Abstract

9-Methyl-7-bromoeudistomin D (MBED), a derivative of eudistomin D isolated from a marine tunicate, induced Ca++ release from the heavy fraction of fragmented sarcoplasmic reticulum (HSR) in the same way as that of caffeine, followed by spontaneous Ca++ reuptake in the Ca++ electrode experiment. The rate of 45Ca++ efflux from HSR vesicles was accelerated markedly by MBED or caffeine in a concentration-dependent manner. The 50% effective concentrations of MBED and caffeine were approximately 1 microM and 1 mM, respectively, indicating that MBED is 1000 times more potent than caffeine in HSR. Procaine, ruthenium red or Mg++ caused concentration-dependent inhibition of MBED-triggered Ca++ release from HSR. The bell-shaped profile of Ca++ dependence for MBED is very similar to that of caffeine. The caffeine-produced maximum response of 45Ca++ efflux was increased further by adenosine-5'-(beta, gamma-methyl-ene)triphosphate, whereas that was not changed by MBED. MBED also caused Ca++ release from sarcoplasmic reticulum (SR) of chemically skinned fibers. These stimulatory effects of MBED on the Ca++ release from skeletal muscle SR were almost indistinguishable from those of caffeine except the difference in potencies. The [3H]ryanodine binding to junctional terminal cisternae membranes was not inhibited by MBED or caffeine. MBED did not cause Ca++ release from the light fraction of fragmented SR and turbidity change of mitochondrial suspension. These observations suggest a most likely idea that MBED binds to the caffeine-binding site in the Ca channel protein and thus produces the potentiation of Ca(++)-induced Ca++ release from SR.(ABSTRACT TRUNCATED AT 250 WORDS)