Functional innervation of pig tracheal smooth muscle: Neural and non- neural mechanisms of relaxation

In pig tracheal smooth muscle (TSM), the isometric tension responses to electrical field stimulation (EFS) were compared to exogenous peptides, vasoactive intestinal polypeptide (VIP) and calcitonin gene-related peptide (CGRP), as well as to the nicotinic agonist dimethylphenyl piperazinium chloride (DMPP). The objectives of this study were to understand the mechanisms of nonadrenergic, noncholinergic relaxations of pig TSM and identify some putative candidate substances mediating the neural inhibitory response. In strips of TSM obtained from 6- to 12-week-old pigs set up in vitro in organ baths in Kreb's solution, EFS resulted in frequency-dependent contractions that were abolished by 1 μM atropine or 0.1 μM tetrodotoxin (TTX). Addition of DMPP (20-100 μM) resulted in a transient, atropine- sensitive contraction, which quickly desensitized. EFS failed to elicit any further contractions, but the tissues responded to carbachol. In tissues exposed to DMPP and contracted with carbachol, EFS elicited frequency- dependent relaxations that were unaffected by 1 μM propranolol, abolished by TTX and partially inhibited by ω-conotoxin. At the peak of carbachol-induced contraction, addition of DMPP resulted in rapid relaxation reversing spontaneously to base line, with no significant relaxation to further addition of DMPP. DMPP-induced relaxations were unaffected by TTX or ω- conotoxin. These tissues readily relaxed to 10^-8 M VIP. In carbachol precontracted tissues, VIP elicited concentration-dependent relaxations that quickly desensitized. However, these tissues readily relaxed to DMPP. CGRP produced only weak relaxations. The results suggest that the pig TSM has predominantly cholinergic excitatory and nonadrenergic and noncholinergic inhibitory innervation, with the latter unlikely to be mediated by VIP or CGRP, as is the DMPP response, and the transmitter is unlikely to be coreleased with acetylcholine. DMPP-induced relaxation is non-neural in origin. Desensitization of DMPP-induced contraction leads to inhibition of acetylcholine release. We speculate that DMPP may release an inhibitory agent from cell types within the airways such as the neuroepithelial cells.