Distinct pharmacology of 2-hydroxycarbazole-induced Ca2+ release in the sea urchin egg

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.