Pharmacological evaluation of a guinea‐pig tracheal epithelium‐derived inhibitory factor (EpDIF)

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Abstract
An epithelium‐derived inhibitory factor (EpDIF) released by guinea‐pig tracheal epithelium was evaluated in a co‐axial bioassay system consisting of an epithelium‐intact guinea‐pig tracheal tube surrounding endothelium‐denuded rat aortic strip. Histamine and several muscarinic agonists induced concentration‐dependent relaxation of phenylephrine‐contracted rat aorta via the release of EpDIF. However, several other agonists did not induce the release of EpDIF from guinea‐pig trachea. These included the nicotinic cholinoceptor agonists nicotine (25 μm), 1,1‐dimethyl‐4‐phenylpiperazinium (DMPP) (25 μm), calcium ionophore A23187 (0.5 μm), bradykinin (0.05‐0.5 μm), substance P (5 μm), platelet activating factor (PAF, 1–100 nm), the leukotrienes (LT) LTC4, LTD4 and LTE4 (0.1–10 nm) as well as hyperosmotic stimuli. Prostaglandin E2 (PGE2) induced concentration‐dependent contraction of endothelium‐denuded rat aortic preparations, indicating that this prostanoid could not be EpDIF. Furthermore, relaxation to histamine and methacholine, mediated via EpDIF, was not significantly altered in the presence of phenidone (50 μm) the cyclo‐oxygenase/lipoxygenase inhibitor with radical scavenging properties or the cytochrome P‐450 inhibitors metyrapone (1 mm) and SKF 525A (25 μm). This suggests that EpDIF is neither a prostanoid nor a cytochrome P‐450 metabolite of arachidonic acid. The soluble guanylate cyclase inhibitor, methylene blue (50 μm), caused small but significant increases in the potencies of both histamine and methacholine in co‐axial assemblies, indicating that EpDIF did not activate this enzyme and therefore was not NO or a related substance. The β‐adrenoceptor antagonist, (−)−propranolol (1 μm), and the PAF‐receptor antagonist, WEB 2086 (50 μm), also failed to alter significantly EpDIF‐modulated relaxations. These data suggest that EpDIF is neither a stimulant of β‐adrenoceptors nor of PAF receptors. The present study provides some evidence that this vascular smooth muscle‐sensitive EpDIF may not be related to the putative EpDIF previously hypothesized to modulate directly spasmogen‐induced airway smooth muscle tone.