CORONARY PRESSURE-FLOW RELATIONSHIP AND EXERCISE - CONTRIBUTIONS OF HEART-RATE, CONTRACTILITY, AND ALPHA(1)-ADRENERGIC TONE

We examined the impeding effects of exercise on coronary blood flow by analyzing exercise-induced changes in the pressure-flow relationship during maximal coronary vasodilation with adenosine in chronically instrumented dogs and assessed the individual contributions produced by heart rate, contractility, and al-adrenergic vasoconstriction. Treadmill exercise that increased heart rate from 118 +/- 6 beats/min at rest to 213 +/- 8 beats/min (P < 0.01) decreased maximum coronary blood flows by decreasing the slope of the linear part of the pressure-flow relationship for coronary pressures greater than or equal to 30 mmHg (slope(p greater than or equal to 30)) from 12.3 +/- 0.9 to 10.9 +/- 0.9 ml.min(-1).g(-1).mmHg(-1) (P < 0.01) and increasing the measured coronary pressure at zero flow (P-zf,P-measured) from 12.6 +/- 1.2 to 23.3 +/- 2.0 mmHg (P < 0.01). Atrial pacing at 200 beats/min caused an increase of P-zf,P-measured from 15.0 +/- 1.6 to 18.3 +/- 2.1 mmHg (P < 0.05) with no change in slope(p greater than or equal to 30.) While pacing continued, infusion of dobutamine (20 mu g.kg(-1) min(-1) iv) increased contractility to levels similar to those during exercise but caused no significant change in coronary blood flow, as a decrease of the slope(p greater than or equal to 30) was compensated for by a slight decrease in P-zf,P-measured. alpha(1)-. Adrenergic blockade with intracoronary prazosin (10 mu g/kg) did not prevent the exercise-induced increase of P-zf,P-measured but abolished the decrease of the slope(p greater than or equal to 30). When the increases in heart rate, contractility, and alpha(1)-adrenergic vasoconstriction were prevented, exercise still increased P-zf,P-measured from 15.8 +/- 2.1 to 21.8 +/- 2.6 mmHg(P < 0.05) but had no effect on the slope(p greater than or equal to 30) . This residual increase in P-zf,P-measured correlated with the concomitant increase in left ventricular filling pressure. In conclusion, exercise-induced decreases of maximum coronary blood flow were explained by increases in heart rate, alpha(1)-adrenergic vasoconstriction, and left ventricular filling pressure, with a minimal contribution of contractility.