Functional effects of intracellular pH alteration in the human uterus: simultaneous measurements of pH and force

Changes in intracellular pH have differing effects on contraction in different types of muscle, and between species. Because of the large number of women requiring a caesarean section due to failure to progress in labour, it is important to know how the human myometrium responds to the changes in intracellular pH that may occur during labour. The pH-sensitive dye, carboxy-SNARF, was used to measure intracellular pH in small strips of human myometrium. Intracellular pH and tension were simultaneously recorded in pregnant and nonpregnant tissue. Intracellular pH was altered by the addition of weak acids and bases. Intracellular alkalinization caused an increase in the frequency and amplitude of contractions. Intracellular acidification led to an initial increase in the frequency and/or the amplitude of the contractions, followed by abolition of contractions. Alterations in intracellular pH had profound effects on contraction in human uterine smooth muscle. Possible mechanisms are discussed whereby pH could influence force production, and changes in contraction are related to the speed and extent of the change in intracellular pH.