The abilities of muscarinic agonists (arecoline, bethanechol, carbachol, McN-A343, methacholine, pilocarpine) to inhibit isoprenaline-induced cyclic AMP production in chopped fragments (via M2 receptors), and to evoke cationic current (Icat) (via M2 receptors) or calcium store release (via M3 receptors) in enzyme-dispersed, single voltage-clamped cells from longitudinal smooth muscle of the guinea-pig small intestine were examined.
All muscarinic agonists (1 – 300 μM) examined inhibited isoprenaline (1 μM)-induced accumulation of cyclic AMP, the IC50 varying from 52 to 248 μM. However, their relative potencies to evoke this M2 effect were not significantly correlated with their ability to evoke Icat, also a M2 effect, whether or not calcium stores were depleted; pilocarpine and McN-A343 inhibited the Icat response to carbachol.
Muscarinic agonists (concentration 300 or 1000 μM), except pilocarpine and McN-A343 which were ineffective, evoked Ca2+-activated K+ current (IK-Ca) resulting from Ca2+ store release (M3 effect). Their effectiveness was tested by estimating residual stored calcium by subsequent application of caffeine (10 mM). The relative potencies to evoke Ca2+ store release (M3) and for Icat activation (M2) were closely correlated (P<0.001).
These data might be explained if M2-mediated adenylyl cyclase inhibition and Icat activation involve different G proteins, or involve different populations of M2 receptors. The observed correlation of agonist potency between Icat activation and Ca2+ store release supports the proposal (Zholos & Bolton, 1997) that M3 activation can potentiate M2-cationic channel coupling through Ca2+-independent mechanisms.
British Journal of Pharmacology (2002) 135, 1765–1775; doi:10.1038/sj.bjp.0704642