α-Bungarotoxin (α-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 α-BTX block indicates synthesis
and insertion of new AChR into the membrane. Studies
using α-BTX show that the AChR is a glycoprotein consisting
of five polypeptide subunits (α, β, γ, δ, and ε).The
complex is a cylindrical unit about 8 nm in diameter that
spans the plasma membrane.
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.,
α-BTX vs. k-BTX).
