Permeability of fetal membranes to calcium and magnesium: possible role in preterm labour

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Abstract
Calcium (Ca2+) and magnesium (Mg2+) are co-factors in the synthetic activity of a variety of enzymes and in the secretory process. Both the binding to fetal membranes and the diffusion through the membranes of these two cations could be important factors in the synthesis and/or action of prostaglandins and generation of nitric oxide (NO) which are believed to regulate myometrial activity particularly for the induction of labour. In the present study, the permeability to Ca2+ and Mg2+ of chorioamniotic membranes obtained from women who had undergone term or preterm labour was examined. Diffusion of Ca2+ and Mg2+ were measured using a system of Plexiglas chambers separated by the mounted fetal membrane. Permeability of Mg2+ and Ca2+ through fetal membranes was calculated using non-linear regression analysis. The data show highly significant differences in the diffusion of Ca2+ and Mg2+ across fetal membranes between preterm and term labour. Transport coefficient K for Ca2+ was 0.203 h–1 and 0.0223 h–1 in term and preterm labour respectively. The corresponding values for Mg2+ were –0.017 h–1 and 0.051 h–1 respectively. It is proposed that a considerable reduction in Ca2+ available to myometrium and placenta would result in down-regulation of nitric oxide synthase (NOS) activity and thereby a reduction in NO production. This together with an effect on intracellular Ca2+ transport resulting from a reduced availability of Mg2+ would lead to increased myometrial activity in preterm labour.

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