Characterization of the Ca2+-gated cation channel in sarcoplasmic reticulum vesicles

Two types of sarcoplasmic reticulum (SR) vesicles, i.e., normal (NSR) and heavy (HSR), were prepared by using a centrifugation technique, and further characterization of the cation transport in the two types of SR vesicles was performed by measuring the choline influx. The choline influx in SR vesicles was measured by following the change in light scattering intensity using a stopped flow apparatus. Analysis of the choline influx gave the following results. (1) There exists a Ca2+-gated cation channel in SR vesicles other than the K+-selective, voltage-dependent cation channel. The Ca2+-gated cation channel was activated by both extravesicular Ca2+ and caffeine, and inhibited by procaine and N,N'-dicyclohexylcarbodiimide. (2) About 70% of the HSR vesicles have Ca2+-gated cation channels, whereas only 20% of NSR vesicles have these channels. On the other hand, 60% of HSR vesicles also have the voltage-gated cation channels and 80% of NSR vesicles have these channels. On the basis of various properties of the Ca2+-gated cation channel, it is suggested that this channel must be the one which mediates the Ca2+-induced Ca2+ release.