Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCA354

Poster Communications

An investigation into the effects of extracellular acidification on mouse uterine contractions

A. M. Almohanna1, K. Noble1, S. Wray1

1. University of Liverpool, Liverpool, United Kingdom.

Introduction: Adequate uterine contraction is crucial to safely delivering the fetus. Acidity can play an important role in affecting myometrial contraction but the results, especially concerning external acidification, have not been consistent. In addition recent work has suggested that repetitive transient acidification of external pH (pHo), can increase force (Alotaibi et al., 2015). This study was designed to determine the effects on myometrium and any gestational differences, of acidic pHo. Methods: C57BL/6J mice were used on day 14 or 19 (term) of gestation. Changes in pHo to 6.9 or 6.5 were made for 10 minutes, by adding HCl to physiological saline. Intracellular acidification at constant external pH (7.4) was made by the addition of the Na butyrate (40 mM, isosmotically substituted). The physiological solutions were buffered with HEPES (pH 7.4 or 6.9) or PIPES (pH 6.5). Spontaneous contractions were measured (frequency, amplitude, duration and area under the curve, AUC) and data analysed using LabScribe 2 and OriginPro 9, respectively. Statistical differences were tested with Student's t test (n=4-7 animals), and taken at P<0.05. Results: At 14 days, extracellular acidification to pH 6.5 significantly increased AUC, mainly due to an increased frequency. Similar but smaller effects were found with pH 6.9. At 19 days, extracellular acidification to 6.5 or 6.9 also significantly increased AUC and frequency, but in addition, amplitude was also significantly increased. In 8 week, non-pregnant mice, as with pregnant ones, extracellular acidification to pH 6.9 also increased the parameters of contraction, however with larger acidification, pH 6.5, contractions were not increased, but rather showed decreases, which in the case of amplitude was significant. To determine if the effects of external acidification were due to induced changes in intracellular pH, the effects of butyrate were examined. Unlike extracellular acidification, at day 14 and day 19, intracellular acidification caused no significant changes. Discussion: In mouse a decrease of pHo stimulated uterine contraction, particularly by increasing amplitude and frequency. These data are consistent with the large increases in force found in non-pregnant guinea-pig myometrium (Naderali and Wray, 1999), and an increased frequency (but not amplitude) in pregnant human myometrium (Pierce et al., 2003). Ion channels sensitive to external acid and conducting inward current (ASICs) have been reported. We have preliminary Western blot data suggesting ASICs are expressed in myometrium, and thus may contribute to increases in force, Although a decrease of pHo will slowly acidify the cytoplasm, the different functional effects of butyrate, i.e. no increase of force, supports the conclusion that there are distinct and stimulatory effects of a decrease in extracellular pH on mouse uterus.

Where applicable, experiments conform with Society ethical requirements