Objective: 

To investigate in vitro the effect of short multiple episodes of hypoxia on rat uterine contractility.

Methods: 

Longitudinal uterine strips (2mm×10mm) from pregnant and non pregnant rats were dissected and mounted in organ bath, bubbled with Hepes buffered, oxygenated physiological saline, for isometric force recording. The effect of repeated 5 min hypoxia was induced by replacing the O₂ for N₂, Paired control experiments were bubbled with O₂ throughout. Contractions were analyzed during and after each hypoxic episode.

Results: 

In stable, spontaneously active preparations, the effect of a single episode of hypoxia was to significantly decrease or abolish the contractions in both pregnant and non pregnant rats (n=12 and 6 respectively). In pregnant rats, there was a gradual increase in force during and after each hypoxic episode. By the 5th hypoxic episode the amplitude of contractions upon re-oxygenation was significantly higher (141+/- 8.7% compared to 100% control) than that of the control period and was unchanged in time matched controls. There was also increased contractile activity during the hypoxic period. In contrast, in non pregnant rats hypoxic episodes always abolished contractions and although there was recovery with re-oxygenation, activity never exceeded that seen at the start of the experiments.

Conclusions: 

We concluded that transient, repeated hypoxic episodes significantly increase the force amplitude produced by pregnant rat uterus. This suggests that hypoxic preconditioning may be present in the uterus. In non pregnant rats, the phenomenon was not seen, suggesting that this could be part of a mechanism switched on in preparation for labour.