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 acti vate both type 1 and type 2 ryanodine receptors in skeletal and cardiac mus cle, respectively. This study was undertaken to evaluate the activity of th is compound in the sea urchin egg homogenate, a model system used to charac terize intracellular Ca2+ mobilization mechanisms. 2-Hydroxycarbazole was f ound to potently release Ca2+ in a concentration-dependent manner via a spe cific mechanism displaying apparent desensitization. Use of selective inhib itors of the Ca2+-mobilizing messengers inositol 1,4,5-trisphosphate, cycli c adenosine diphosphate ribose, and nicotinic acid adenine dinucleotide pho sphate, as well as desensitization of homogenates to each of these molecule s, failed to inhibit the response to 2-hydroxycarbazole. However, the respo nse to 2-hydroxycarbazole was competitively antagonized by caffeine. Invest igation of the Ca2+ stores accessed by 2-hydroxycarbazole revealed Ca2+ rel ease from a thapsigargin-insensitive pool. Finally, 2-hydroxycarbazole fail ed to enhance [H-3]ryanodine binding, suggesting the operation of a nonryan odine receptor mechanism. These results demonstrate that 2-hydroxycarbazole is acting to modulate a Ca2+ release mechanism with distinct pharmacologic al properties to those previously reported in the sea urchin egg.