Ca²⁺ wave dispersion and spiral wave entrainment in Xenopus laevis oocytes overexpressing Ca²⁺ ATPases

Complex, spatiotemporal patterns of intracellular Ca²⁺ release in Xenopus oocytes can be accounted for by the operation of two fundamental processes: Ca²⁺ release from the endoplasmic reticulum (ER) via the inositol 1,4,5-trisphosphate receptor (IP₃R) with its inherent dependency on cytosolic Ca²⁺, and Ca²⁺ uptake via Ca²⁺ ATPases. Overexpression of sarco-endoplasmic reticulum Ca²⁺ ATPases (SERCAs) in Xenopus oocytes, increases IP₃-induced Ca²⁺ wave frequency and amplitude [ 1-3]. This effect can be attributed to an increased removal of cytoplasmic Ca²⁺ and more efficient refilling of Ca²⁺ stores. By overexpressing SERCA isoforms, we report here that Ca²⁺ waves exhibit dispersion [4-6]. At wavelengths greater than 60 μm, wave velocity is constant. However, wave velocity and amplitude progressively decreases at smaller wavelengths. Below λ -20 μm, Ca²⁺ waves disperse and fail to propagate. In oocytes exhibiting both spiral and target patterns of Ca²⁺ release, spiral waves had higher frequencies and showed entrainment of the surrounding regions. These properties are characteristic of a classical excitable medium [4-6].