Proceedings of The Physiological Society
University of Edinburgh (2011) Proc Physiol Soc 25, PC36
The effects of transient multiple episodes of hypoxia on spontaneous rat uterine contractility: does hypoxic preconditioning occur?
M. Alotaibi1, S. Wray1
1. Physiology, Liverpool University, Liverpool, United Kingdom.
During labour the myometrium experiences transient hypoxic episodes as contractions occlude blood vessels, yet labour progresses and contractions increase. We have therefore investigated the effect of short multiple episodes of hypoxia on uterine contractility. Methods: Longitudinal uterine strips from pregnant and non pregnant rats were dissected and mounted in organ bath, bubbled with Hepes buffered, oxygenated physiological saline (2 mM Ca, pH 7.4), for isometric force recording. The effect of repeated 5 min hypoxia was induced by replacing the oxygen for nitrogen. In other protocols uterine preparations from pregnant rats were subjected to5 min episodes of repeated extracellular acidosis(from pH 7.4 to 6.9) or to hypoxia and 0-Ca (EGTA) to determine their role in the effects produced by hypoxia. Result: 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. Repeated 5 min episodes of 0-Ca in the presence of Oxygen abolished force during the episode and force rapidly recovered upon changing back to normal solution and although there was a transient increase in force amplitude activity never exceeded control (99+/-0.08% compared to 100% control, n= 7). However with repeated 0-Ca and hypoxia simultaneously we found a sustained gradual increase in force upon reoxygenation which was significantly different from that found in presence of Ca (159+/-9%,compared to 100% control, n= 10). We also found a gradual increase in force amplitude after each acidotic episode (after return to normal pH) (139+/-5% compared to 100% control, n= 8). CONCLUSIONS: We concluded that transient, repeated hypoxic episodes significantly increase the amplitude of force 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. We also concluded that the extracellular acidosis associated with hypoxia could be a part of mechanism involved in hypoxic preconditioning and Ca entry may not be involved.
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