Identification of Ca2+-Release Channels in Smooth Muscle and Isolated Membranes

Ca2+ plays an important role in muscle contraction. Two major Ca2+ release channels have been suggested to be present in smooth muscle by functional studies: one is sensitive to inositol 1,4,5-trisphosphate (IP3) and the other is operated by Ca2+ and also sensitive to ryanodine. The existence of these two channels in smooth muscle has been confirmed recently by radioligand binding studies using subcellular membrane and named IP3 and ryanodine receptors. The isolated IP3 and ryanodine receptors from smooth muscles show a channel activity and can be operated by IP3 and ryanodine, respectively. Some similarities of these two receptors have been noted at the molecular level, but they are most likely to be two separate proteins. Although both channels are believed to be involved in regulating cytosol Ca2+ level in smooth muscle, their relative importance in different smooth muscles is still not very clear.