| Abstract: | The effects of leukotriene D4 and methacholine on cyclic nucleotide content and isoproterenol-induced relaxation were examined in the isolated opossum trachea. Although leukotriene D4 (-log EC50 = 6.70) was a more potent contractile agent than methacholine (-log EC50 = 5.78), the maximal response to leukotriene D4 was only 65% of the maximum response to methacholine. Contraction of tracheal strips with leukotriene D4 was accompanied by a 3-fold increase in cyclic GMP accumulation. Methacholine-induced contraction was not associated with an increase in cyclic GMP. Neither agent altered basal cyclic AMP content. Additional experiments were carried out to examine functional inhibitory interactions between bronchoconstricting and bronchodilating pathways. In these studies, cumulative isoproterenol concentration-response curves were constructed in tracheal strips contracted with three different concentrations of methacholine and in tissues contracted with three corresponding equieffective concentrations of leukotriene D4. Although the relaxant response to isoproterenol decreased as tissues were contracted with higher concentrations of either agent, the inhibitory effect of methacholine on isoproterenol-induced relaxation was much greater than the inhibitory effect of leukotriene D4. Previous studies from our laboratory suggested that a potential explanation for the greater inhibitory effect of methacholine on the mechanical response to isoproterenol was that methacholine may inhibit isoproterenol-stimulated cyclic AMP accumulation whereas leukotriene D4 may not. However, neither methacholine nor leukotriene D4 inhibited isoproterenol-stimulated cyclic AMP accumulation in the opossum trachea. The results of this study indicate that the sensitivity of airway smooth muscle to beta adrenoceptor agonists is influenced both by the initial contractile state of the tissue and by the type of agent used to induce tone. |
