Differential relaxant effects of isoproterenol on methacholine- versus leukotriene D4-induced contraction in the guinea-pig trachea.

The ability of isoproterenol to relax guinea-pig trachea was examined in tissues precontracted with three concentrations of methacholine or three corresponding equieffective concentrations of leukotriene D4. Although increasing the concentration of either contractile agent progressively shifted isoproterenol concentration-response curves to the right, the shifts produced by methacholine were greater than those produced by leukotriene D4. Thus, the sensitivity of tracheal smooth muscle to beta-agonists depends upon the initial contractile state of the tissue, and the type of agent used to induce tone.