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Sperm Initiate a Ca²⁺ Wave in Frog Eggs that is More Similar to Ca²⁺ Waves Initiated by IP₃ than by Ca²⁺

Andrej Bugrim^*, Ray Fontanilla, Bridget B. Eutenier, Joel Keizer^* and Richard Nuccitelli

^* Institute of Theoretical Dynamics and Section of Molecular and Cellular Biology, University of California, Davis, California 95616 USA

Correspondence: Address reprint requests to Richard Nuccitelli, RPN Enterprises, Inc., 144 Carroll St., New Britain, CT 06053.

We have measured the initial propagation velocity of the sperm-induced Ca²⁺ wave in the egg of Xenopus laevis and have compared it with the initial propagation velocities of the inositol triphosphate (IP₃)-induced and Ca²⁺-induced Ca²⁺ waves. The initial mean propagation velocity of the sperm-induced wave (13 µm/s) is very similar to that of the IP₃-induced waves (12.3 µm/s) and two times faster than the mean Ca²⁺-induced wave velocity (6.6 µm/s). We have generated realistic simulations of the fertilization wave in the frog egg using a computational technique based on the finite difference method. Modeling refinements presented here include equations for the production, degradation, and diffusion of IP₃, a description for Ca²⁺ dynamics in the endoplasmic reticulum, and a highly concentrated endoplasmic reticulum in the egg cortex. We conclude that models incorporating sperm-induced IP₃ generation fit the data best and those involving the influx of either Ca²⁺ or a diffusible sperm factor fit the data poorly. This independence from Ca²⁺ influx is also supported by electrophysiological data indicating that Ca²⁺ influx is not needed to maintain open Cl^- channels that generate the fertilization potential.