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The classical site of muscarinic receptors is at the postganglionic parasympathetic neuroeffector junction in smooth muscle, heart, and exocrine glands. However, muscarinic receptors also exist on effectors cells, even in the absence of cholinergic innervation.Prototype agonists include muscarine and ACh.he classical site of muscarinic receptors is at the postganglionic parasympathetic neuroeffector junction in smooth muscle, heart, and exocrine glands. However, muscarinic receptors also exist on effectors cells, even in the absence of cholinergic innervation. Prototype agonists include muscarine and ACh.
In single cells, muscarinic receptor activation evokes calcium release from stores which raises the internal free calcium concentration and causes opening of calcium-activated potassium channels. The rise in internal calcium suppresses the voltage-dependent inward calcium current.A third important effect is the opening of channels which cause depolarization of the membrane and so increase action potential discharge and contraction in the whole muscle.These channels were studied by voltage-clamp of single cells from longitudinal muscle of rabbit small intestine.n single cells, muscarinic receptor activation evokes calcium release from stores which raises the internal free calcium concentration and causes opening of calcium-activated potassium channels. The rise in internal calcium suppresses the voltage-dependent inward calcium current.