Bara M, Ibrahim B, Leveteau J, Guiet-Bara A
Laboratory of Physiopathology of Development, University P.M. Curie, Paris, France.
Magnes Res. 1997 Mar;10(1):3-10.
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.
人绒毛膜胎盘血管平滑肌细胞的膜电位（MP）是解释兴奋-收缩偶联的主要因素。实际上，这些血管的平滑肌细胞可被视为紧张性的，因为它们仅通过分级去极化对兴奋性刺激作出反应。相反，相性平滑肌产生动作电位，有些还产生电慢波。平滑肌细胞膜的去极化可能是多种因素的结果，特别是内皮源性去极化因子（EDDF）的分泌。细胞外介质离子组成的改变会干扰膜电位值。[Mg2+]o（Cl-或SO4[2-]）的增加会使膜去极化。这种体外效应似乎与细胞外Mg2+已知的血管舒张作用不一致。为了解释这一结果，提出了一个关于局部和全身Mg2+作用可能靶点的假设方案：细胞外Mg2+离子通过分泌使膜去极化的内皮去极化因子对膜电位产生直接作用；通过调节K+和Ca2+通道对膜电位产生间接作用；对细胞内Mg2+浓度产生作用；与细胞内Ca2+浓度相互作用并调节Na+/Ca2+交换体。对Mg2+离子对平滑肌作用的数据进行了综述，以证实该方案。
