4, 6‐Dibromo‐3‐hydroxycarbazole (an analogue of caffeine‐like Ca²⁺ releaser), a novel type of inhibitor of Ca²⁺‐induced Ca²⁺ release in skeletal muscle sarcoplasmic reticulum

4,6‐Dibromo‐3‐hydroxycarbazole (DBHC) was synthesized as an analogue of bromoeudistomin D (BED), a powerful Ca²⁺ releaser, and its pharmacological properties were examined. In Ca²⁺ electrode experiments, DBHC (100 μm) markedly inhibited Ca²⁺ release from the heavy fraction of sarcoplasmic reticulum (HSR) induced by caffeine (1 mm) and BED (10 μm). DBHC (0.1 to 100 μm) inhibited ⁴⁵Ca²⁺ release induced by Ca²⁺ from HSR in a concentration‐dependent manner. DBHC (100 μm) abolished ⁴⁵Ca²⁺ release induced by caffeine (1 mm) and BED (10 μm) in HSR. Inhibitory effects of calcium‐induced calcium release (CICR) blockers such as procaine, ruthenium red and Mg²⁺ on ⁴⁵Ca²⁺ release were clearly observed at Ca²⁺ concentrations from pCa 7 to pCa 5.5, and were decreased at Ca²⁺ concentrations higher than pCa 5.5 or lower than pCa 7. However, DBHC decreased Ca²⁺ release induced by Ca²⁺ over the wide range of extravesicular Ca²⁺ concentrations. [³H]‐ryanodine binding to HSR was suppressed by ruthenium red, Mg²⁺ and procaine, but was not affected by DBHC up to 100 μm. [³H]‐ryanodine binding to HSR was enhanced by caffeine and BED. DBHC antagonized the enhancement in a concentration‐dependent manner. 9‐[³H]‐Methyl‐7‐bromo‐eudistomin D, an ³H‐labelled analogue of BED, specifically bound to HSR. Both DBHC and caffeine increased the K_D value without affecting the B_max value, indicating a competitive mode of inhibition. These results suggest that DBHC binds to the caffeine binding site to block Ca²⁺ release from HSR. This drug is a novel type of inhibitor for the CICR channels in SR and may provide a useful tool for clarifying the Ca²⁺ releasing mechanisms in SR. 1995 British Pharmacological Society