Ionic selectivity of the acetylcholine-activated ionic channel of frog endplate membranes to various organic cations has been studied. The ratio of test cation permeability (PX) to sodium permeability (PNa) was estimated by two methods, one based on the measurements in test cation solutions of the amplitude of transient depolarization induced by iontophoretic application of acetylcholine, and the other on the measurements of the reversal potential for the membrane current induced by iontophoretic application of acetylcholine under voltage-clamp conditions. The endplate channel is relatively nonselective to various test cations. The permeabilities relative to Na are ammonium (1.71), formamidine (1.49), methylamine (1.39), hydrazine (1.35), and Li (0.76), as measured from the reversal potential for acetylcholine currents, and guanidine (0.74), aminoguanidine (0.20), methylguanidine (0), and choline (0) as measured from the amplitude of acetylcholine potential. Methylguanidine and aminoguanidine block the endplate channel with the apparent dissociation constants of 0.5 and 15 mM, respectively. Based on these data, the dimensions of selectivity filter of acetylcholine-activated channel appear to be slightly larger than those of the sodium channel of frog nodes and smaller than those of the epithelial membrane of gallbladder of frogs and rabbits.

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