Effects of FK224, a NK₁ and NK₂ receptor antagonist, on plasma extravasation of neurogenic inflammation in rat airways

Background: Asthma is an inflammatory disease because there are many inflammatory changes in the asthmatic airways. Axon reflex mechanisms may be involved in the pathogenesis of asthma. Sensory neuropeptides are involved in this inflammation, which is defined as neurogenic inflammation. Substance p, neurokinin A, and neurokinin B may be main neuropeptides of neurogenic inflammation in airways. These tachykinins act on neurokinin receptors. Three types of neurokinin receptors, such as NK₁, NK₂, and NK₃, are currently recognized, at which substance p, neurokinin A, and neurokinin B may be the most relevant natural agonist of neurogenic inflammation in airways. The receptor subtypes present in several tissues have been characterized on the basis of differential sensitivity to substance p, neurokinin A, and neurokinin B. Plasma extravasation and vasodilation are induced by substance p more potently than by in A, indicating NK₁ receptors on endothelial cells mediate the response. But airway contraction is induced by neurokinin A more potently than by substance P, indicating the NK₂ receptors in airway smooth muscles. These receptors are used to evaluate the pathogenesis of bronchial asthma. FK224 was identified from the fermentation products of Streptomyces violaceoniger. FK224 is a dual antagonist of both NK₁ and NK₂ receptors. Purpose: For a study of pathogenesis of bronchial asthma, the effect of FK224 on plasma extravasation induced by vagal NANC electrical stimulation was evaluated in rat airway. Method: Male Sprague-Dawley rats weighing 180 ~ 450 gm were anesthetized by i.p. injection of urethane. Plasma extravasation was induced by electrical stimulation of cervical vagus NANC nerves with 5 Hz, 1 mA, and 5 V for 2 minutes (NANC2 group) and for sham operation without nerve stimulation (control group). To evaluate the effect of FK224 on plasma extravasation in neurogenic inflammation, FK224 (1 mg/kg, Fujisawa Pharmaceutical Co., dissolved in dimethylsulphoxide; DMSO, Sigma Co.) was injected 1 min before nerve stimulation (FK224 group). To assess plasma exudation, Evans blue dye (20 mg/kg, dissolved in saline) was used as a plasma marker and was injected before nerve stimulation. After removal of intravascular dye, the Evans blue dye in the tissue was extracted in formamide (37°C, 24h) and quantified spectrophotometrically by measuring dye absorbance at 629 nm wavelength. Tissue dye content was expressed as ng of dye per mg of wet weight tissue. The amount of plasma extravasation was measured on the part of airways in each groups. Results: 1) Vagus nerve (NANC) stimulation significantly increased plasma leakage in trachea, main bronchus, and peripheral bronchus compared with control group, 14.1 ± 1.6 to 49.7 ± 2.5, 17.5 ± 2.0 to 38.7 ± 2.8, and 12.7 ± 2.2 to 19.1 ± 1.6 ng of dye per mg of tissue (mean ± SE), respectively (p < 0.05). But there was not significantly changed in lung parenchyma (p > 0.05). 2) FK224 had significant inhibitory effect upon vagal nerve stimulation-induced airway plasma leakage in any airway tissues of rat, such as trachea, main bronchus, and peripheral bronchus compared with vagus nerve stimulation group, 49%, 58%, and 70%, respectively (p < 0.05). Inhibitory effect of FK224 on airway plasma leakage in neurogenic inflammation was revealed the more significant in peripheral bronchus, but no significant in lung parenchyma. Conclusion: These results suggest that FK224 is a selective NK receptor antagonist which effectively inhibits airway plasma leakage induced by the endogenous neurotransmitters released by neurogenic inflammation in rat airway. Tachykinin receptor antagonists may be useful in the treatment of bronchial asthma.