Thapsigargin activates a calcium influx pathway in the unfertilized mouse egg and suppresses repetitive calcium transients in the fertilized egg.

Abstract

At fertilization, the sperm initiates development of the mouse egg by inducing a large transient increase in the intracellular Ca2+ concentration ([Ca2+]i), which is followed by repetitive transient increases in [Ca2+]i. To determine how the repetitive Ca2+ transients are produced, thapsigargin, an inhibitor of the endoplasmic reticulum Ca-ATPase, was used to deplete intracellular Ca2+ stores within the egg. In the unfertilized egg, thapsigargin (1-50 microM) caused a slowly rising and falling transient increase in [Ca2+]i with or without extracellular Ca2+. An influx pathway for Ca2+ is activated by thapsigargin, since an immediate increase in [Ca2+]i occurred when Ca2+ was added to eggs after thapsigargin treatment in a Ca2+, Mg(2+)-free medium. This suggests that Ca2+ entry in the mouse egg may be coupled to the emptying of an intracellular store. The magnitude of the first Ca2+ transient at fertilization was reduced by as much as 84% in eggs pretreated with thapsigargin. Reduction of extracellular Ca2+, by addition of a Ca2+ chelator, suppressed the repetitive Ca2+ transients following fertilization. The Ca2+ transients also require filling of an intracellular store; they were suppressed when thapsigargin was added before or after fertilization. These results support the hypothesis that the first sperm-induced Ca2+ transient at fertilization depletes an intracellular Ca2+ store, triggering an increase in plasma membrane Ca2+ permeability, and that the enhanced Ca2+ influx causes repetitive Ca2+ transients due to the periodic filling and emptying of an intracellular Ca2+ store.