Myocardial salvage after regional beta-adrenergic blockade

Impaired function of inhibitory G proteins during acute myocardial ischemia of canine hearts and its reversal during reperfusion and a second period of ischemia. Possible implications for the protective mechanism of ischemic preconditioning

A brief antecedent period of myocardial ischemia and reperfusion can delay cellular injury during a subsequent ischemic condition. Recent observations suggest that this protective mechanism depends on the continued activation of adenosine A1 receptors and Gi proteins. During acute myocardial ischemia, sufficient amounts of adenosine for maximal activation of adenosine A1 receptors are released, independent of a preconditioning ischemia. Hence, the protective mechanism of ischemic preconditioning may not exclusively be explained by activation of adenosine A1 receptors. As a working hypothesis, an increased responsiveness of Gi proteins toward receptor-mediated activation, leading to an increased response of Gi-regulated effectors, was tested in this study. In 47 anesthetized dogs, ischemia was induced by proximal ligation of the left anterior descending coronary artery. Animals underwent either a single period of 5 minutes of ischemia (n = 9), a single period of 15 minutes of ischemia (n = 10), 5 minutes of ischemia followed by 15 minutes of reperfusion (n = 8), 15 minutes of ischemia followed by 60 minutes of reperfusion (n = 5), or 5 minutes of ischemia followed by 15 minutes of reperfusion and a second period of 5 minutes of ischemia (n = 15). Sarcolemmal membranes were prepared from the central ischemic area and from the posterior left ventricular wall, which served as the control. During ischemia, carbochol-stimulated GTPase decreased by 38% (control, 33.5 +/- 17.7; ischemia, 24.2 +/- 15 pmol.min-1.mg protein-1; n = 9; P < .001). The decrease in carbachol-stimulated GTPase activity was associated with a 45% decrease in carbachol-mediated inhibition of adenylyl cyclase (control, 28.9 +/- 2.4% maximal inhibition; ischemia, 15.1 +/- 2.6% maximal inhibition; n = 5; P < .001). Prolongation of the ischemic period to 15 minutes did not lead to a further reduction of the Gi-mediated signal transduction. The binding properties of muscarinic receptors were not affected by ischemia. Furthermore, as demonstrated by carbachol-stimulated binding of [gamma-35S]GTP to sarcolemmal membranes, high- and low-affinity binding sites for the muscarinic antagonist carbachol, the EC50 for carbachol-stimulated GTPase activity and the substrate dependency of the high-affinity GTPase, the interaction between muscarinic receptors and inhibitory G proteins, and GTP binding to G proteins were not altered (n = 14). Immunoblotting with alpha 1- and alpha 2-specific antibodies did not indicate a loss of Gi proteins during ischemia that could explain the reduced GTPase activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Local beta-adrenergic blockade does not reduce infarct size after coronary occlusion and reperfusion: a study of coronary venous retroinfusion of metoprolol

Previous studies have demonstrated pronounced ischemic zone myocardial concentrations of metoprolol following coronary venous retroinfusion in pigs with coronary artery ligation. The effect of coronary venous retroinfusion of metroprolol on myocardial infarct size was studied in 16 pentobarbital-anesthetized open-chest pigs undergoing 60-minute occlusion of the left anterior descending coronary artery followed by 3 hours of reperfusion. Pigs in the experimental group (n = 8) were given 0.4 mg/kg (1.0 mg/ml) of metroprolol via the anterior interventricular vein over a period of 5 minutes, beginning immediately after coronary occlusion followed by 0.2 mg/kg/hr intravenously. Control pigs (n = 8) received the same volume of saline as the treated group. The risk area and the necrotic area were assessed by monastral blue dye and triphenyl tetrazolium chloride staining, respectively. Metoprolol did not influence hemodynamics. Plasma concentrations of metoprolol were within therapeutic levels. The administration of the beta-blocker resulted in a trend toward reduced norepinephrine concentrations, both in the aorta and coronary vein after coronary occlusion, but it did not prevent norepinephrine overflow following reperfusion. Infarct size expressed as a percentage of the risk area was 77 +/- 11% in the control group and 75 +/- 12% (mean +/- SD; NS) in the treated group. Thus, metoprolol retroinfusion did not reduce infarct size and did not prevent catecholamine overflow after reperfusion. It is concluded that the beneficial effects of metroprolol in acute infarction are probably unrelated to local beta-adrenergic blockade, at least in the pig, an animal with a paucity of coronary collateral blood flow.

