Evidence for facilitatory and inhibitory muscarinic receptors on postganglionic sympathetic nerves in mouse isolated atria

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Abstract
McNeil A 343 (10 μm‐30 μm) enhanced the fractional stimulation‐induced (S‐I) outflow of radioactivity from mouse isolated atria which had been incubated with [3H]‐noradrenaline. The enhancing effect of McNeil A 343 was not altered by hexamethonium (300 μm) suggesting that it was not due to an action at nicotinic receptors. It is also unlikely that McNeil A 343 enhanced the S‐I outflow of radioactivity in mouse atria by blocking neuronal reuptake of noradrenaline since the effect persisted in the presence of cocaine (30 μm). The facilitatory effect of McNeil A 343 on the S‐I outflow of radioactivity was attenuated by atropine (0.3 μm), pirenzepine (0.2 μm or 1.0 μm), dicyclomine (1.0 μm) and methoctramine (1.0 μm) and was thus due to activation of muscarinic receptors. In contrast to the effect of McNeil A 343, another muscarinic receptor agonist, carbachol (3.0 μm) significantly decreased the S‐I outflow of radioactivity. The receptors through which McNeil A 343 acts to enhance the S‐I outflow of radioactivity appear to be distinct from inhibitory prejunctional muscarinic receptors. The relatively M1‐selective antagonist, pirenzepine (0.2 μm), attenuated the facilitatory effect of McNeil A 343 whereas a higher concentration (1.0 μm) was required to block the inhibitory effect of carbachol. Conversely, the relatively M2‐selective antagonist, methoctramine (0.1 μm), blocked the inhibitory effect of carbachol but a higher concentration of methoctramine (1.0 μm) was required to block the facilitatory effects of McNeil A 343. These results tentatively ascribe facilitatory muscarinic receptors as belonging to the M1 subtype and inhibitory muscarinic receptors as belonging to the M2 subtype. The non‐selective muscarinic receptor antagonist, atropine, enhanced the S‐I outflow of radioactivity, suggesting that there was tonic activation of inhibitory prejunctional muscarinic receptors by endogenous acetylcholine released from parasympathetic nerves. However, pirenzepine (0.03 μm–1.0 μm) did not decrease the S‐I outflow of radioactivity, suggesting that under the conditions of the present study, facilitatory muscarinic receptors are not tonically activated by endogenous acetylcholine.