The action of (±)L-660,863 [(±)3-(3-amino-1,2,4-oxadiazole-5-yl)-quinuclidine] at muscarinic receptor subtypes in vitro

1. The muscarinic pharmacology of a novel oxadiazole muscarinic agonist, (±) L-660,863, [±3-(3-amino- 1,2,4-oxadiazole-5-yl)-quinuclidine] has been studied using pharmacological, radioligand binding and biochemical techniques, in vitro. 2. In isolated tissue experiments, (±)L-660,863 was a more potent agonist than carbachol in all preparations studied, being most potent at muscarinic receptors mediating negative chronotropy in guinea-pig right, spontaneously beating atria and least potent at receptors mediating contractions in canine saphenous vein and endothelial denuded rabbit aorta (-log EC50) values were 8.8, 6.6 and 6.3, respectively. The apparent affinities (-log KA) of (±)L-660,863, estimated by receptor inactivation, showed some selectivity toward the atrial M2 muscarinic receptor (-log KA = 7.6) in comparison to the M1 or M3 muscarinic receptors (-log KA = 5.4 and 6.2), respectively. This degree of selectivity was also observed in competition radioligand binding studies. 3. At M3 muscarinic receptors mediating inositol phosphates (IPs) accumulation in longitudinal muscle of guinea-pig ileum, the potency of (00B1;)L-660,863 (-log EC50 value = 6.2) was similar to the apparent affinity calculated at M3 muscarinic receptors in the functional studies (see above). In contrast, at muscarinic receptors mediating IPs accumulation in guinea-pig atria and ventricles, the potency for (±)L-660,863 (-log EC50 = 6.2 and 6.4, respectively) was lower than the apparent affinity calculated at M2 muscarinic receptors from inotropic and binding studies in cardiac tissue (see above). These data suggest that the concentration-occupancy curve for (±)L-660,863 at cardiac muscarinic receptors mediating IPs accumulation lies to the right of the concentration-response curve. Similar conclusions may be made with regard to contraction by (±)L-660,863 of endothelial denuded rabbit aorta mediated by M2 receptors (Jaiswal et al. 1991). These findings may imply differences between muscarinic receptors mediating negative inotropy and IPs accumulation in guinea-pig myocardium or contraction of endothelial denuded rabbit aorta. 4. (±)L-660,863, in conclusion, acted as a potent muscarinic agonist, with some degree of M2 muscarinic receptor selectivity M2 VS M1 - 160 fold; M2 vs M3 - 25 fold \3- the first muscarinic agonist with, such a profile of activity. This compound may provide a useful tool with which to characterize muscarinic receptor function, as evidenced by the biochemical studies in cardiac tissue.