Two novel types of calcium release from skeletal sarcoplasmic reticulum by phosphatidylinositol 4,5-bisphosphate

In both the heavy and light fractions of fragmented sarcoplasmic reticulum (SR) vesicles from the fast skeletal muscle, about 27 min after beginning the active Ca2+ uptake, the extravesicular Ca2+ concentration suddenly increased to reach a steady level (delayed Ca2+ release). Phosphatidylinositol 4,5-bisphosphate (PIP2) not only shortened the rime to delayed Ca2+ release but also induced prompt Ca2+ release from the heavy fraction of SR. Delayed Ca2+ release and prompt Ca2+ release stimulated by 100 mu M PIP2 were not modified by ruthenium red. PIP2 (>0.1 mu M) markedly accelerated the rate of Ca-45(2+) efflux from SR vesicles in a concentration-dependent manner. The PIP2-induced Ca-45(2+) efflux was potentiated by ruthenium red but profoundly inhibited by La3+. The concentration-response curve for Ca2+ or Mg2+ in PIP2-induced Ca-45(2+) release was clearly different from that in the Ca2+-induced Ca2+ release. PIP2 caused a concentration-dependent increase in Ca2+ release from SR of chemically skinned fibers from skeletal muscle. Furthermore, [H-3]ryanodine or [H-3]methyl-7-bromoeudistomin D (MBED) binding to SR was increased by PIP2 in a concentration-dependent manner. These observations present the first evidence that PIP2 most likely activates two types of SR Ca2+ release channels whose properties are entirely different from those of Ca2+-induced Ca2+ release channels (the ryanodine receptor 1).