Acetylcholine acts through muscarinic and nicotinic receptors, which have different signaling structures. The passage also describes an endothelial mediator of smooth muscle relaxation.
Receptor structure
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Muscarinic receptors are G protein-coupled receptors.
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Nicotinic receptors are ligand-gated cation channels.
Endothelial mediator
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Activation of the endothelial receptors described releases endothelial-derived relaxing factor, identified as nitric oxide.
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Nitric oxide diffuses to smooth muscle and causes relaxation.