a-Bungarotoxin (a-BTX), isolated from snake venom, is a protein that binds selectively and irreversibly to the AChR of muscle. Because binding of the toxin is irreversible, recovery from a-BTX block indicates synthesis and insertion of new AChR into the membrane. Studies using a-BTX show that the AChR is a glycoprotein consisting of five polypeptide subunits (a, (3,7,8, and e).The complex is a cylindrical unit about 8 nm in diameter that spans the plasma membrane (Fig. 28.2).
Histrionicotoxin, obtained from a Panamanian frog, is a toxin that attaches to a high-affinity site within the pore of the AChR complex and results in muscular blockade. Agents that have a similar effect include local anesthetics, barbiturates, and phencyclidine. They reduce the flow of ions and shorten the duration of time the channel is open.
The AChR in muscle is distinct from the AChRs found in the central and autonomic nervous systems (neuronal AChRs). Although both are activated by nicotine, each is blocked by a different antagonist (e.g., a-BTX vs. k-BTX).
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