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 in bovine tracheal smooth muscle in vitro. In addition, the putative contribution of muscarinic M(2) receptors mediating inhibition of beta-adrenoceptor-induced biochemical responses to this functional antagonism was investigated with the selective muscarinic antagonists, pirenzepine (M(1) over M(2)), AF-DX 116 and gallamine (M(2) over M(3)), and hexahydrosiladiphenidol (M(3) over M(2)). 2 By use of isotonic tension measurement, contractions were induced with various concentrations of methacholine or histamine, and isoprenaline concentration-relaxation curves were obtained in the absence or presence of the muscarinic antagonists. Antagonist concentrations were chosen so as to produce selective 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-DX 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 contractile 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 concentration of the contractile agonist used. Thus, isoprenaline pD(2) (-log EC(50)) values were decreased 3.7 log units as a result of increasing cholinergic tone from 22 to 106%, and 2.4 log units by increasing histamine tone over a similar range. Furthermore, maximal relaxability of cholinergic tone decreased gradually from 100% at low to only 1.3% at supramaximal contraction levels, whereas with histamine almost complete relaxation was maintained at all concentrations applied. As a result, isoprenaline relaxation was clearly hampered with methacholine compared to histamine at equal levels of contractile tone. 4 In the presence of gallamine, isoprenaline relaxation was facilitated for most concentrations of methacholine, and for all concentrations of histamine. These changes could be explained by the decreased contraction levels for both contractile agonists in the presence of gallamine. 5 Isoprenaline-induced relaxation of cholinergic contraction was also facilitated by AF-DX 116 as well as by pirenzepine and hexahydrosiladiphenidol, and these (small) changes were again related to the (small) decreases in cholinergic contraction levels that were present in these experiments despite the additional administration of the agonist to readjust contractile tone. Similarly, changes in isoprenaline relaxation of histamine-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, they stress the importance of taking into account non-cholinergic controls as well as contraction levels in these experiments.