Effect of magnesium sulfate on thrombus formation following partial arterial constriction: implications for coronary vasospasm.

The antithrombogenic effect of magnesium sulfate in vivo is shown at the site of endothelial damage induced by partial coronary and carotid artery constriction. The left anterior descending coronary artery of dogs and the right common carotid artery of rabbits were subjected to partial constriction with suture thread (40-60% reduction in transluminal diameter). Distal blood flow, as measured by electromagnetic flow probe, was not reduced. Scanning electron-microscopic examination of vessels fixed by glutaraledhyde perfusion and dried by the critical-point technique showed endothelial damage at the site of partial constriction ranging from crater- and balloon-like vesicular defects to cellular desquamation. Marked platelet deposition on exposed subendothelium and microthrombi could be seen with the maximum degree of luminal protrusion reaching 30% of the luminal diameter. Animals pretreated with magnesium sulfate (50 mg/kg, i.v.) showed platelet deposition restricted to a maximum of 1 or 2 discontinuous layers of platelets with most vessels showing only isolated platelets on exposed subendothelium. Microthrombi were not seen in any of the magnesium-treated animals. It is suggested that the therapeutic implications of magnesium in ischemic heart disease might be extended from its use in certain tachyarrhythmias and in arterial spasm associated with Prinzmetal's angina to the more classic episodes of ischemic heart disease where thrombus formation plays an unequivocally major role.

Intimal changes associated with arterial spasm induced by periarterial application of calcium chloride.

Arterial spasm was induced by application of calcium chloride to the adventitial surface of the rabbit common carotid artery in vivo. Sodium chloride (NaCl) was applied to the contralateral vessel as control. Vessels were fixed in situ by intravascular perfusion after 15 min, 1 hr, or 24 hr and prepared for light and scanning electron microscopy (SEM). With SEM, the luminal surface at the site of calcium application showed severe longitudinal folding accompanied by endothelial desquamation with extensive platelet deposition on exposed subendothelium. The luminal cross-sectional area was reduced by 53 +/- 19.5% after 15 min and by 44 +/- 12% after 1 hr as compared with the contralateral control. Furthermore, the luminal area at the site of calcium application was found to be reduced by 42 +/- 8% after 1 hr when compared with segments of the same vessel distal to the site of calcium application. Blood flow rate, as measured by electromagnetic flow probe, was not reduced. Vessels examined after 24 hr showed a significant increase in luminal cross-sectional area as compared with contralateral control vessels (136 +/- 70%). Control vessels (NaCl) showed no significant change in luminal cross-sectional area and no endothelial desquamation or platelet deposition after 15 min, 1 hr, or 24 hr. Examination of histologic sections showed calcium precipitation within the attached thrombus after 15 min with calcium deposits also adherent to the adjacent luminal aspect of the internal elastic lamina (IEL). By 24 hr, this precipitation extended throughout the media. Marked deposition of leukocytes was seen after 24 hr which showed a preferential attachment for areas of endothelial damage and discontinuity of IEL.

Endothelial cell damage and thrombus formation after partial arterial constriction: relevance to the role of coronary artery spasm in the pathogenesis of myocardial infarction.

The left anterior descending coronary artery of four dogs and the right common carotid artery of 15 rabbits were subjected to 40-60% reduction in transluminal diameter for 1 hour by partial ligation with suture thread. Scanning electron microscopic examination of the luminal surface of these vessels revealed endothelial craters and balloons, fragmentation and desquamation on the proximal slope of the constriction. Platelet attachment to exposed subendothelial tissues was clearly evident, and microthrombi were seen at the point of maximum constriction. Blood flow, as measured by electromagnetic flow probe, was virtually unchanged upon partial ligation. In control studies, where a second ligature was placed proximal to and before the first to reduce blood to the distal constriction site substantially or totally, endothelial desquamation was found in only one of 14 animals, and the number of craters and balloons was significantly reduced. We suggest that endothelial damage and thrombus formation may occur at the site of focal arterial constriction even when the reduction in transluminal diameter is insufficient to alter substantially the rate of flow.

Effect of magnesium sulfate on platelet deposition in rabbits following temporary arterial occlusion with surgical clips.

The effect of magnesium sulfate on thrombus formation in vivo was studied by scanning electron microscopy utilizing the model of endothelial damage by temporary arterial occlusion with a Heifetz clip. Examination of arterial segments compressed by the surgical clip for 30 minutes, followed by 2 hours of reflow, revealed marked attachment of platelets and deposition of erythrocytes, with very few leukocytes, to exposed subendothelial tissues. This thrombotic deposition protruded as much as 150 mu into the vascular lumen (15% of luminal diameter). By contrast, in animals pretreated with magnesium sulfate, this deposition was limited to isolated areas of single or very few platelets with few erythrocytes or leukocytes. The difference in overall quantity of thrombus formation between nontreated and magnesium sulfate-treated groups was much less obvious after 24 hours; however, fewer leukocytes and erythrocytes were seen to be attached to the platelet carpet in the treated group as compared with the nontreated group. The effect of magnesium sulfate on the overall quantity of thrombus formation corresponded inversely to the measured levels of serum magnesium in these groups.