In this study, we explored the possible mechanism of cooling-induced relaxation of the isolated guinea-pig trachea. A rapid cooling (-4°C/min) from 37 ± 0.5°C to 25 ± 0.5°C induced a transient and small contraction followed by a sustained cooling-relaxation. This relaxation was not blocked by propranolol or tetrodotoxin. Various concentrations of four contractile agonists (histamine, carbachol, 5-HT and ryanodine) all enhanced cooling-relaxation in a concentration-dependent manner which correlated well with their increase in the developed muscular tension, suggesting an inherent counterbalance between cooling-relaxation and the bronchoconstriction. Treating with either indomethacin or nordihydroguaiaretic acid (NDGA) did not affect the contractile properties of histamine, carbachol and 5-HT except ryanodine, but reversed cooling-relaxation into sustained cooling-contraction. Indomethacin partially inhibited but NDGA abolished cooling-relaxation induced by ryanodine. Moreover, ryanodine, but not the other three contractile agonists, could antagonize indomethacin in inducing cooling-contractions by various agonists. From above findings, we can conclude that eicosanoids including prostaglandins particularly leukotrienes, which would be produced by the elevated Ca2+-release from the ryanodine sensitive Ca2+-store, play prominent roles in inducing cooling-relaxation.
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