In vitro characterisation of the muscarinic receptor partial agonist, sabcomeline, in rat cortical and heart membranes

Abstract

We have investigated the pharmacology of the functionally selective muscarinic M 1 receptor partial agonist, sabcomeline [SB-202026 ( R-( Z)-(+)-α-(methoxyamino)-1-azabicyclo[2.2.2] octane-3-acetonitrile)], in rat cortex and heart using radioligand binding and functional studies. The Quinuclidinyl benzilate/Oxotremorine-M acetate ratio from radioligand binding studies suggested that sabcomeline and xanomeline [3(3-hexyloxy-1,25-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine] are muscarinic receptor partial agonists in cortical and heart membranes. In [ 35 S ]GTPγS binding studies in rat cortex, carbachol stimulated binding via muscarinic M 2/M 4 receptors which could be blocked by sabcomeline with a p A 2 of 7.2. In rat heart membranes, carbachol also stimulated [ 35 S ]GTPγS binding studies through muscarinic M 2 receptors. Sabcomeline caused a small stimulation of basal [ 35 S ]GTPγS binding in both rat and heart tissues. Sabcomeline did not stimulate phosphoinositide hydrolysis in rat cortical slices, but did block the muscarinic M 1 receptor-mediated response caused by carbachol with apparent p K b of 6.9. Xanomeline and milameline also had no effect on phosphoinositide hydrolysis up to 100 μM. In adenylyl cyclase studies in rat atria, sabcomeline inhibited forskolin-stimulated adenylyl cyclase activity to a similar extent to that of carbachol, xanomeline and milameline. The present study, using the techniques of radioligand binding, supports previous publications which have claimed that sabcomeline is a muscarinic receptor partial agonist. As expected, this study shows that the functional actions of this compound at muscarinic receptor subtypes and in different tissues will depend on receptor reserve.