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Modulation of force induced by pH in the guinea-pig uterus examined at two stages of the oestrous cycle

Changes in pH have a marked influence on uterine contractility. Changes in uterine pH occur during pregnancy and labour, when marked endocrine changes are occurring. As hormonal status can also influence contractility, this study investigated whether pH-induced modulation of uterine force is influenced by hormonal changes. The effects of altering intracellular and extracellular pH on uterine contractions were studied in guinea-pigs on day 7 (high progesterone) and day 15 (low progesterone) of the oestrous cycle. Resting values of pH were significantly more acidic on day 15 compared with day 7, and more force was produced on day 15. Changing external pH produced similar changes in intracellular pH and force on both days. External acidification was associated with a large increase in force. In contrast, intracellular acidification, at constant external pH, reduced force. In conclusion, the stage of the oestrous cycle has a large effect on resting pH in the myometrium but only small effects on the pH-induced modulation of force, and the link between pH and force is complex.