Activation of pig oocytes by intracytoplasmic injection of strontium and barium

Abstract

Ovulated mouse oocytes are activated by exposure to culture medium containing Sr²⁺ or Ba²⁺ or by intracytoplasmic injection of the divalent cations. It is known that in vitro matured pig oocytes are activated by the intracytoplasmic injection of Ca²⁺. In this study, we examined the effect of exposure and of intracytoplasmic injection of Sr²⁺ or Ba²⁺ on in vitro matured pig oocytes (MII-oocytes). When MII-oocytes were exposed to the medium containing divalent cations, no oocytes were activated. However, in the case of oocytes that were injected with Sr²⁺, Ba²⁺ and Ca²⁺, at 6 h after injection, 64%, 71% and 86% of the oocytes had been released from MII-arrest, and 51%, 67% and 84% formed female pronuclei, respectively. The initial transient in intracellular Ca²⁺ concentration ([Ca²⁺]_i) was measured by the Ca²⁺ indicator dye fluo-4 dextran. Microinjection of Sr²⁺, Ba²⁺ or Ca²⁺ induced a rapid elevation of [Ca²⁺]_i. The exocytosis of cortical granules was examined by staining with fluorescein isothiocyanate (FITC)-labelled peanut agglutinin. After an injection of divalent cations, a release of cortical granules was observed in the oocytes. Maturation promoting factor (MPF) activity declined to a low level after 6 h in all the oocytes injected with divalent cations. To test their developmental ability, injected oocytes were treated with cytochalasin B and then cultured for 168 h in NCSU23 medium. After 168 h, 29% (Sr²⁺), 29% (Ba²⁺) and 51% (Ca²⁺) of the oocytes had developed to the blastocyst stage. These results indicate that intracytoplasmic injection of Sr²⁺ and Ba²⁺, like that of Ca²⁺, induces in vitro matured pig oocytes to be released from MII-arrest and leads them into a series of events related to oocyte activation.