Porcine oocyte activation: Differing roles of calcium and pH

Intracellular pH has recently been shown to increase during parthenogenetic activation of the porcine oocyte. In the following set of experiments, int racellular pH was monitored during activation and pronuclear development wa s assessed following activation treatments with calcium, in the absence of calcium, and in oocytes loaded with the calcium chelator BAPTA-AM in calciu m-free medium. Intracellular pH increase was not different among groups whe n treating with 7% ethanol or 50 muM calcium ionophore, or during treatment with thimerosal for 12 or 25 min. Activation with thimerosal (200 muM, 12 min) followed by 8 mM dithiothreitol (DTT, 30 min) resulted in a decreased pronuclear development in calcium-free medium with or without BAPTA-AM load ed oocytes as compared to controls. Activation with 50 muM calcium ionophor e resulted in pronuclear development that was different between the calcium -free and BAPTA-AM loaded oocytes in calcium-free medium. Similar incidence s of pronuclear formation were observed in all ethanol treatment groups. It was concluded that external calcium as well as large changes in intracellu lar free calcium are not necessary for the increase in intracellular pH, bu t normal intracellular calcium signaling is critical for normal levels of p ronuclear development. Finally, oocytes were measured for intracellular pH changes for 30 min following subzonal sperm injection. Intracellular pH did not increase, although pronuclear formation was observed 6 hr post SUZI. T his suggested that major differences were still present between sperm-induc ed and parthenogenetic activation of the porcine oocyte. Mol. Reprod. Dev. 59:227-234, 2001. (C) 2001 Wiley-Liss, Inc.