Heart rate modulates the disposition of neurally released norepinephrine in cardiac tissues.

Abstract

We determined the effect of heart rate on the disposition of neurally released norepinephrine from the cardiac tissues in open-chest, anesthetized dogs with complete atrioventricular block. After injecting desipramine to block the neuronal uptake of norepinephrine, we stimulated the cardiac sympathetic nerves supramaximally for about 3 minutes at a mean frequency of 2.7 Hz. In one series of experiments, we allowed coronary sinus blood flow to change spontaneously in response to cardiac pacing and sympathetic stimulation. The mean coronary sinus blood flows just before cessation of sympathetic stimulation were 67.4 +/- 4.2 (SE) and 37.4 +/- 2.7 ml/min during pacing at a high (150/min) and a low (60/min) frequency, respectively. The mean norepinephrine overflow into the coronary sinus blood during steady state sympathetic stimulation was 45.0 +/- 14.2% greater during high frequency pacing than during low frequency pacing. After cessation of neural stimulation, the norepinephrine overflow decayed more rapidly during high frequency pacing than during low frequency pacing. This difference in decay rates was mainly ascribable to the difference in coronary blood flows. In a second series of experiments, we adjusted the coronary sinus blood flow during sympathetic stimulation so that the flows were not significantly different during pacing at the two frequencies. During the initial 30-second period, after cessation of sympathetic stimulation, the mean decrements of norepinephrine overflow were 95.9 +/- 21.4 and 59.2 +/- 17.3 ng/min at the high- and low-pacing frequencies, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of sympatho-vagal interaction on ventricular electrophysiology and their modulation during beta-blockade

“…Given that the heart rate at baseline did not change during metoprolol infusion and washout, thereby suggesting the absence of tonic sympathetic activity, the current study could not conclusively discount the possibility of a different tonic sympathetic activity between sinoatrial and ventricular levels. On the other hand, the possibility of local release of norepinephrine at the pacing site could not be excluded in this study especially in the context of high-frequency pacing [34]. Furthermore, it is also conceivable that metoprolol may possess an unrecognised direct effect on ion channels at the ventricular level to account for the higher VFT at baseline.…”