Muscarinic receptors mediate their effects by activating guanosine triphosphate (GTP)-binding proteins (G proteins). These receptors have seven protein helixes that transcend the plasma membrane, creating four extracellular domains and four intracellular domains (Fig. 17.3). The extracellular domain of the receptor contains the binding site for ACh. The intracellular domain couples with G proteins to initiate biochemical changes that result in pharmacological action from receptor activation.
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