Modulatory effect of imetit, a histamine H₃ receptor agonist, on C-fibers, cholinergic fibers and mast cells in rabbit lungs in vitro

Abstract

The pharmacological mechanisms involved in the interactions between C-fibers, cholinergic fibers and mast cells were investigated in tracheally perfused rabbit lungs by measuring the simultaneous release of substance P and histamine in lung effluents. The amounts of substance P and histamine released in lung superfusates were measured by radioimmunoassay (RIA) after administration of capsaicin and carbachol. Capsaicin (10⁻⁴ M) induced a simultaneous increase in substance P (273±56% of baseline) and histamine (460±138%) release. Similarly, carbachol (10⁻⁴ M) caused an increase in the release of both substance P (367±111%) and histamine (1379±351%). The effect of capsaicin was prevented by pretreating the lungs with the tachykinin NK₁ receptor antagonist SR 140333 (10⁻⁷ M), and atropine (10⁻⁶ M). SR 140333 prevented the carbachol-induced release of substance P but not of histamine. Exogenous substance P induced an increase in histamine release (136±7%) which was significantly greater in lungs perfused with the neutral endopeptidase inhibitor, thiorphan (10⁻⁵ M) (272±35%). This effect was prevented by atropine (10⁻⁶ M). Pretreatment of lungs with imetit (5×10⁻⁸ M), a selective H₃ receptor agonist, prevented the capsaicin-induced release of both mediators. Imetit also blocked the carbachol-induced release of substance P but not of histamine. Exogenous substance P-evoked histamine release was inhibited by imetit. Therefore, it can be concluded that substance P released through the action of capsaicin can activate cholinergic fibers, leading to cholinoceptor stimulation with subsequent activation of C-fibers and mast cells. While the presence of presynaptic H₃ receptors modulating substance P-induced acetylcholine release was only surmised, the existence of modulating histamine H₃ receptors on C-fibers was confirmed.

Introduction

Substance P is a tachykinin localized in C-fibers which belongs to the excitatory non-adrenergic, non-cholinergic nervous system, known for its bronchoconstrictor effects. These fibers innervate airways of several species and are found beneath the epithelium, around blood vessels (Lundberg et al., 1985; Hislop et al., 1990) and close to parasympathetic ganglia and mast cells (Dimitriadou et al., 1994; Domeij et al., 1996). Substance P can be released by various irritants, infectious agents and inflammatory mediators (Saria et al., 1988; Martins et al., 1991a; Hazbun et al., 1993). Substance P activates tachykinin receptors, resulting in an increase in vascular permeability (Rogers et al., 1988; Delaunois et al., 1993), bronchoconstriction (Advenier et al., 1987), vasodilatation (Delay-Goyet et al., 1992) and submucosal secretion (Rogers et al., 1989).

The interactions between C-fibers, cholinergic fibers and mast cells are potentially important in the development of neurogenic inflammation and have been demonstrated in various experimental models. The release of substance P, after stimulation of C-fibers by capsaicin, histamine and methacholine, has been described in tracheally perfused guinea pig lungs (Martins et al., 1991aMartins et al., 1991b). Similarly, Saria et al. (1988)have shown that substance P is released by histamine and vagal stimulation in vascularly perfused guinea pig lungs. The release of substance P plus thiorphan, following capsaicin stimulation of guinea pig isolated tracheal and bronchial segments, has also been recorded (Manzini et al., 1989). In turn, substance P can induce the release of histamine, as has been demonstrated for human pulmonary mast cells (Louis and Radermacker, 1990; Heany et al., 1995; Cross et al., 1996) and for tracheally perfused guinea pig lungs (Lilly et al., 1995). However, the simultaneous determination of substance P and histamine in the same experimental model, in order to investigate the interactions between C-fibers and mast cells, has never been performed.

The pathophysiological consequences of the interactions between C-fibers, cholinergic fibers and mast cells for endothelial permeability and lung mechanics have been studied in vascularly perfused rabbit lungs (Delaunois et al., 1993, Delaunois et al., 1994a, Delaunois et al., 1995b, Delaunois et al., 1996). The protective role of histamine H₃ receptors against acetylcholine- and capsaicin-induced pulmonary edema, under these conditions, has been described (Delaunois et al., 1995a). However, the localization of such receptors on C-fibers, mast cells or cholinergic fibers has not been resolved definitively.

Accordingly, the aim of the present study was two-fold: (1) to investigate the interactions between C-fibers, cholinergic fibers and mast cells in tracheally perfused rabbit lungs by measuring the simultaneous release of substance P and histamine in lung effluents and by using a specific tachykinin NK₁ receptor antagonist, SR 140333; and (2) to localize the histamine H₃ receptors on these structures by using a selective histamine H₃ receptor agonist, imetit.