxations of histamine-induced contractions in airways pretreated with 10(-6) M atropine and 10(-6) M phentolamine. Relaxations were blocked by 10 x 10(-6) M tetrodotoxin but were unaffected by either 10(-6) M propranolol or 10(-5) M guanethidine. The response of airways to exogenous ATP depended on the degree of active tone. ATP caused contractions in resting airways and airways with moderate tone. In contrast, ATP relaxed airways with high active tone. We conclude that a nonadrenergic system constitute the primary inhibitory innervation of baboon airways, but whether ATP might act as the neurotransmitter could not be determined.