Modulation of force induced by pH in the guinea-pig uterus examined at twostages 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 s tudy investigated whether pH-induced modulation of uterine force is influen ced by hormonal changes. The effects of altering intracellular and extracel lular 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 simil ar changes in intracellular pH and force on both days. External acidificati on was associated with a large increase in force. In contrast, intracellula r acidification, at constant external pH, reduced force. In conclusion, the stage of the oestrous cycle has a large effect on resting pH in the myomet rium but only small effects on the pi-I-induced modulation of force, and th e link between pH and force is complex.