We examined the importance of decreased heart rate in the beneficial effect of beta-
adrenergic blockade on exercise-induced regional
myocardial ischemia and contractile dysfunction in conscious dogs with single vessel coronary
stenosis (
ameroid constrictor). Studies were performed during control treadmill exercise, which produced regional
myocardial ischemia (blood flow measured with
microspheres) and wall dysfunction (measured using sonomicrometers). A second run was performed after the administration of
atenolol (0.3-1.0 mg/kg i.v.), and the reduced heart rate caused by
atenolol during early steady-state running was then prevented by atrial pacing during the latter portion of the run.
Atenolol reduced the exercise heart rate from 217 +/- 25 beats per minute (SD, n = 9) to 166 +/- 15, and ischemic zone wall thickening during systole improved from 27 +/- 22% of the resting value in the control run to 50 +/- 25% of the resting value in the
atenolol run (p less than 0.01). Atrial pacing then increased heart rate to 217 +/- 23 beats per minute, and regional wall thickening deteriorated to 15 +/- 25% of the resting value. Regional subendocardial blood flow in the ischemic zone during atrial pacing with
atenolol was slightly less than that observed in the control run, in both ischemic and control zones, indicating no remaining beneficial effect of
atenolol when heart rate reduction was eliminated. We conclude that the only significant mechanism for the improvement in exercise-induced
ischemia and wall motion produced by
atenolol is a reduction in the exercise heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)