The Sources of Ca2+ for Muscarinic Receptor-induced Contraction in the Rat Ileum

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Abstract
The contractile responses obtained by activation of different muscarinic receptor subtypes in the longitudinal muscle of the rat ileum and especially the responses of this muscle to acetylcholine in a Ca2+-free medium have been investigated. In Ca2+-containing solution, acetylcholine elicited similar concentration-dependent contractile responses in the duodenum, jejunum and ileum strips of the rat intestine. The response to a maximal concentration of the agonist (1 μM) consisted of a rapid phasic response followed by a slower tonic one. Nifedipine completely relaxes or inhibits the sustained response and only partially diminishes the phasic one, which suggests that the phasic contraction depends on the release of internal Ca2+ as well as Ca2+ entry from the extracellular space through voltage-dependent Ca2+ channels, but that the tonic contraction only depends on the influx of the external ion. In Ca2+-free medium, acetylcholine (1 μM) induced phasic contractions that depend on the release of this ion from internal stores. Participation of different subtypes of receptors (M1, M2 and M3) in these responses depends on the inhibitory action shown by methoctramine, 4−DAMP and atropine but not by pirenzepine in two different experimental models.