Functional interactions between capsaicin-sensitive and cholinergic nerves in the guinea pig bronchus

Electrical stimulation of the right vagus nerve causes a biphasic contraction of the guinea pig isolated right bronchus. The ''first-phase'' is blocked by hexamethonium or atropine and the ''second-phase'' is eliminated by capsaicin pretreatment. We investigated a potential interaction between capsaicin-sensitive nerves and cholinergic nerves in the guinea pig bronchus. Hexamethonium (100 μM) abolished the first-phase contraction but had no effect on the capsaicin-sensitive second-phase contraction. In the presence of hexamethonium, atropine (0.1 μM) significantly decreased the amplitude of the second-phase contraction by 28%. Similar results were observed with the M3-selective muscarinic receptor antagonist, 4-diphenyl-acetoxy-M-methylpiperadine, but not with the M2 muscarinic antagonist, AFDX-116. Atropine also reduced contractions induced by exogenously applied neurokinin A. We then analyzed the effect of stimulating capsaicin-sensitive fibers with electrical field stimulation on vagus nerve evoked cholinergic contractions. By reducing the stimulus intensity we were able to evoke vagus nerve-mediated contractions that were exclusively cholinergic in nature. The cholinergic contractions were significantly increased after stimulation of capsaicin-sensitive fibers by about 50%. By contrast, contractions elicited by exogenous methacholine were unaffected after field stimulation of capsaicin-sensitive responses. Our findings indicate that the contractions of the guinea pig bronchus elicited by stimulation of capsaicin-sensitive nerves are due in part to muscarinic cholinergic receptor activation. Secondly, our data demonstrate that the cholinergic contractions elicited by vagal preganglionic nerve stimulation are potentiated by neurotransmitter(s) released from capsaicin-sensitive fibers in bronchus.