Coronary pressure-flow relationship and exercise: Contributions of heart rate, contractility, and α₁-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 α₁-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 ≥ 30 mmHg (slope(P ≥ 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,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,measured) from 15.0 ± 1.6 to 18.3 ± 2.1 mmHg (P < 0.05) with no change in slope(P ≥ 30). While pacing continued, infusion of dobutamine (20 μg · 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 ≥ 30) was compensated for by a slight decrease in P(zf,measured). α₁- Adrenergic blockade with intracoronary prazosin (10 μg/kg) did not prevent the exercise-induced increase of P(zf,measured) but abolished the decrease of the slope(P ≥ 30). When the increases in heart rate, contractility, and α₁-adrenergic vasoconstriction were prevented, exercise still increased P(zf,measured) from 15.8 ± 2.1 to 21.8 ± 2.6 mmHg (P < 0.05) but had no effect on the slope(P ≥ 30). This residual increase in P(zf,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, α₁-adrenergic vasoconstriction, and left ventricular filling pressure, with a minimal contribution of contractility.