Spiral Calcium Wave Propagation and Annihilation in <i>Xenopus laevis</i> Oocytes

Intracellular calcium (Ca 2+ ) is a ubiquitous second messenger. Information is encoded in the magnitude, frequency, and spatial organization of changes in the concentration of cytosolic free Ca 2+ . Regenerative spiral waves of release of free Ca 2+ were observed by confocal microscopy in Xenopus laevis oocytes expressing muscarinic acetylcholine receptor subtypes. This pattern of Ca 2+ activity is characteristic of an intracellular milieu that behaves as a regenerative excitable medium. The minimal critical radius for propagation of focal Ca 2+ 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 Ca 2+ release with cellular automata, the absolute refractory period for Ca 2+ 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.