Functional antagonism between muscarinic receptor and beta-adrenergic receptor agonists in equine trachealis muscle in vitro

Functional antagonism between the muscarinic receptor agonist acetylcholine (ACh) and the {dollar}betasb2{dollar}-adrenoceptor agonist isoproterenol in normal equine trachealis muscle strips was characterized in order to elucidate general mechanisms by which these stimuli transduce opposing signals to the contractile machinery. Isoproterenol attenuated ACh- and PDB-induced tension with equal efficacy and potency, presumably because these contractions were mediated by PKC-induced increases in Ca{dollar}sp{lcub}++{rcub}{dollar}-sensitivity of the contractile apparatus. Isoproterenol was less effective at attenuating KCl-induced tension, presumably because muscle cells had a limited capacity to attenuate increases in myoplasmic free calcium ion concentration (Ca{dollar}sp{lcub}++{rcub}rbracksb{lcub}rm i{rcub}{dollar} during chronic depolarization. To confirm these conclusions, tension and Fura-2 fluorescence ratio (F{dollar}sb{lcub}340{rcub}{dollar}/F{dollar}sb{lcub}380{rcub}{dollar}; an index of (Ca{dollar}sp{lcub}++{rcub}rbracksb{lcub}rm i{rcub}{dollar}) were recorded simultaneously in trachealis muscle strips during concomitant ACh and isoproterenol exposure. High concentrations of isoproterenol attenuated ACh-induced increases in the fluorescence ratio and ACh-induced tension was reduced for a given suprabasal fluorescence ratio. These results suggested the conclusion that isoproterenol attenuates ACh-induced increases in both (Ca{dollar}sp{lcub}++{rcub}rbracksb{lcub}rm i{rcub}{dollar} and Ca{dollar}sp{lcub}++{rcub}{dollar}-sensitivity of the contractile apparatus. Because high concentrations of ACh ({dollar}{lcub}>{rcub}100 mu{dollar}M) overcame isoproterenol-induced antagonism, selective muscarinic receptor antagonists were used to functionally characterize the subtypes of muscarinic receptors that mediate ACh-induced contraction in equine trachealis muscle. I conclude that in equine, like in other species, (1) trachealis muscle M{dollar}sb3{dollar}-receptors mediate contraction, (2) trachealis muscle M{dollar}sb2{dollar}-receptor-mediated events may facilitate contraction, and (3) activation of prejunctional M{dollar}sb2{dollar}-receptors on nerves innervating trachealis muscle attenuates ACh release. This latter finding is particularly important because (1) dysfunctional prejunctional autoinhibitory M{dollar}sb2{dollar}-receptors may result in elevated levels of ACh in the airways in response to activation of irritant receptors, (2) dysfunctional prejunctional M{dollar}sb2{dollar}-receptors have been implicated in the pathogenesis of human asthma ( J. Appl. Physiol. 67:2461-2465, 1989; Life Sciences 52:529-536, 1993), and (3) atopic horses suffer from a chronic obstructive pulmonary disease ("heaves") that has been proposed as a model for human asthma.