BETA ADRENERGIC BLOCKADE ON INTRAMYOCARDIAL DISTRIBUTION OF CORONARY BLOOD FLOW

¹ Department of Pharmacology, Warner-Lambert Research Institute, Morris Plains, New Jersey

Intramyocardial tissue perfusion was determined in the subepicardium (epi) and subendocardium (endo) of the canine left ventricle by a polarographic hydrogen clearance technique, using electrodes at a depth of 3 and 9 mm, respectively. Control epi perfusion (88 ml/min/100 g) was approximately 5% higher than endo perfusion (84 ml/min/100 g). Beta adrenergic blockade produced by i.v. propranolol (0.5 mg/kg), bunolol (0.3 mg/kg) or practolol (1.0 mg/kg) resulted in a decrease in heart rate, average coronary blood inflow and epi tissue perfusion. Endo perfusion was maintained. The epi/endo ratio decreased from 1.03 to 0.79. When heart rate was elevated to the predrug rate by atrial pacing, epi perfusion and average coronary blood inflow increased, endo perfusion decreased and the epi/endo ratio returned to control. Similarly, no changes in blood flow distribution were observed when heart rate was not permitted to fall after beta blockade. A decrease in heart rate by crushing of the sinoatrial node did not significantly lower the epi/endo ratio. In addition, beta blockade did not affect the epi/endo ratio after maximal coronary vasodilatation and the abolition of autoregulation by chromonar, despite a large decrease in heart rate. It is concluded that beta adrenergic blockade causes a redistribution of blood flow from the epi to the endo of the left ventricle. Among the factors involved are a decrease in heart rate and the ability to autoregulate and increase arteriolar resistance in the epicardium. Other factors as yet undetermined are also involved.

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