Bromo-eudistomin D, a novel inducer of calcium release from fragmented sarcoplasmic reticulum that causes contractions of skinned muscle fibers

Bromo-eudistomin D induced a contraction of the chemically skinned fibers from skeletal muscle at concentrations of 10 μM or more. This contractile response to bromo-eudistomin D was completely blocked by 10 mM procaine. The extravascular Ca²⁺ concentrations of the heavy fractions of the fragmented sarcoplasmic reticulum (HSR) were measured directly by a Ca²⁺ electrode to examine the effect of bromo-eudistomin D on the sarcoplasmic reticulum. After the HSR was loaded with Ca²⁺ by the ATP-dependent Ca²⁺ pump, the addition of 10 μM bromo-eudistomin D caused Ca²⁺ release that was followed by spontaneous Ca²⁺ reuptake. In the presence of 2 μM ruthenium red or 4 mM MgCl₂, no Ca²⁺ release was induced by 20 μM bromo-eudistomin D. The rate of ⁴⁵Ca²⁺ efflux from HSR, which had been passively preloaded with ⁴⁵Ca²⁺, was accelerated 7 times by 10 μM bromo-eudistomin D. The concentration of bromo-eudistomin D for half-maximum effect on the apparent efflux rate was 1.5 μM, while that of caffeine was 0.6 mM. The bromo-eudistomin D-evoked efflux of ⁴⁵Ca²⁺ was abolished by 2 μM ruthenium red or 0.5 mM MgCl₂. Bromo-eudistomin D was found to be 400 times more potent than caffeine in its Ca²⁺-releasing action but was similar in its action in other respects. These results indicate that bromo-eudistomin D may induce Ca²⁺ release from the sarcoplasmic reticulum through physiologically relevant Ca²⁺ channels.