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

Abstract

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