The Effect of the Pattern of Cardiac Sympathetic Activity on Myocardial Contractile Force and Norepinephrine Overflow in the Dog Heart

The left or right cardiac sympathetic nenes in open-chest, anesthetized dogs were stimulated at mean frequencies of 2 or 4 Hz. The stimuli were applied intermittently, in patterns with repetition rates of either 60/min or 15/min, to simulate the spontaneous patterns of sympathetic neural activity that occur synchronously with the cardiac or respiratory cycles, respectively. With either repetition rate, intermittent stimulation of the left sympathetic nerves was about 10-20% less effective in enhancing myocardial contractile force (CF) and about 10% less effective in increasing coronary sinus blood flow than was steady stimulation at the same mean frequency. With right-sided stimulation, there was no appreciable difference between steady and intermittent stimulation patterns with respect to the effect on heart rate. With either left-or right-sided stimulation, the rate of norepinephrine (NE) overflow into the coronary sinus blood was 20-40% less with intermittent than with steady stimulation. Cocaine administration did not materially affect this difference in NE overflow. It was concluded that the higher instantaneous frequencies that prevail during intermittent stimulation result in a reduction in the rate of NE release at the sympathetic postganglionic nerve endings in the heart.