Matthew L. Schlossberg

Timing of Magnesium Therapy Affects Experimental Infarct Size

BACKGROUND Controversy exists regarding the use of magnesium in the treatment of acute myocardial infarction (AMI) because of apparent conflicting results from clinical trials. One hypothesis to explain the various clinical observations proposes that the timing of magnesium administration significantly influences its therapeutic effect; ie, supraphysiological levels of Mg2+ must be present at the time of reperfusion for magnesium to produce clinical benefit. METHODS AND RESULTS These experiments evaluated the effect of varying the timing of magnesium administration during AMI. Female Yorkshire swine (34 to 42 kg) underwent thoracotomy and 50 minutes of left anterior descending coronary artery (LAD) occlusion, followed by 3 hours of reperfusion. In the first group, MgSO4 (250 mg of magnesium diluted in 60 cm3 saline) was infused into the LAD over 12 minutes, beginning immediately with the onset of reperfusion (n = 6, Mg-early group). In the second group, MgSO4 was given after 1 hour of reperfusion (n = 6, Mg-late group). Six pigs received saline instead of magnesium and served as the control group. Lethal arrhythmias were significantly reduced in the Mg-early group. Infarct size was determined by vital staining. Infarct size was 0.16 +/- 0.05 g/kg body wt (Mg-early), 0.35 +/- 0.08 g/kg (Mg-late), and 0.42 +/- 0.04 g/kg for the control group. Compared with the control group, significant (P = .029) reduction in infarct size occurred in the Mg-early group but not in the Mg-late group. CONCLUSIONS We conclude that intracoronary MgSO4 delivered during reperfusion can significantly diminish infarct size in swine, but the timing of administration is critical.

Short-term low dose intracoronary diltiazem administered at the onset of reperfusion reduces myocardial infarct size

BACKGROUND Currently, controversy exists regarding the use of calcium-channel blockers in the treatment of acute myocardial infarction (AMI), due to apparent conflicting results from clinical trials and animal models. One hypothesis to explain such a discrepancy proposes that the timing and duration of drug administration might influence its cardioprotective effect. Pretreatment with calcium-channel blockers or their administration during coronary artery occlusion is associated with the diminished infarct size in animal models. While verapamil failed to reduce infarct size when the drug was given at the onset of reperfusion, similar effects of low dose diltiazem are not known. METHODS AND RESULTS This experiment evaluated the effect of intracoronary short term low dose diltiazem administration given immediately with postischemic myocardial reperfusion. Yorkshire swine underwent thoracotomy and 50 min of left anterior descending (LAD) occlusion, followed by 3 h of reperfusion. In the first group, diltiazem (2.5 mg diluted in 60 cc saline) was infused into the LAD over 12 min, beginning with the onset of reperfusion (n=8). In the second group, animals received saline instead of diltiazem and served as controls (n=6). Infarct size was 0.13+/-0.06 g/kg of body weight for diltiazem group, and 0.42+/-0.04 g/kg for controls (P=0.01). CONCLUSIONS Short-term low dose diltiazem delivered exclusively during early reperfusion can significantly diminish infarct size in swine. Local intracoronary diltiazem may be valuable adjunct in patients subject to myocardial ischemia/reperfusion during coronary artery bypass grafting, primary angioplasty for AMI, or thrombolysis for AMI if given immediately after restoration of coronary blood flow.

Regional and Systemic Platelet Function Is Altered by Myocardial Ischemia-Reperfusion.

Structured AbstractBackground: Myocardial reperfusionafter short durations of ischemia causes prolonged contractile dysfunction (myocardial stunning). Recently it has also been suggested that ischemia-reperfusion results in impaired coronary endothelial function. Since platelet function is, in part, regulated by an intact functioning endothelium, platelet function could be expected to change during ischemiareperfusion. However, the effect of ischemia and reperfusion on regional and systemic platelet function is unknown. The purpose of this study was to determine the effect of a brief period of myocardial ischemia followed by reperfusion on regional and systemic platelet function.Methods: Fourteen swine in an open-chest model underwent left anterior descending coronary artery (LAD) occlusion for 15 minutes followed by 120 minutes of reperfusion. Platelet aggregability in response to 5 μ M ADP was determined simultaneously in the femoral (systemic; N=14) and great cardiac (regional; N=9) venous blood at baseline, during occlusion, and at 40 and 90 minutes after reperfusion. LAD blood flow and regional myocardial function were determined by standard methods.Results: Hemodynamics remained stable in all animals. During LAD occlusion platelet aggregability increased only in the regional coronary circulation (126% of baseline, p=.0001). At 40 minutes of reperfusion systemic platelet aggregability decreased (86% of baseline, p=.0001) and subsequently increased at 90 minutes of reperfusion in both the systemic (127% of baseline, p=.0001) and regional circulations (156% of baseline, p=.0001). Ischemia was evident by the absence of distal LAD flow during occlusion that returned during reperfusion and a typical response of myocardial stunning in each animal (stunning time=47.7 +5.2 minutes).Conclusions: This study demonstrates that platelet function is not static during ischemia-reperfusion. Instead, during ischemia regional platelet aggregability is increased. Systemic and regional platelet aggregability also increase during myocardial reperfusion. The mechanism of these responses is unknown but may be related to regional endothelial dysfunction created by ischemia. The response observed could also be explained by the release of proaggregatory mediators in the coronary and/or systemic circulation during ischemia-reperfusion. The relative hyeraggregability observed following reperfusion may be relevant for further investigations of coronary artery reocclusion occurring after the relief of myocardial ischemia.

