Membrane potential of smooth muscle cells of human placental chorionic vessels. Comparative effects of MgCl2 and MgSO4.

Magnesium has been shown to produce various effects on vascular smooth muscle, in that its absence elicits an increase in muscle tone and increases sensitivity to a number of stimulatory agonists. In the present study, the influence of MgCl2 and MgSO4 was investigated in human placental chorionic muscle cells (from arteries and veins with or without endothelium), especially on the membrane potential, Um. Classically, Um was obtained from microelectrodes inserted into smooth muscle cells through the endothelium or directly. In arteries with endothelium, the smooth muscle cells were depolarized by MgSO4 (threshold: 6 mM) and MgCl2 (threshold: 8 mM). In endothelium-free arteries, the smooth muscle cells were also depolarized by MgSO4 (threshold: 8 mM) and MgCl2 (threshold: 6 mM). Identical results were obtained with veins, but thresholds were different. The different thresholds may indicate that MgCl2 influences the cell membrane potential directly, while MgSO4 interferes first with the endothelial cells, which may act as an intermediary between the Mg2+ ions and the membrane of the smooth muscle cells.