In situ pH measurements of the Syrian hamster uterus during early pregnancy to determine the role of pH in zone pellucida loss in vivo

The mechanisms of zona pellucida (ZP) loss in peri-implantation hamster emb ryos in vivo versus in vitro are distinctly different. To investigate if ZP loss in vivo is the result of transient uterine pH changes, the luminal pH of the pregnant uterus was measured during the ZP loss period. Prior to ZP loss, pH was 7.30 +/- 0.05 (mean +/- SE; left uterine horn) and 7.35 +/- 0 .03 (right horn). During ZP loss, prl was 7.26 +/- 0.07 (left) and 7.35 +/- 0.03 (right), and after embryo attachment, 7.25 +/- 0.02 (left) and 7.27 /- 0.02 (right). None of these values are statistically different. The pseu dopregnant uterine pH was 7.30 +/- 0.04 (left) and 7.31 +/- 0.04 (right), n ot statistically different from each other or from pregnant uteri. Blastocy st ZP loss in vitro (pH 3.0-8.5) occurred only at pH 3.0. Loss of ZP occurr ed in uterine flushings from pregnant ol pseudopregnant hamsters, evidence that ZP loss is related to uterine factors. Complete ZP loss occurred at pH 6.8, but was incomplete at pH 6.6, 7.0 and 7.2. No ZP loss occurred in ute rine flushings from nea-mated females. rn summary: (i) a change in uterine pH does not cause ZP loss in vivo in the Syrian hamster; (ii) a pH-sensitiv e factor ill pregnant and pseudopregnant uterine fluid is responsible for Z P loss.