Fig. 1. Sketch of muscle acetylcholine receptor channels (  right ) and tracings of cell patch records of receptor channel openings (  left ). The mature, innervated, or junctional receptor consists of two α1 subunits, and one each of the β1, δ, and ε subunits. The immature, extrajunctional or fetal form consists of two α1 subunits and one each of the β1, δ, and γ subunits. The ε subunit is therefore substituted by the γ subunit in the immature receptor. The subunits are arranged around the central cation channel. The mature isoform containing the ε subunit shows shorter open times and high-amplitude currents; hence, it is called  high-conductance channel . The immature isoform containing the γ subunit is called  low-conductance channel because it shows long open times and low-amplitude currents. The immature receptors can be depolarized with smaller concentrations of acetylcholine or succinylcholine. Therefore, they can be depolarized more easily even with decreasing concentrations of succinylcholine during its continued metabolism after systemic administration. The fact that these immature channels remain open for a longer time and are up-regulated in muscles in certain pathologic states increases the chance that intracellular potassium ions will leak out and increase the plasma levels of potassium.