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Current Research in magnesium, edited by M.J. Halpern and J. Durlach
©1996 John Libbey & Company Ltd./7th Intemational Magnesium Symposium, pp. 197-198.


Chapter 45

Contribution to the physiology of the human placental vessels: effects of Mg2+ on membrane potential of smooth muscle cell vessels

1B. Ibrahim, 1A. Guiet-Bara, 2J. Leveteau, 1J.C. Challier, 1C. Vervelle, 3J. Durlach and 1M. Bara

1Laboratoire de Physiopathogie du Développement, Université P.M. Curie, 4 Place Jussieu 75252-Paris Cedex 05, France; 2Neurophysiologie comparée, Université P.M. Curie, France; 3SDRM, Hôpital Saint-Vincent-de-Paul, France


Introduction

A wide range of data has been collected concerning the effect of extracellular magnesium ions on the blood vessel smooth muscle tone and reactivity. The aim of the present study was to contribute to the physiology of the human placental vessels, investigating the influence of magnesium ions (MgCl2 and MgSO4) on membrane potential of the smooth muscle cells of the human placental chorionic arteries with endothelium and without endothelium (endothelium-free).

Material and methods

Short segments (about 10 mm long) of human chorionic placental arteries were carefully dissected after delivery from 12 normal term vaginal deliveries. The preparations were left to equilibrate for 60 min in Earle's solution (37±1°C) and bubbled with carbogen (95 per cent O2 - 5 per cent CO2; the final pH was 7.4 ± 0.05). Recordings of membrane potential of the arterial smooth muscle cells was made using glass microelectronics (KCl 3M).

Results are expressed as means ± SD. The data statistical processing was made with the one way analysis of variance test (ANOVA), the Tukey-Kramer multiple comparisons test and the Student test. P values from 0.05 were considered significant.

Results

Effects Of MgCl2 and MgSO4 on the membrane potential

(a) Arteries with endothelium

The addition of MgCl2 elicited a significant depolarization of the smooth muscle cell membrane from 8 mM (P < 0.05). An identical effect was observed with MgSO4 from 6 mM (P < 0.05).

(b) Endothelium-free arteries

A significant depolarization of the smooth muscle cell membrane was observed with MgCl2 from 6 mM (P < 0.05) and with MgSO4 from 8 mM (P < 0.05).

Discussion

The results show that the magnesium ions associated with Cl- or with SO42- exert a direct contribution to the membrane potential and certainly, regulate the viability of the K+ channels implicated in the origin of the membrane potential1. These data suggest that the importance of thedepolarizing effect of Mg2+ seem to be mediated through the endothelium since the depolarization thresholds are different in arteries with endothelium and in endothelium-free arteries. Endothelial cells exert a protective effect towards MgCl2 compared to MgSO4. The endothelial cells seem act as an intermediate between the Mg2+ ions and the membrane of the smooth muscle cells. In the endothelial cells, two mechanisms may be involved: (1) outside Mg2+ ions exert important effects on the precise location and concentration of internal Ca2+ and Mg2+ which affect the tone in the vascular smooth muscle cells2 ; (2) the depolarizing effect due to a lower concentration of MgSO4 may be the consequence of endothelial secretion. Indeed, the vascular endothelium plays a functional role in modulating the tone of underlying smooth muscle by releasing relaxing factors such as nitric oxide and prostacyclin or constricting factors such as endothelin3. The effect of MgCl2 is as follows: the high depolarization threshold indicates a protective role of endothelial cells towards smooth muscle cells. What is the mechanism? Mgi variation, Mg2+ uptake?

In endothelium-free arteries, MgSO4 has a direct effect on the smooth muscle cell membrane potential, but at high concentrations, since there is not secretion of various factors by the endothelial cells. On the other hand, MgCl2 interferes directly with the smooth cell membrane and the depolarization threshold is lower. The depolarization of the vascular smooth muscle cells implicates a vasoconstrictor effect. in vivo, in preeclampsia, MgSO4 has a vasodilator action4 . Our studies indicate that the addition of MgCl2 and MgSO4 has a possible vasoconstrictor action on the human placental arterial smooth muscle cells and reveal the important role of the endothelial cells.

Figure 1.


References

I . Bara, M. & Guiet-Bara, A. (1993): Magnesium effect on the membrane potential of smooth muscle cells of human placental chorionic vessels. Magnes. Res. 6, 329-331.

2. Adams, D.J. (1994): Ionic channels in vascular endothelial cells. Trends Cardiovasc. Med. 4, 18-26.

3. Yanagisawa, M., Kurihara, H., Kimura, S., Tonnobe, Y., Kobayashi, M., Mitsui, Y., Yasaki, Y., Goto, K. & Masaki, T. (1988): A novel potent vasoconstrictor peptide produced by vascular endothelial cells. Nature 332, 411-415.

4. Andrews, W.W., Gant, N.F. & Magness, R.R. (1994): Effect of magnesium sulfate on eicosanoid levels in women with pregnancy-induced hypertension. Hypertens. Pregnancy 13, 71-82.


All articles by Dr. Durlach are copyrighted, and permission is granted to Web users only to make single hard copies for personal use. Additional reprints should be obtained from the originating journals. Excerpts may be used by the media with attribution to Dr. Durlach.


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