A wave of IP sub(3) production accompanies the fertilization Ca super(2+) wave in the egg of the frog, Xenopus laevis: theoretical and experimental support

• Published in: Cell calcium (Edinburgh) 2004-05, Vol.36 (5), p.433-447
• Main Authors: Wagner, J, Fall, C P, Hong, F, Sims, CE, Allbritton, N L, Fontanilla, R A, Moraru, I I, Loew, L M, Nuccitelli, R
• Format: Article
• Language: English
• Subjects: Anura; Xenopus laevis
• ISSN: 0143-4160
• DOI: 10.1016/j.ceca.2003.10.009

The fertilization Ca super(2+) wave in Xenopus laevis is a single, large wave of elevated free Ca super(2+) that is initiated at the point of sperm-egg fusion and traverses the entire width of the egg. This Ca super(2+) wave involves an increase in inositol-1,4,5-trisphosphate (IP sub(3)) resulting from the interaction of the sperm and egg, which then results in the activation of the endoplasmic reticulum Ca super(2+) release machinery. The extraordinarily large size of this cell (1.2 mm diameter) together with the small surface region of sperm-receptor activation makes special demands on the IP sub(3)-dependent Ca super(2+) mobilizing machinery. We propose a detailed model of the fertilization Ca super(2+) wave in Xenopus eggs that requires an accompanying wave of IP sub(3) production. While the Ca super(2+) wave is initiated by a localized increase of IP sub(3) near the site of sperm-egg fusion, the Ca super(2+) wave propagates via IP sub(3) production correlated with the Ca super(2+) wave, possibly via Ca super(2+)-mediated PLC activation. Such a Ca super(2+)-mediated IP sub(3) production wave has not been required previously to explain the fertilization Ca super(2+) wave in eggs; we argue this is necessary to explain the observed IP sub(3) dynamics in Xenopus eggs. To test our hypothesis, we have measured the IP sub(3) levels from 20 nl "sips" of the egg cortex during wave propagation. We were unable to detect the low IP sub(3) levels in unfertilized eggs, but after fertilization, [IP sub(3)] ranged from 175 to 430 nM at the sperm entry point and from 120 to 700 nM 90 degree away once the Ca super(2+) wave passed that region about 2 min after fertilization. Prior to the Ca super(2+) wave reaching that region the IP sub(3) levels were undetectable. Since significant IP sub(3) could not diffuse to this region from the sperm entry point within 2 min, this observation is consistent with a regenerative wave of IP sub(3) production.