Initiation, Persistence, and Cessation of the Series of Intracellular Ca²⁺ Responses during Fertilization of Bovine Eggs

Abstract

A long-lasting series of transient rises in intracellular Ca²⁺ concentration ([Ca²⁺]_i) were recorded with Ca²⁺ imaging during fertilization of bovine eggs matured in vivo or in vitro. The first Ca²⁺ response could be recorded in zona pellucida-free eggs within 1 min after sperm-egg contact, when a preceding slow [Ca²⁺]_i rise reached 150-170 nM. The response had the peak of 565 ±32 nM (n=15; maximum 780 nM) and the duration of 4-6 min, being significantly larger and longer than any succeeding response (359 ± 12 nM, n=68; 2 min). The series of Ca²⁺ transients occurred at constant intervals of 19-20 min without attenuation in the magnitude for more than 9 h. Ca²⁺ transients became smaller progressively during pronuclear migration and nuclear envelope breakdown, and no response occurred in 2-cell embryo. In unfertilized eggs, injection of inositol 1, 4, 5-trisphophate (InsP₃) induced a Ca²⁺ transient and the sulfhydryl reagent thimerosal caused repetitive Ca²⁺ transients. These responses were markedly inhibited or blocked by preinjection of a monoclonal antibody to the mouse InsP₃ receptor. However, sperm-induced responses were not blocked in 2/3 of eggs examined. Another mechanism may be involved in addition to InsP₃-induced Ca²⁺ release from intracelluar stores.