Muscarinic agonist potencies at three different effector systems linked to the M₂ or M₃ receptor in longitudinal smooth muscle of guinea‐pig small intestine

Abstract

1 The abilities of muscarinic agonists (arecoline, bethanechol, carbachol, methacholine, pilocarpine) to inhibit isoprenaline-induced cyclic AMP production in chopped fragments (via M 2 receptors), and to evoke cationic current (I cat ) (via M 2 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. 2 All muscarinic agonists (1 ± 300 mM) examined inhibited isoprenaline (1 mM)-induced accumulation of cyclic AMP, the IC 50 varying from 52 to 248 mM. However, their relative potencies to evoke this M 2 eect were not signi®cantly correlated with their ability to evoke I cat , also a M 2 eect, whether or not calcium stores were depleted; pilocarpine and McN-A343 inhibited the I cat response to carbachol. 3 Muscarinic agonists (concentration 300 or 1000 mM), except pilocarpine and McN-A343 which were ineective, evoked Ca 2+ -activated K + current (I K-Ca ) resulting from Ca 2+ store release (M 3 eect). Their eectiveness was tested by estimating residual stored calcium by subsequent application of caeine (10 mM). The relative potencies to evoke Ca 2+ store release (M 3 ) and for I cat activation (M 2 ) were closely correlated (P50.001). 4 These data might be explained if M 2 -mediated adenylyl cyclase inhibition and I cat activation involve dierent G proteins, or involve dierent populations of M 2 receptors. The observed correlation of agonist potency between I cat activation and Ca 2+ store release supports the proposal (Zholos & Bolton, 1997) that M 3 activation can potentiate M 2 -cationic channel coupling through Ca 2+ -independent mechanisms.

Phospholipase C Involvement in Activation of the Muscarinic Receptor-Operated Cationic Current in Guinea Pig Ileal Smooth Muscle Cells

“…This is supported by the observation that M2-receptor antagonists inhibited carbachol-evoked I cat in a competitive manner, while M3-receptor antagonists severely depressed its maximum response, under conditions where [Ca 2+ ] i was buffered to a certain level (5). Under the same conditions, significant correlations of agonist potencies between for I cat activation and M3-receptor stimulation have also been found (16). However, the mechanisms underlying the synergistic activation of I cat by M2 and M3 receptors remain to be elucidated.…”

“…Upon its application, I K-Ca lasting 3 -10 s was elicited, as described (6,16) (Fig. 1A), whose amplitude amounted to 3.90 ± 0.63 nA on the average (n = 6).…”

“…Single intestinal smooth muscle cells were prepared as previously described (16). Briefly, male guinea pigs weighing 350 -450 g were killed by a stunning blow followed by immediate exsanguination, from which the ileum of 15 -20 cm in length was removed and the longitudinal muscle layer peeled from the underlying tissue.…”

Two Types of Cation Channel Activated by Stimulation of Muscarinic Receptors in Guinea-Pig Urinary Bladder Smooth Muscle

“…1). A similar outward I K-Ca (mean amplitude: 2866 ± 414 pA, n = 5) was also evoked by a potent Ca 2+ -store releaser, caffeine (10 mM), which was capable of depleting stored Ca 2+ in guinea-pig ileal myocytes (20). After the initial outward I K-Ca evoked by caffeine, brief, spontaneous outward I K-Ca no longer occurred, indicating depletion of internal Ca 2+ stores.…”

Carbachol Induces Ca2+-Dependent Contraction via Muscarinic M2 and M3 Receptors in Rat Intestinal Subepithelial Myofibroblasts

“…3B, 3C, 5A, and 5B, AF-DX116 (30 nM) and 4-DAMP (10 nM) each exerted more than 50% inhibition of both carbachol-induced IMF contraction and [Ca 2+ ] i elevation. A recent study reported that activation of muscarinic M 3 receptor can potentiate the muscarinic M 2 receptor signal by opening of cationic channels in smooth muscle cells (30). Another report suggested that muscarinic M 2 receptor activation strengthens muscarinic M 3 -mediated contraction by decreasing K + efflux (15).…”