General Principles Of Pharmacology/Types of receptors
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Intracellular receptors
Lipophilic substances capable of crossing the plasma membrane may activate intracellular receptors: Sex steroids, mineralocorticoids, glucocorticoids, and thyroid hormones all activate specific intracellular receptors
Ion-channel-linked receptors
There are two general classes of ion channels: voltage gated and ligand gated. Voltage-gated ion channels are activated by alterations in membrane voltage. For example, voltage-gated sodium (Na+) channels open when the membrane is depolarized to a threshold potential and contribute to further membrane depolarization by allowing Na+ influx into the cell.
Ligand-gated ion channels are activated after binding to specific ligands or drugs. Many neurotransmitters and drugs activate membrane- bound ligand ion -gated channels, including several types of glutamate receptors
G-protein-linked receptors
G-protein-linked receptors compose a large class of membrane-bound receptors. The protein structure of these receptors includes a common seven-membered transmembrane domain. In general, receptors linked to G proteins greatly amplify the biologic signal because they activate G proteins, which in turn activate ion channels or, more commonly, other enzymes (e.g., adenylate cyclase), leading to stimulation of still other enzymes (e.g., protein kinase A)." This amplification system, which generally involves an extended duration of activation of the G protein relative to the binding of drug to the receptor, may explain why maximal pharmacologic effects are often observed when only a small proportion of receptors are activated.
Enzyme-linked receptors
Enzyme-linked receptors have only one transmembrane domain per protein subunit, with "an enzymatic catalytic site on the cytoplasmic side of the receptor. Dimerization of activated receptors provides the conformational change required for expression of enzymatic activity. The catalytic sites are commonly protein kinases that phosphorylate tyrosine.
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