Inositol trisphosphate is required for the propagation of calcium waves in Xenopus oocytes

Stimuli which act through the second messenger inositol 1,4,5-trisphosphate (InsP₃) often increase free intracellular Ca²⁺ concentration ([Ca²⁺]_i) in a localized subcellular area. Actively propagated Ca²⁺ waves then extend this focal Ca²⁺ signal to other parts of the cell. To understand how cells may control the spatial distribution of Ca²⁺, we investigated the mechanism by which Ca²⁺ waves propagate through the cytoplasm of Xenopus oocytes. Heparin, which inhibits the binding of InsP₃ to its receptor, prevented the migration of Ca²⁺ waves induced by a poorly metabolized InsP₃ (InsP₃S₃). This result suggested that Ca²⁺ waves move through the cell via the serial release of Ca²⁺ from InsP₃-sensitive stores. Interventions which caused a localized increase in [Ca²⁺]_i without elevations of InsP₃ did not trigger Ca²⁺ waves. In the presence of a Ins-P₃S₃, however, endogenously released or locally injected Ca²⁺ elicited Ca²⁺ waves. A cooperative interaction between Ca²⁺ and InsP₃ may therefore be responsible for the propagation of Ca²⁺ waves.