Alpha 2-adrenoceptor mechanisms in guinea-pig trachea

In the presence of propranolol tracheal strips from guinea-pigs were markedly contracted by norepinephrine and clonidine but only slightly by phenylephrine. The contractile responses to clonidine and norepinephrine were inhibited by yohimbine but not by prazosin. The specific binding of [3H]p-aminoclonidine to the microsomal fractions from guinea-pig tracheal muscle was much greater than that of [125I]BE2254. These results indicate that alpha 2-adrenoceptors are the predominant subtype of alpha-adrenoceptor in the guinea-pig trachea. The response to norepinephrine was abolished by cyclooxygenase inhibitors, suggesting the release of excitatory prostaglandins by norepinephrine. These results raise the possibility that norepinephrine interacts with alpha 2-adrenoceptors and releases prostaglandins to contract tracheal muscle.