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

Abstract

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 time 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 µM PIP2were not modified by ruthenium red. PIP2(>0.1 µM) markedly accelerated the rate of45Ca2+efflux from SR vesicles in a concentration-dependent manner. The PIP2-induced45Ca2+efflux was potentiated by ruthenium red but profoundly inhibited by La3+. The concentration-response curve for Ca2+or Mg2+in PIP2-induced45Ca2+release was clearly different from that in the Ca2+-induced Ca2+release. PIP2caused a concentration-dependent increase in Ca2+release from SR of chemically skinned fibers from skeletal muscle. Furthermore, [3H]ryanodine or [3H]methyl-7-bromoeudistomin D (MBED) binding to SR was increased by PIP2in a concentration-dependent manner. These observations present the first evidence that PIP2most 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).Key words: phosphatidylinositol 4,5-bisphosphate, sarcoplasmic reticulum, calcium release, ryanodine receptor, ryanodine.