Co‐axial bioassay of a smooth muscle relaxant factor released from guinea‐pig tracheal epithelium

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Abstract
1 The ability of guinea-pig trachea to release an epithelium-derived relaxant factor (EpDRF) was assessed in a co-axial bioassay system. 2 Histamine (100 μm) and methacholine (25 μm) caused endothelium-dependent relaxation of rat isolated aorta, presumably via the release of endothelium-derived relaxant factor (EDRF). In contrast, endothelium-denuded rat aorta did not relax in response to these agents. 3 EDRF release was detected in response to methacholine in a co-axial bioassay system, consisting of intact rabbit aorta tube (EDRF donor) and endothelium-denuded rat aorta strip (assay preparation). These results indicated the transfer of EDRF from a donor to an assay preparation, thereby validating the co-axial bioassay method. 4 Substitution of endothelium-intact rabbit aorta tube by epithelium-intact guinea-pig tracheal tube tissue in co-axial assemblies, still allowed the assay preparation to relax in response to histamine or methacholine. Removal of the intact tracheal tube from the system, or removal of the epithelium from the donor tracheal tube in co-axial preparations, abolished such relaxant responses. These observations are consistent with histamine- or methacholine-induced release of an epithelium-derived relaxant factor (EpDRF) from the trachea. 5 In the co-axial assembly comprising intact guinea-pig trachea and endothelium-denuded rat aorta, histamine and methacholine produced concentration-dependent, EpDRF-induced aortic relaxation. Mean concentrations of histamine and methacholine producing 50% of the maximum relaxation (EC50) were 39.8 μm and 2.7 μm respectively. Histamine-induced relaxation was inhibited in the presence of mepyramine (2 μm) and responses to methacholine were inhibited by atropine (0.1 μm). 6 Methylene blue (50 μm) had no effect on such relaxant responses, indicating that EpDRF does not activate guanylate cyclase. Furthermore, the cyclo-oxygenase inhibitor indomethacin (5 μm), the cyclo-oxygenase/lipoxygenase inhibitor BW 755C (150 μm) and the leukotriene receptor antagonist FPL 55712 (10 μm) each failed significantly to alter EpDRF-mediated relaxation of vascular smooth muscle suggesting that EpDRF is not a prostanoid. Platelet activating factor (Paf) failed to cause relaxation of endothelium-denuded rat aorta, indicating that this mediator was also not EpDRF. 7 EpDRF was also released from human bronchial segments. 8 This study provides direct evidence for the release of an EpDRF from non-diseased airway tissue and further suggests that healthy airway reactivity to spasmogens is modulated by the release of an endogenous protective, spasmolytic substance. The bronchial hyperreactivity of asthma may be partly caused by attenuated production of such an inhibitory signal.