Calcium channel activity in a purified dihydropyridine-receptor preparation of skeletal muscle.
A purified dihydropyridine-receptor complex (DHPR) of skeletal muscle consisting of a major polypeptide of Mr 150K under reducing conditions induces divalent cation selective channels when incorporated into planar lipid bilayers. Channels were inserted into preformed planar bilayers by two techniques: (i) direct dilution of detergent-solubilized DHPR into the aqueous chambers adjacent to the bilayer membrane or (ii) reconstitution of DHPR into phospholipid vesicles followed by fusion of the preformed vesicles to the planar bilayer membrane. Unlike native membrane preparations of t-tubules, which only have one major Ca channel type of slope conductance of 12 pS in symmetrical 100 mM Ba, the purified DHPR complex induced at least two channel types with conductances of 12-14 and 22 pS. Some recordings suggest that these two channels are statistically coupled in time, i.e., that they may correspond to substrates of the same DHPR channel. Activity was found to occur spontaneously in the absence of the Ca channel agonist Bay k 8644. The 12-14-pS channel from DHPR exhibits voltage-dependent kinetics, is highly selective for barium ions, and was inhibited by micromolar nitrendipine. The 12-14-pS DHPR channel appears to be identical with functional Ca channels previously described in native t-tubules.