Spiral calcium wave propagation and annihilation in Xenapus laevis oocytes

Intracellular calcium (Ca²⁺) is a ubiquitous second messenger. Information is encoded in the magnitude, frequency, and spatial organization of changes in the concentration of cytosolic free Ca²⁺. Regenerative spiral waves of release of free Ca²⁺ were observed by confocal microscopy in Xenopus laevis oocytes expressing muscarinic acetylcholine receptor subtypes. This pattern of Ca²⁺ activity is characteristic of an intracellular milieu that behaves as a regenerative excitable medium. The minimal critical radius for propagation of focal Ca²⁺ waves (10.4 micrometers) and the effective diffusion constant for the excitation signal (2.3 × 10^-6 square centimeters per second) were estimated from measurements of velocity and curvature of circular wavefronts expanding from foci. By modeling Ca²⁺ release with cellular automata, the absolute refractory period for Ca²⁺ stores (4.7 seconds) was determined. Other phenomena expected of an excitable medium, such as wave propagation of undiminished amplitude and annihilation of colliding wavefronts, were observed.