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

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.