The Ca channel in skeletal muscle is a large pore

The permeability of Ca channels to various foreign cations has been investigated in the absence of external Ca²⁺. All physiological metal cations are clearly permeant, including Mg²⁺. The large organic cation n-butylamine⁺ is sparingly permeant or impermeant, but its larger derivative 1,4-diaminobutane²⁺ is highly permeant. Among the cations of the methylated ammonium series, permeability diminishes in a graded fashion as ion size increases. Tetramethylammonium, the largest cation found to be permeant, has a diameter of about 6 Å; hence, the aqueous pore of the Ca channel at its narrowest point can be no smaller. That the pore is so large strengthens our view that, under physiologic conditions, the high selectivity of Ca channels is due to selective binding of Ca²⁺ rather than to rejection of other cations by, for example, a sieving mechanism.