NPC 15669, an antiinflammatory leucine derivative, reduces in vitro platelet aggregability in both swine and human plasma

Background: Leumedins inhibit cell adhesion to endothelium via blockage of integrin binding. We tested a hypothesis that the novel leucine derivate NPC 15669 will affect in vitro platelet aggregability (PA) in both human and swine plasma.Methods and Results: Platelet-rich plasma (PRP) was incubated with 200 μ-g and 400 μg of NPC 15669. Then PA was induced by ADP, collagen, thrombin, and ristocetin in the PRP without NPC 15669 and in NPC 15669 treated samples. We have found that PRP incubation with 200 μg of NPC 15669 significantly decreases PA compared to baseline in all three experimental groups in response to all agonists tested. When PRP was treated with 400 μg of NPC 15669, dose-dependent reduction of PA was observed only in the human control and swine groups, but not in patients with coronary atherosclerosis.Conclusions: Leumedins, known for their antiinflammatory properties, may have clinical applications related to their effect on platelet function. The mechanism of these effects is unknown, but may be related to the inhibition of platelet-endothelial binding.

Effects of a novel leumedin NPC 15669 on myocardial stunning and preconditioned infarction size in swine

Background: NPC is a member of the leumedins and is an inhibitor of leukocyte adhesion to endothelium via blockage of integrin binding. NPC 15669 also may have anti-platelet effects. We tested the efficacy of the novel leukocyte recruitment inhibitor NPC 15669 on myocardial stunning (MS) and preconditioned myocardial infarction (MI).Methods and Results: In an open-chested swine model, NPC 15669 (10 mg NPC/kg loading dose followed by constant infusion at 6 mg/kg/hr) was administered in six animals. Myocardial thickening (MT) was determined by epicardial ultrasound. The left anterior descending artery was occluded for 8 minutes followed by 90 minutes of reperfusion, during which myocardial MT was recorded at regular intervals. We have found that treatment with NPC 15669 increases myocardial contractility and significantly decreases MS time compared to controls (26.7±4.0 minutes vs. 50.0±4.3 minutes, p=.0026). In NPC 15669-treated animals we observed a reduction of MI size (23.4±6.7% of tissue at risk became necrotic compared to 53.0±6.6% in controls, p=.0102).Conclusions: Our data suggest that NPC 15669 significantly reduces myocardial injury in both the stunning and infarction models.

Pretreatment with an inhibitor of Mac-1 alters regional and systemic platelet function during ischemia-reperfusion in swine

Myocardial ischemia-reperfusion alters regional and systemic platelet function. The aim of our study was to elucidate the role of the Mac-1 receptor in changes of platelet function by using the leumedin, NPC-15669, an inhibitor of Mac-1 upregulation. In an open-chest swine model (n = 15), the treatment group (n = 6) received NPC-15669 (10 mg/kg loading dose over 12 min at the rate of 5 ml/min at the onset of left-anterior descending coronary artery occlusion, followed by constant infusion at 6 mg kg-1 h-1 during 90 min of reperfusion). Regional platelet aggregation (response to 5 microM ADP) increased after 15 min occlusion (126% of baseline) and at 90 min of reperfusion (156% of baseline). This increase in platelet aggregability was inhibited by NPC-15669 (83% of baseline after 15 min occlusion and 98% of baseline at 90 min reperfusion, both p < 0.001 compared to control). Systemic platelet function was not affected by NPC-15669 after 15 min occlusion (102% of baseline vs. 96% of baseline for control, p = NS). At 90 min of reperfusion platelet function was increased in controls (131% of baseline) and not affected by NPC-15669 (126% of baseline, p = NS). Myocardial neutrophil accumulation did not differ between the control and treatment groups. Inhibition of Mac-1 upregulation by NPC-15669 attenuates the increased regional platelet aggregability resulting from myocardial ischemia-reperfusion, with a less marked effect on the systemic response. The data suggest that Mac-1 may modulate regional platelet responses induced by ischemia-reperfusion. The increased aggregability of platelets during ischemia and reperfusion and its attenuation by inhibition of Mac-1 may be relevant for future strategies to reduce coronary arterial occlusion by platelet thrombi.

