Role of ancillary properties of beta-adrenoceptor antagonists in protecting the heart from anoxia

Early intravenous administration of metoprolol enhances myocardial salvage by thrombolysis with recombinant tissue-type plasminogen activator after thrombotic coronary artery occlusion in the dog by improvement of the collateral blood flow to the area at risk

OBJECTIVES

We studied the effects of beta 1-adrenergic blockade preceding thrombolysis on hemodynamic variables, myocardial blood flow and infarct size in a canine model of thrombotic occlusion of the left anterior descending coronary artery.

BACKGROUND

Previous work suggested a reduction in infarct size and improvement in left ventricular function by intravenous beta-blockade preceding thrombolysis.

METHODS

Experiments were conducted in 34 anesthetized dogs; 17 received 0.975 mg/kg body weight of metoprolol intravenously starting 15 min after occlusion, and thrombolysis was initiated 60 min after occlusion. Seventeen dogs received saline solution followed by thrombolysis. Coronary blood flow was measured by radioactive microspheres, infarct size by a dye method, hemodynamic variables by catheter-tipped pressure transducers and cardiac output by the thermodilution method.

RESULTS

Infarct size in metoprolol- and placebo-treated dogs was 23.62 +/- 18.04% and 41.50 +/- 16.03% of area at risk, respectively (p < 0.01). Before occlusion, myocardial blood flow and hemodynamic variables were similar. Sixty minutes after occlusion, cardiac output (1.94 +/- 0.41 vs. 2.32 +/- 0.68 liters/min, p < 0.01) was lower in the metoprolol-treated dogs. Collateral flow to the area at risk (17.27 +/- 7.44 vs. 10.25 +/- 5.33) and to its epicardial (21.68 +/- 8.04 vs. 11.5 +/- 6.10), midmyocardial (14.30 +/- 8.63 vs. 7.35 +/- 4.94) and endocardial (13.18 +/- 8.21 vs. 6.26 +/- 5.34 cm3/min per 100 g) layers was higher (p < or = 0.05) in the metoprolol-treated dogs. The ratio of epicardial flow area at risk/circumflex territory was inversely correlated to infarct size (r = -0.69, p < 0.01). After 5 min of occlusion, collateral flow was comparable in the five dogs of each group; over the next 55 min it remained constant in the metoprolol group but decreased in the placebo dogs.

CONCLUSIONS

Intravenous metoprolol, administered before thrombolysis, enhances infarct size limitation, partly by improvement of collateral flow to area at risk.

Cardioprotective effect of d-propranolol in ischemic-reperfused isolated rat hearts

In the isolated, perfused working rat heart, ischemia (15 min) decreased mechanical function and also the tissue levels of ATP and creatine phosphate, and increased the tissue levels of lactate and free fatty acids including arachidonic acid. Reperfusion (20 min) did not restore mechanical function, but restored changes of metabolites incompletely except for free fatty acids, which changed further during reperfusion. Drugs were given 5 min before ischemia until the end of ischemia or for the first 10 min after reperfusion. Both dl- and d-propranolol (10 and 30 microM) decreased mechanical function, accelerated the recovery of mechanical function during reperfusion following ischemia, and attenuated ischemia reperfusion-induced metabolic changes. The attenuation of reperfusion-induced metabolic changes was more marked when these drugs were present during reperfusion. d-Propranolol showed a cardioprotection similar to that by dl-propranolol. Timolol (50 microM) did not accelerate the recovery of mechanical function during reperfusion, and did not attenuate the reperfusion-induced metabolic changes. These results suggest that d-propranolol, like dl-propranolol, has a cardioprotective effect which is probably due to its membrane stabilizing (or sodium channel blocking) action.

Cardioprotective effect of pindolol in ischemic-reperfused isolated rat hearts

The effects of pindolol and timolol on ischemia reperfusion damage were studied in isolated working rat hearts. Ischemia (15 min) decreased the mechanical function and the energy state, and increased the tissue levels of free fatty acids (FFA). During reperfusion (20 min), the mechanical function did not recover, but the energy state recovered incompletely, whereas FFA increased further. Pindolol (50 microM) accelerated recovery of the mechanical function and the energy state that had been decreased by ischemia during reperfusion, and inhibited the accumulation of FFA during ischemia and reperfusion, especially when it was applied during the whole period of reperfusion. Timolol (50 microM), however, did not accelerate recovery of the mechanical function and the energy state during reperfusion, although it attenuated FFA accumulation during reperfusion. The pindolol-induced recovery of the mechanical function during reperfusion was reduced by timolol. The results suggest that the intrinsic sympathomimetic activity of pindolol may play an important role, at least in part, in producing the cardioprotective effect, especially during reperfusion.

Pharmacologic characterization of beta blockers with special reference to the significance of nonspecific membrane effects

Along with the beneficial effects of beta blockers in the ischemic derangements of myocardial energy metabolism are nonspecific membrane effects, which are thought to derive from the physicochemical interaction of the drugs with the cell membrane. These interactions are observed in various forms (e.g., membrane stabilizing action, quinidine-like action, cardiodepressant action), depending on the intensity of the interaction and on the method used for their detection. There may be still other manifestations that have not been detected so far, owing to our ignorance of their existence and our lack of an adequate method for their detection. The importance of the presence of specific chemical moieties in certain beta blockers with ability to trap peroxy radicals is also discussed.