Effect of clonidine on cardiac norepinephrine spillover in isolated rat heart.

The purpose of this study is to determine the effect of clonidine on cardiac norepinephrine spillover utilizing an isolated rat heart preparation with attached cardiac sympathetic nerves. Following a 20-minute stabilization period, the sympathetic ganglion for each heart preparation was electrically stimulated with 10V and 2 Hz for 30 seconds (S1: 60 pulses). Heart rate, left ventricular developed pressure, and coronary perfusion pressure was allowed to return to baseline and the perfusate was randomly switched to Krebs buffer containing one of two treatments: placebo or clonidine (1 microM). After 10 minutes of treatment, the sympathetic ganglion was again electrically stimulated with 10V and 2 Hz for 30 seconds (S2: 60 pulses). The perfusate exiting the heart before, during, and after each electrical stimulation was collected for the determination of cardiac norepinephrine spillover. Clonidine administration significantly reduced cardiac norepinephrine spillover by approximately 50% (P < 0.05) and was associated with a 36% reduction in heart rate (P < 0.05). These findings provide evidence that clonidine can directly suppress NE spillover from cardiac sympathetic nerve terminals. Thus, suppression of cardiac NE by clonidine may be due to stimulation of presynaptic alpha2-adrenergic receptors or imidazoline subtype I receptors located on cardiac sympathetic nerve terminals. Results from our study demonstrate a reduction in cardiac NE spillover by clonidine and provide additional evidence that it can directly suppress peripheral sympathetic activity in that our results were obtained utilizing an isolated perfused heart preparation with attached cardiac sympathetic nerves devoid of any CNS input.