In vitro-matured porcine oocytes were given injections of 0.1 M CaCl2
and after 6 h evaluated for signs of early and late activation events.
CaCl2 injection caused cortical granule exocytosis in 75% (3 of 4) of
the oocytes tested. It also induced cell cycle resumption as monitore
d by the histone H1 kinase assay: the phosphorylation rate of histone
H1 decreased to 36.7% of the original value. Treated oocytes completed
meiosis, extruded the second polar body, and progressed to first inte
rphase: 79.4% of them formed one or more pronuclei. The elevated intra
cellular Ca2+ level resulted in activation-related changes in the prot
ein synthetic profile in 90% (9 of 10) of the oocytes. Furthermore, 14
.7% (9 of 61) of the treated oocytes developed to the compact morula/e
arly blastocyst stage after a 7-day culture in ligated porcine oviduct
, and one blastocyst hatched from the zona pellucida. Control oocytes
given injections of 0.1 M MgCl2 or carrier medium (10 mM Hepes) did no
t show the changes mentioned. The results strengthen the idea that Ca2
+ is a cell messenger that plays a central part in oocyte activation;
it is concluded that elevated intracellular Ca2+ level caused by a sin
gle injection of CaCl2 leads to both early and late events of porcine
oocyte activation.