Heart rate and coronary flow effects on the cardiac response to sympathetic stimulation

We studied the effects of changes in heart rate and coronary blood flow on the decay of the cardiac inotropic response to sympathetic stimulation in 10 anesthetized dogs. After we induced complete heart block, we paced the ventricles at rates of 90, 120 and 150.min-1. At each pacing frequency, we perfused the left main coronary artery with blood at a baseline flow and at a flow 85% above the baseline rate. At each combination of pacing frequency and flow, we stimulated the left ansa subclavia supramaximally at a frequency of 5 Hz for 2 min. To assess the rate of norepinephrine removal from the ventricular myocardium, we measured the 50% decay time of the left ventricular inotropic response [(dP/dt)max] immediately after cessation of sympathetic stimulation. We found that the inotropic response decayed faster (P = 0.03) when the coronary artery blood flow was high than when the flow was baseline. The effect of heart rate on the decay of the left ventricular inotropic response depended on the level of the coronary blood flow. When the blood flow was baseline, the inotropic response decayed more rapidly (P = 0.001) when the heart rate was 150.min-1 than when it was 90 or 120.min-1. However, when the flow was high, heart rate did not affect the response decay significantly. We conclude that the mechanical contraction of the ventricles facilitates the washout of norepinephrine from the cardiac interstitium and into the coronary circulation by virtue of a massaging action.