Characterization of RyR1-slow, a ryanodine receptor specific to slow-twitch skeletal muscle

Two distinct skeletal muscle ryanodine receptors (RyR1s) are expressed in a fiber type-specific manner in fish skeletal muscle (11). In this study, we compare [H-3] ryanodine binding and single channel activity of RyR1-slow f rom fish slow-twitch skeletal muscle with RyR1-fast and RyR3 isolated from fast-twitch skeletal muscle. Scatchard plots indicate that RyR1-slow has a lower affinity for [H-3] ryanodine when compared with RyR1-fast. In single channel recordings, RyR1-slow and RyR1-fast had similar slope conductances. However, the maximum open probability (P-o) of RyR1-slow was threefold les s than the maximum P-o of RyR1-fast. Single channel studies also revealed t he presence of two populations of RyRs in tuna fast-twitch muscle (RyR1-fas t and RyR3). RyR3 had the highest P-o of all the RyR channels and displayed less inhibition at millimolar Ca2+. The addition of 5 mM Mg-ATP or 2.5 mM beta, gamma -methyleneadenosine 5'-triphosphate (AMP-PCP) to the channels i ncreased the P-o and [H-3] ryanodine binding of both RyR1s but also caused a shift in the Ca2+ dependency curve of RyR1-slow such that Ca2+-dependent inactivation was attenuated. [H-3] ryanodine binding data also showed that Mg2+-dependent inhibition of RyR1-slow was reduced in the presence of AMP-P CP. These results indicate differences in the physiological properties of R yRs in fish slow- and fast-twitch skeletal muscle, which may contribute to differences in the way intracellular Ca2+ is regulated in these muscle type s.