Characterization of type 3 ryanodine receptor (RyR3) of sarcoplasmic reticulum from rabbit skeletal muscles.

We investigated type 3 isoform (RyR3) of ryanodine receptor in rabbit skeletal muscles using an antibody specific for RyR3. By Western blot analysis and by immunoprecipitation, a single polypeptide for RyR3 was detected in sarcoplasmic reticulum vesicles from rabbit diaphragm but not in those from back muscle. The molecular mass was slightly smaller than that of RyR1, the major isoform in skeletal muscles. Each of RyR1 and RyR3 formed a homotetramer in rabbit diaphragm. RyR3 had a single class of [3H]ryanodine binding sites of high affinity (KD = 1.6 nM). From the Bmax of the binding, the content of RyR3 was estimated to be only 0.6% of RyR1 in rabbit diaphragm. -3H-Ryanodine binding to RyR3 was biphasically dependent on Ca2+, as is true of RyR1, and was stimulated further by adenine nucleotide, caffeine, or high salt concentration. Procaine and ruthenium red inhibited the binding. RyR3 was more resistant to Mg2+ inhibition than RyR1. Interestingly, RyR3 showed about a 7-fold lower Ca2+ sensitivity for activation than RyR1. Comparison with the counterparts in bullfrog skeletal muscles indicates that the Ca2+ sensitivities of RyR3 homologs are similar to each other, whereas those of RyR1 homologs are species-specific.