Unique Vasocontraction of Okadaic Acid Isolated from Black Sponge, Independent of Extracellular Ca ²⁺

Abstract

Okadaic acid (OA) isolated from black sponge ( Halichondria) caused tonic contractions of human umbilical arteries and rabbit aorta both in the presence and absence of Ca²⁺. This tonic contraction was not affected by Ca²⁺ chelator, Ca²⁺ entry blockers and La³⁺. In addition, the antagonists of alpha-adrenoceptors, histamine, serotonin and ACh receptors had no effect on the OA-induced contraction. High K, ouabain and indomethacin failed to inhibit the response to OA. However, the combination of anaerobic conditions and absence of glucose abolished the response to OA. OA had no effect on the myosin B ATPase and saponin-treated skinned fibers of rabbit aorta. The contractile action of OA may not also be related to calmodulin-related PDE and mitochondrial respiration. In conclusion, although the precise mode of action is not evident at the present time, OA, in its unique pharmacological action – that of producing sustained contraction independent of extracellular Ca²⁺ -may alter the handling of Ca²⁺ to intracellular store sites.