Differences in apparent pore sizes of low and high voltage-activated Ca2+ channels

Pore size is of considerable interest in voltage-gated Ca2+ channels because they exemplify a fundamental ability of certain ion channels: to display large pore diameter, but also great selectivity for their ion of choice. We determined the pore size of several voltage-dependent Ca2+ channels of known molecular composition with large organic cations as probes. T-type channels supported by the Ca(V)3.1, Ca(V)3.2, and Ca(V)3.3 subunits; L-type channels encoded by the Ca(V)1.2,beta(1), and alpha(2)delta(1) subunits; and R-type channels encoded by the Ca(V)2.3 and beta(3) subunits were each studied using a Xenopus oocyte expression system. The weak permeabilities to organic cations were resolved by looking at inward tails generated upon repolarization after a large depolarizing pulse. Large inward NH4+ currents and sizable methylammonium. and dimethylammonium currents were observed in all of the channels tested, whereas trimethylammonium permeated only through L- and R-type channels, and tetramethylammonium currents were observed only in L-type channels. Thus, our experiments revealed an unexpected heterogeneity in pore size among different Ca2+ channels, with L-type channels having the largest pore (effective diameter = 6.2 Angstrom), T-type channels having the tiniest pore (effective diameter = 5.1 Angstrom), and R-type channels having a pore size intermediate between these extremes. These findings ran counter to first-order expectations for these channels based simply on their degree of selectivity among inorganic cations or on the bulkiness of their acidic side chains at the locus of selectivity.