NO EVIDENCE FOR A ROLE OF MUSCARINIC M(2) RECEPTORS IN FUNCTIONAL ANTAGONISM IN BOVINE TRACHEA

1 The functional antagonism between methacholine- or histamine-induced contraction and beta-adrenoceptor-mediated relaxation was evaluated i n bovine tracheal smooth muscle in vitro. In addition, the putative co ntribution of muscarinic M(2) receptors mediating inhibition of beta-a drenoceptor-induced biochemical responses to this functional antagonis m was investigated with the selective muscarinic antagonists, pirenzep ine (M(1) over M(2)), AF-DX 116 and gallamine (M(2) over M(3)), and he xahydrosiladiphenidol (M(3) over M(2)). 2 By use of isotonic tension m easurement, contractions were induced with various concentrations of m ethacholine or histamine, and isoprenaline concentration-relaxation cu rves were obtained in the absence or presence of the muscarinic antago nists. Antagonist concentrations were chosen so as to produce selectiv e blockade of M(2) receptors (AF-DX 116 0.1 mu M, gallamine 30 mu M), or half-maximal blockade of M(3) receptors (pirenzepine 0.1 mu M, AF-D X 116 0.5 mu M, hexahydrosiladiphenidol 0.03 mu M). Since these latter antagonist concentrations mimicked K-B values towards bovine tracheal smooth muscle M(3) receptors, antagonist-induced decreases in contrac tile tone were compensated for by doubling the agonist concentration. 3 It was found that isoprenaline-induced relaxation of bovine tracheal smooth muscle preparations was dependent on the nature and the concen tration of the contractile agonist used. Thus, isoprenaline pD(2) (-lo g EC(50)) values were decreased 3.7 log units as a result of increasin g cholinergic tone from 22 to 106%, and 2.4 log units by increasing hi stamine tone over a similar range. Furthermore, maximal relaxability o f cholinergic tone decreased gradually from 100% at low to only 1.3% a t supramaximal contraction levels, whereas with histamine almost compl ete relaxation was maintained at all concentrations applied. As a resu lt, isoprenaline relaxation was clearly hampered with methacholine com pared to histamine at equal levels of contractile tone. 4 In the prese nce of gallamine, isoprenaline relaxation was facilitated for most con centrations of methacholine, and for all concentrations of histamine. These changes could be explained by the decreased contraction levels f or both contractile agonists in the presence of gallamine. 5 Isoprenal ine-induced relaxation of cholinergic contraction was also facilitated by AF-DX 116 as well as by pirenzepine and hexahydrosiladiphenidol, a nd these (small) changes were again related to the (small) decreases i n cholinergic contraction levels that were present in these experiment s despite the additional administration of the agonist to readjust con tractile tone. Similarly, changes in isoprenaline relaxation of histam ine-induced tone could be explained by different contraction levels. 6 These results can be explained by the sole involvement of muscarinic M(3) receptors, and provide no evidence for a role of muscarinic M(2) receptors in functional antagonism in bovine trachea. Furthermore, the y stress the importance of taking into account non-cholinergic control s as well as contraction levels in these experiments.