Pharmacokinetic analysis of coronary sinus retroinfusion in pigs. Ischemic myocardial concentrations in the left circumflex coronary arterial area using metoprolol as a tracer

Myocardial availability of drug, using metoprolol as a tracer, in acutely ischemic myocardium in the area of the left circumflex (LCX) coronary artery was compared following standard intravenous (IV) administration and after coronary sinus retroinfusion. Seven open-chest farm pigs were subjected to a 21-minute occlusion of the LCX coronary artery. All animals received simultaneous IV tritium-labeled metoprolol (0.2 mg/kg) and unlabeled metoprolol (0.2 mg/kg) retrogradely into the coronary sinus. The drug administration was started after 1 minute of coronary artery occlusion and continued for 5 minutes. Intravenously administered metoprolol resulted in a higher peak plasma concentration of metoprolol (382 +/- 52 nmol/l) than coronary sinus retroinfusion (276 +/- 47 nmol/l). The nonischemic myocardial metoprolol concentration was of similar magnitude (393-454 pmol/g), whether infused IV or into the coronary sinus. Coronary sinus retroinfusion, however, resulted in a substantial accumulation of metoprolol in the ischemic myocardium (2887-5863 pmol/g). Coronary sinus retroinfusion resulted in a pronounced and specific accumulation in the ischemic myocardium in the territory of the LCX coronary artery.

Coronary venous retroinfusion of deferoxamine reduces infarct size in pigs

The efficacy of coronary venous retroinfusion of the iron chelator deferoxamine was studied in 24 pentobarbital-anesthetized open chest pigs with a 60 min occlusion of the left anterior descending coronary artery followed by 3 h of reperfusion. Eight retrogradely treated pigs were given 10 mg/kg body weight of deferoxamine by way of the anterior interventricular vein and eight systemically treated pigs received the same doses of deferoxamine intravenously. Drug infusions lasted for 5 min, beginning 15 min before reperfusion. Eight control pigs received systemic intravenous saline solution. Myocardial area at risk and necrotic area were assessed by the monastral blue dye and the triphenyltetrazolium chloride staining method, respectively. There were no significant differences in hemodynamics or regional myocardial function (sonomicrometry) among the groups. Infarct size expressed as percent of risk area was 73.9 +/- 13.5% in the control group, 70.6 +/- 16.4% in the systemically treated group and 48.5 +/- 21.4% (p less than 0.05) in the retrogradely treated group. In conclusion, deferoxamine significantly reduced infarct size after coronary occlusion only when given regionally by way of the coronary vein. Because there was no significant hemodynamic effect caused by deferoxamine infusion, it is suggested that this drug prevents postischemic reperfusion injury by a direct cardioprotective effect.

Retrograde coronary venous administration of recombinant tissue-type plasminogen activator: a unique and effective approach to coronary artery thrombolysis

Recent studies of interventional therapy by way of the coronary venous system have demonstrated that it can protect acutely ischemic myocardium. To evaluate the efficacy of coronary venous retroinfusion compared with systemic intravenous administration of recombinant tissue-type plasminogen activator (rt-PA), 14 dogs were studied with a copper coil-induced thrombus in the left anterior descending coronary artery. The rt-PA (24,000 fluorescence units/kg) was administered continuously, either intravenously (n = 8) or retrogradely (n = 6), for 30 min beginning 60 min after coronary occlusion. Thrombolysis was determined by repetitive coronary angiography. All dogs were killed 3 h after termination of rt-PA infusion and infarct size was measured by the triphenyltetrazolium chloride staining technique. Complete thrombolysis occurred in five of the six dogs in the retroinfusion group and four of the eight dogs in the systemic intravenous infusion group. Partial lysis was achieved in two dogs treated by intravenous infusion. Lysis did not occur in one dog treated with retroinfusion and in two dogs treated with intravenous infusion. Time to thrombolysis was 13.4 +/- 2.3 min in the retroinfusion group versus 27.8 +/- 4.8 min in the intravenous group (p less than 0.001). Myocardial functional recovery in the ischemic zone measured by two-dimensional echocardiography 60 min after reperfusion was significant only in the retroinfusion group (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial protection with autoperfusion during prolonged coronary artery occlusion

Passive transcatheter coronary arterial perfusion, i.e., autoperfusion, has been introduced for clinical use to ameliorate short episodes of myocardial ischemia during percutaneous transluminal coronary angioplasty. The primary goal of this study was to evaluate the cardioprotective effect of autoperfusion after prolonged coronary artery occlusion. Accordingly, in 24 anesthetized dogs, either the left anterior descending or left circumflex coronary artery was occluded for 6 hours. The dogs were randomized to a control group subjected to coronary artery occlusion alone (n = 13) or to a group treated with transcatheter autoperfusion (n = 11). The hypoperfused zone, i.e., risk area and infarct size, were measured by autoradiography and triphenyltetrazolium chloride staining, respectively. The hypoperfused zone was 30 +/- 2% and 29 +/- 2% in the control and treated (NS) groups, respectively. When infarct size was expressed as a percent of the hypoperfused zone, it was 84 +/- 5% in the control group and 25 +/- 9% in the group treated with transcatheter autoperfusion (p less than 0.001), showing a reduction of 70%. In addition, an in vitro study showed pressure-dependent flow during autoperfusion as reflected by close linear relationship between perfusion pressure and flow (Flow = 0.54 X Pressure + 16.16, r = 0.99, n = 16). These data suggest that although passive coronary arterial perfusion for 6 hours after coronary occlusion does not prevent myocardial necrosis, it markedly reduces myocardial infarction in the canine model.