Thromboxane contributes to submaximal coronary dilation during myocardial ischemia

OBJECTIVES

Thromboxane has been shown to contribute to coronary constriction in conduit coronary arteries during platelet aggregation at the site of a critical stenosis. Previous studies from our laboratory suggest that following a critical coronary stenosis, persistent vasomotor tone occurs. We tested the hypothesis that thromboxane is responsible for that increased tone.

METHODS

To test this hypothesis, 14 mongrel dogs of either sex were anesthetized and subjected to a critical coronary stenosis where distal coronary perfusion pressure was reduced to 36 +/- 2 mm Hg. During the coronary stenosis, SQ 29,548 (thromboxane/endoperoxide receptor antagonist) was administered intravenously (0.2 mg/kg and 2.0 mg/kg). Coronary microvascular responses were observed by directly visualizing the epicardial microcirculation. Diameters were measured using intravital microscopy coupled to stroboscopic epi-illumination and jet ventilation to compensate for cardiac and respiratory-induced motion.

RESULTS

Coronary microvessels were divided into small (< 150 microns) and large (> 150 microns) arterioles. During SQ 29,548 administration, small coronary arterioles demonstrated no additional dilation during a critical coronary stenosis. In contrast, coronary microvessels > 150 microns demonstrated a dose-dependent vasodilation to SQ 29,548 (0.2 mg/kg: 6 +/- 2%; 2.0 mg/kg: 11 +/- 4%; P < 0.05 vs. no change). A time control study in six additional animals demonstrated no significant microvascular diameter changes following a critical stenosis over the time course of SQ 29,548 administration.

CONCLUSION

Endoperoxides contribute to poststenotic microvascular vasoconstriction in vessels 150-300 microns.

Protection of reperfused ischemic canine myocardium by CI-922, a new inhibitor of leukocyte activation

There is increasing evidence that activated neutrophils cause myocardial injury during reperfusion of ischemic myocardium. This study was performed to assess the effect of Ci-922, an inhibitor of neutrophil activation, in a canine preparation of myocardial infarction. Dogs received 15-min infusions of CI-922 1 mg/kg or 5% dextrose beginning 30 min before occlusion of the left circumflex coronary artery. After occlusion for 90 min and reperfusion for 6 h, infarct size was determined by ex vivo perfusion of the left circumflex coronary artery with triphenyltetrazolium chloride. The percentage of the area at risk infarcted was: control, 42 +/- 5; and CI-922, 23 +/- 4 (p less than 0.05 vs. control). There were no significant inter-group differences in heart rate or mean arterial pressure, and CI-922 did not enhance collateral blood flow to the ischemic bed. After incubation with CI-922 (100 microM), production of superoxide anions by canine neutrophils activated by opsonized zymosam decreased from 3.5 +/- 0.2 to 2.0 +/- 0.4 nmol/10 min/10(6) cells (p less than 0.05). Thus, inhibition of neutrophil-mediated damage may explain the cardioprotective effect of CI-922.

Electrophysiologic and antiarrhythmic actions of sulphinpyrazone and its sulfide metabolite G25671

In anesthetized dogs, the cumulative intravenous administration of 1.0-40.0 mg/kg sulphinpyrazone failed to alter the ventricular excitation threshold, ventricular refractory period and ventricular fibrillation threshold determined during nonobstructed coronary blood flow. Sulphinpyrazone, however, did attenuate the reduction in the ventricular fibrillation threshold occurring during transient myocardial ischemia. G25671, the sulfide metabolite of sulphinpyrazone, failed to alter ventricular refractoriness and 'nonischemic' ventricular fibrillation thresholds, and was minimally effective in reducing the decrease in 'ischemic' fibrillation thresholds when administered in cumulative intravenous doses of 5.0-20.0 mg/kg. In conscious dogs in the subacute phase of anterior myocardial infarction, the administration of a cumulative 10.0-40.0 mg/kg sulphinpyrazone failed to alter the mode of induction, rate or morphology of ventricular tachyarrhythmias initiated by programmed ventricular stimulation. These data suggest that neither sulphinpyrazone nor its sulfide metabolite possess primary electrophysiologic properties which might contribute directly to significant antiarrhythmic or antifibrillatory activity.

