Implication of external Mg2+ ions in the depolarization of smooth muscle cell membrane of the human chorionic placental vessels: an hypothesis.

The membrane potential (MP) of the smooth muscle cells of human allantochorial placental vessels is the major factor which explains the excitation-contraction coupling. Indeed, the smooth muscle cells of these vessels may be considered as tonic because they only respond to excitatory stimuli with graded depolarization. On the contrary, the phasic smooth muscles generate action potentials and some generate electrical slow waves as well. The depolarization of smooth muscle cell membranes may be a consequence of various factors, particularly the secretion of endothelium-derived depolarizing factors (EDDF). The modifications of the ionic composition of external medium interfere with the membrane potential values. The increase of [Mg2+]o (Cl- or SO4[2-]) depolarizes the membrane. This effect in vitro seems inconsistent with the known vasodilator action of external Mg2+. To explain this result, an hypothetical scheme is proposed on the probable targets of local and systemic Mg2+ effects: a direct action of external Mg2+ ions on the membrane potential by secretion of depolarizing endothelial factors which depolarize the membrane; an indirect action on the membrane potential by regulation of K+ and Ca2+ channels; an action on the concentration of internal Mg2+; an interaction with internal Ca2+ concentration and regulation of the Na+/Ca2+ exchanger. A data review of Mg2+ ions effects on smooth muscles is realized to corroborate this scheme.