Abstract

2-Hydroxycarbazole, a compound structurally related to the Ca²⁺-mobilizing marine toxin 9-methyl-7-bromoeudistomin, has recently been proposed to activate both type 1 and type 2 ryanodine receptors in skeletal and cardiac muscle, respectively. This study was undertaken to evaluate the activity of this compound in the sea urchin egg homogenate, a model system used to characterize intracellular Ca²⁺ mobilization mechanisms. 2-Hydroxycarbazole was found to potently release Ca²⁺ in a concentration-dependent manner via a specific mechanism displaying apparent desensitization. Use of selective inhibitors of the Ca²⁺-mobilizing messengers inositol 1,4,5-trisphosphate, cyclic adenosine diphosphate ribose, and nicotinic acid adenine dinucleotide phosphate, as well as desensitization of homogenates to each of these molecules, failed to inhibit the response to 2-hydroxycarbazole. However, the response to 2-hydroxycarbazole was competitively antagonized by caffeine. Investigation of the Ca²⁺ stores accessed by 2-hydroxycarbazole revealed Ca²⁺ release from a thapsigargin-insensitive pool. Finally, 2-hydroxycarbazole failed to enhance [³H]ryanodine binding, suggesting the operation of a nonryanodine receptor mechanism. These results demonstrate that 2-hydroxycarbazole is acting to modulate a Ca²⁺ release mechanism with distinct pharmacological properties to those previously reported in the sea urchin egg.