Multiple types of Ca2+ channels in visceral smooth muscle cells

Single-channel currents were recorded from two classes of Ca²⁺ channels in visceral smooth muscle cells isolated from the stomach of the toad, Bufo marinus: a class of small-conductance channels (approximately 11 pS) and a class of large-conductance channels (approximately 26 pS). Small-conductance channels were present in a majority of patches and gave rise to a slowly inactivating current (t_1/2 250 ms at 0 mV). Openings of large-conductance channels could be unequivocally resolved only in the presence of the dihydropyridine Ca²⁺ agonist Bay K 8644. Two subtypes of the large-conductance channels were found — those with a very slow rate of decay (> 500 ms) and those with a faster one (< 100ms). Large-conductance channels resemble L-type Ca²⁺ channels of other preparations. Small-conductance channels do not fit unambiguously into the other existing categories (i.e., N or T). Correspondence between single-channel and macroscopic Ca²⁺ currents is discussed.