Molecular mechanisms of intracellular calcium excitability in X. laevis oocytes

Following receptor activation in Xenopus oocytes, spiral waves of intracellular Ca²⁺ release were observed. We have identified key molecular elements in the pathway that give rise to Ca²⁺ excitability. The patterns of Ca²⁺ release produced by GTP-γ-S and by inositol 1,4,5-trisphosphate (IP₃) are indistinguishable from receptor-induced Ca²⁺ patterns. The regenerative Ca²⁺ activity is critically dependent on the presence of IP₃ and on the concentration of intracellular Ca²⁺, but is independent of extracellular Ca²⁺. Broad regions of the intracellular milieu can be synchronously excited to initiate Ca²⁺ waves and produce pulsating foci of Ca²⁺ release. By testing the temperature dependence of wavefront propagation, we provide evidence for an underlying process limited by diffusion, consistent with the elementary theory of excitable media. We propose a model for intracellular Ca²⁺ signaling in which wave propagation is controlled by IP₃-mediated Ca²⁺ release from internal stores, but is modulated by the cytoplasmic concentration and diffusion of Ca²⁺.