Intracoronary magnesium and diltiazem affect to a similar extent certain hemostatic factors during acute myocardial infarction in swine

The use of calcium antagonists and magnesium (Mg) in the treatment of acute myocardial infarction is controversial. We compared changes in hemostasis during acute myocardial infarction after either low-dose intracoronary Mg or diltiazem infusion in 20 Yorkshire swine undergoing thoracotomy and coronary artery occlusion for 50 min, followed by 3 h of reperfusion. The first group received MgSO4 (250 mg), delivered at the onset of reperfusion, the second group received diltiazem (2.5 mg) at the beginning of reperfusion. Six controls received saline. Plasma antithrombin III, protein C, total protein S, fibronectin, endothelin 1, and metabolites of thromboxane and prostacyclin were measured at baseline, twice during occlusion, and three times during reperfusion. Compared to controls, Mg and diltiazem infusion diminished endothelin 1 (32.9 vs. 34.5%) and fibronectin. (21.7 vs. 23.2%), but increased protein C (31.9 vs. 29.3%). Intracoronary Mg, like diltiazem, improved hemostasis in swine.

Hemostatic changes after early versus late intracoronary magnesium during acute myocardial infarction in swine.

There has been some debate regarding the benefit of magnesium (Mg) in the treatment of acute myocardial infarction (AMI) because of conflicting results from recent clinical trials. Several different hypotheses have been advanced to explain the cardioprotective properties of Mg, including the influence of the timing of Mg administration during AMI. This experiment was designed to assess the effect of intracoronary Mg on certain hemostatic parameters that are known to change during an AMI. Yorkshire swine underwent thoracotomy and 50 min left anterior descending artery (LAD) occlusion, followed by 3 h of reperfusion. In the early group, 250 mg of MgSO4 was delivered at the onset of reperfusion (n = 6, Mg-early group). In the second group, MgSO4 was given after 1 h of reperfusion (n = 6, Mg-late group). Six animals received saline instead of Mg and served as controls. The dynamics of plasma antithrombin-III (AT-III), protein C, total protein S, fibronectin, endothelin-1 (ET-1), as well as the stable metabolites of thromboxane (TXB2) and prostacyclin (6-keto-PGFla) were determined at baseline, twice during occlusion, and three times during reperfusion. Mg given at reperfusion onset was associated with a diminished ET-1 (32.9%), decreased fibronectin level (21.7-25.2%), and increased protein C concentrations (31.9-52.3%) when compared with both the control and late Mg group. In summary, intracoronary Mg administered at the onset of reperfusion favorably influenced hemostasis in swine. The beneficial effects of early Mg supplementation in an expanding array of clinical conditions, including AMI, may be directly related to the improved hemostatic profile in such patients.

Effects of intracoronary diltiazem on certain hemostatic parameters during acute myocardial infarction in swine

Controversy currently exists regarding the use of diltiazem in the treatment of acute myocardial infarction (AMI). due to conflicting results from clinical trials and animal studies. The purpose of this project was to evaluate the changes in the hemostatic profile during AMI following low dose intracoronary diltiazem infusion. Fourteen Yorkshire swine underwent thoracotomy and 50 min LAD occlusion, followed by 3 h of reperfusion. The first group (n = 8) received 2.5 mg of diltiazem intracoronary at a rate of 5.6 micrograms kg min-1 at the onset of reperfusion. The second group (n = 6) received 0.9% saline intracoronary at the onset of reperfusion and served as the control. The dynamics of plasma antithrombin-III (AT-III), Protein C, total Protein S, fibronectin, endothelin-1 (ET-1), and the stable metabolites of thromboxane (TxB2) and prostacyclin (6-keto-PGF1a) were determined at baseline, then twice during occlusion and finally three times during reperfusion. Diltiazem infusion resulted in diminished ET-1 (34.5%), fibronectin (23.2%), and TxB2 (35.6%); and elevated Protein C (29.3%) when compared with controls. We conclude that intracoronary diltiazem favorable influences hemostasis during AMI in swine. The cardioprotective effects of diltiazem during AMI may be related to the improved hemostatic profile and the reduced incidence of thrombotic complications in such patients.