Objective: 

Dysfunctional labour is one of the commonest causes for emergency caesarean section (CS) and is characterised by weak, uncoordinated uterine contractions. Lactate has been previously shown to increase in the myometrium during dysfunctional labour (Quenby et al 2004) but currently no work has been done to investigate the effect of lactate on the myometrium. We have therefore examined its effect on contractility.

Methods: 

Myometrial strips were taken from late pregnant rats or biopsies obtained with consent from women undergoing elective CS at term. The effects of contractility in response to lactate (1–20mM) and other weak acids (Butyrate, propionate and pyruvate, 5–20mM) were recorded. In some experiments, simultaneous force and intracellular pH analysis were carried out.

Results: 

Lactate significantly decreased contractility (n=8 rat, n=4 human) A dose dependent decrease in integral force of contraction was seen, which was significant at 5mM (rat=36%±14%, human=44%±17%) relative to control (100%). Other weak acids also significant affected contractions in a dose dependent manner. Measurements of pHi suggest this response is associated with intracellular acidification.

Conclusions: 

Lactate in the physiological range and other weak acids potently decrease myometrial contractility. Initial pHi measurements suggest its effect is due to increasing acidity of the myometrium, which will decrease Ca²⁺ current. This could therefore cause the decreased contractions seen in dysfunctional labour. Ongoing investigations of Ca²⁺ signalling will shed further mechanistic insight.