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Elevated intracellular calcium (Ca2+) concentrations impair hamster embryo metabolism and viability (Lane M and Bavister B 1998 Biol. Reprod. 59, 1000–1007). Extracellular magnesium (Mg2+) regulates intracellular Ca2+ by controlling its uptake and release. In the present study, we examined the effects of altering Ca2+ and Mg2+ ion concentrations in Purdue Porcine Medium (PPM1) on porcine embryo mitochondrial distribution, metabolic (glycolytic and Krebs cycle) activity, and in vitro developmental potential. Cumulus-oocyte complexes collected from abattoir ovaries were matured for 40–42h, inseminated with 5×105 sperm mL−1 for 5h, and initially cultured in 1:0.4 or 2:1 ratio of Ca2+ to Mg2+ (concentrations in mM) at 38.7°C, in 6% CO2, 10% O2, balance N2. At 22–26, 46–50, and 70–74h post-insemination, 2-, 4-, and 8-cell embryos, respectively, were removed from culture to evaluate mitochondrial distribution (confocal microscopy after tetramethylrhodamine methyl ester staining) and glycolytic and Krebs cycle activity (5-[3H]-glucose and 2-[C14]-pyruvate, respectively). Remaining embryos were further cultured to determine developmental competence (2:1, n=548; 1:0.4, n=560). Cleavage was assessed on Day 3 (2:1, n=552; 1:0.4, n=560) of culture. All data were analyzed using GLM ANOVA, except mitochondrial distribution data which were analyzed using GLIMMIX. A majority (P&amp;lt;0.05) of 2-cell (65%, 13/20) and 4-cell (67%, 22/33) embryos cultured in 2:1 displayed a homogeneous mitochondrial distribution. More (70%, 21/30; P&amp;lt;0.05) 8-cell embryos cultured in 2:1 had a perinuclear mitochondrial distribution. When cultured in 1:0.4, a majority (61%, 14/23; P&amp;lt;0.05) of 2-cell embryos displayed a cortical mitochondrial distribution, whereas most (P&amp;lt;0.05) 4-cell (66%, 19/29) and 8-cell embryos (69%, 18/26) displayed a homogeneous distribution. Glycolytic and Krebs cycle activities were similar (P&amp;gt;0.05) between treatments and across all cell stages examined. Treatment had no effect (P&amp;gt;0.05) on cleavage or blastocyst total cell number. Unlike hamster embryos, culturing pig embryos in a higher Ca2+ concentration resulted in more embryos developing to the blastocyst stage. Culture medium containing 2mM Ca2+ and 1mMMg2+ best supports in vitro blastocyst development, possibly by supporting a more correct mitochondrial distribution. These results are not mediated via changes in glycolytic or Krebs cycle activity, thus suggesting that another cellular mechanism plays a key role in developmental competence in early pig embryos.Table 1Effects of Ca2+:Mg2+ on porcine embryonic development and metabolic activity (mean±SEM).

147ION COMPOSITION OF CULTURE MEDIUM INFLUENCES MITOCHONDRIAL DISTRIBUTION AND BLASTOCYST DEVELOPMENT OF PREIMPLANTATION PORCINE EMBRYOS