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

Calcium (Ca2+) and magnesium (Mg2+) are co-factors in the synthetic activit y 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 catio ns could be important factors in the synthesis and/or action of prostagland ins and generation of nitric oxide (NO) which are believed to regulate myom etrial activity particularly for the induction of labour. In the present st udy, the permeability to Ca2+ and Mg2+ of chorioamniotic membranes obtained from women who had undergone term or preterm labour was examined. Diffusio n of Ca2+ and Mg2+ were measured using a system of Plexiglas chambers separ ated by the mounted fetal membrane. Permeability of Mg2+ and Ca2+ through f etal membranes was calculated using non-linear regression analysis. The dat a show highly significant differences in the diffusion of Ca2+ and Mg2+ acr oss 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 resp ectively. 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+ avail able to myometrium and placenta would result in down-regulation of nitric o xide synthase (NOS) activity and thereby a reduction in NO production. This together with an effect on intracellular Ca2+ transport resulting from a r educed availability of Mg2+ would lead to increased myometrial activity in preterm labour.