Magnesium regulation of Ca2+ channels in smooth muscle and endothelial cells of human allantochorial placental vessels.

In human allantochorial placental vessels, the vascular tone is regulated by membrane potential controlled by the ion flux through K+ and Ca2+ channels. The effects of MgCl2 and MgSO4 were studied on the membrane potential of vascular smooth muscle cells (VSMCs) and of endothelial cells (VECs). The membrane potential was the main factor of the excitation-contraction coupling of placental vessels. The VSMCs and VECs were predepolarized by high external K+, which blocked voltage-sensitive K+ channels, and depolarized by serotonin addition which activated Ca2+ influx through voltage-gated Ca2+ channels. Addition of MgCl2 or MgSO4 in the external medium induced a depolarization level lower than the previous level, as nifedipine a Ca2+ blocker, corresponding to a Ca2+ influx reduction in VSMCs and VECs and inducing relaxation of the cells. The effect of MgCl2 and MgSO4 was the same on VSMCs and VECs, but the depolarization level reduction was more important with MgCl2 than MgSO4 on VSMCs. These data suggested that Mg salts regulated the Ca2+ influx through voltage-gated Ca2+ channels in VSMCs and VECs and consequently the tonus of human allantochorial placental vessels and that there was a difference between Mg salts.