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

Abstract

The mechanisms of zona pellucida (ZP) loss in peri-implantation hamster embryos 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, pH 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 pseudopregnant uterine pH was 7.30 0.04 (left) and 7.31 0.04 (right), not statistically different from each other or from pregnant uteri. Blastocyst ZP loss in vitro (pH 3.0–8.5) occurred only at pH 3.0. Loss of ZP occurred in uterine flushings from pregnant or 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 uterine flushings from non-mated females. In summary: (i) a change in uterine pH does not cause ZP loss in vivo in the Syrian hamster; (ii) a pH-sensitive factor in pregnant and pseudopregnant uterine fluid is responsible for ZP loss.