Antiarrhythmic and electrophysiologic actions of clofilium in experimental canine models

Clofilium was studied in three experimental models. In non-ischemic and chronically infarcted canine hearts, clofilium (0.5-2 mg/kg) produced a dose-dependent increase in electrical ventricular fibrillation threshold (VFT), but prolonged the effective refractory period (ERP) of normal myocardium in only the non-ischemic heart. When chronically infarcted hearts were subjected to programmed electrical stimulation, 1 mg/kg of clofilium inhibited the re-induction of either ventricular tachycardia or ventricular fibrillation in 5 of 6 animals and slowed the rate of the induced tachycardia in the sixth. Clofilium, however, failed to alter ventricular refractory periods of normal myocardium at either twice diastolic threshold current (176 +/- 5 ms control vs. 187 +/- 9 ms post-clofilium, P greater than 0.05) or at 10 mA (134 +/- 6 ms control vs. 137 +/- 13 ms post-clofilium, P greater than 0.05). In addition, chronic administration of clofilium (2 mg/kg, i.v., followed by 1 mg/kg every 12 h) was ineffective in decreasing mortality in a canine model of sudden coronary death. Of 10 saline-treated conscious animals subjected to an electrically-induced intimal lesion of the left circumflex coronary artery in the presence of a previous ischemic insult, all 10 died suddenly of ventricular fibrillation within 173 +/- 45 min after current application. Under similar conditions, 7 clofilium-treated animals died suddenly within 249 +/- 88 min (P greater than 0.05) after current application while 3 animals survived (P greater than 0.10). Clofilium did, however, elevate the effective refractory period in these animals (150 +/- 3 ms saline-treated vs. 195 +/- 7 ms clofilium-treated). It is concluded from our data that there is little relationship between clofilium's electrophysiologic actions in normal myocardium and antiarrhythmic effects. Furthermore, simple prolongation of refractoriness in normal non-ischemic myocardium may be insufficient for the prevention of ventricular fibrillation which develops in response to a transient ischemic event superimposed on a chronically injured myocardium.

Effects of diltiazem upon experimental ventricular dysrhythmias

The antidysrhythmic and antifibrillatory actions of the calcium entry blocker diltiazem were examined in three experimental canine models: 1) ventricular fibrillation thresholds in the anesthetized dog; 2) programmed electrical stimulation in the conscious dog during the subacute phase of myocardial infarction; and 3) a conscious canine model of sudden coronary death wherein ventricular fibrillation was produced by acute myocardial ischemia in the presence of previous anterior myocardial infarction. Diltiazem administration failed to alter ventricular fibrillation thresholds determined under normal physiologic conditions and during acute occlusion of the distal left anterior descending coronary artery. Diltiazem did not prevent ventricular tachyarrhythmias produced by programmed electrical stimulation in the subacute phase of myocardial infarction. The coupling intervals of premature beats producing ventricular tachycardia and the cycle length of the ventricular tachycardias were unchanged by diltiazem administration. In a conscious canine model of sudden coronary death, left circumflex coronary artery thrombosis was produced in the presence of previous anterior myocardial infarction. Acute myocardial ischemia as evidenced by ST-segment changes in saline-treated animals (145 +/- 26 min) was followed by the development of premature ventricular beats (150 +/- 27 min) and ventricular tachycardia (154 +/- 26 min). Ventricular fibrillation occurred in 9 of 10 animals at 156 +/- 28 min. Diltiazem administration did not affect the time to development of ST-segment changes (196 +/- 38 min), premature ventricular beats (202 +/- 37 min) or ventricular tachycardia (209 +/- 37 min). Although the development of ventricular fibrillation was not prevented by diltiazem (9 of 10 animals developed ventricular fibrillation), the time to onset of the fatal arrhythmia was delayed significantly (P less than .01). These data fail to suggest a protective action for the calcium entry blocker diltiazem.

Ventricular fibrillation in a conscious canine model--its prevention by UM-272

The antifibrillatory properties of UM-272 (dimethylpropranolol; Pranolium) were evaluated in a conscious canine model of sudden coronary death. The initial preparation of the animal model was carried out under surgical anesthesia and involved the intraluminal implantation of a Teflon-coated silver wire into the circumflex coronary artery so that 3 mm of the bared electrode was in contact with the endothelial surface. The left anterior descending coronary artery then was occluded for a period of 90 min and reperfused in the presence of a critical stenosis. Three days after myocardial infarction, they were randomized into two groups. One group (n = 10) served as controls and received saline. The second group (n = 10) received UM-272 in a dose of 5 mg/kg every 6 h. On day 4, a 150 microA current was applied to the intimal surface of the left circumflex coronary artery, resulting in transient or permanent alterations in circumflex coronary blood flow accompanied by electrocardiographic evidence of regional myocardial ischemia. The time to onset of ST-segment changes in the saline control group was 99 +/- 34 min and was followed by the appearance of premature ventricular complexes (111 +/- 34 min) and subsequent ventricular tachycardia (131 +/- 37 min) which terminated in ventricular fibrillation in each of the 10 dogs. Animals treated with UM-272 likewise developed ST-segment changes (156 +/- 28 min) and premature ventricular complexes (168 +/- 29 min), but 4 of 10 animals failed to develop ventricular fibrillation (P less than 0.05 vs. saline). These results demonstrate that UM-272, the dimethyl quaternary analog of propranolol, is effective in reducing the incidence of ventricular fibrillation in a conscious canine model in which the superimposition of a transient ischemic event upon an already jeopardized heart leads to the development of sudden death.