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Figure 3 Arterial (open bars) and coronary sinus (solid bars) plasma norepinephrine at control values and in response to 50 and 100 µg of clonidine. There is a significant reduction in the transcardiac gradient of plasma norepinephrine in response to 100 µg of clonidine. This results from a significantly greater decrease in the coronary sinus plasma norepinephrine level as compared with the arterial plasma norepinephrine level. *p < 0.05 versus control. p < 0.01 versus control. p < 0.05 for comparison between the norepinephrine transcardiac gradient (coronary sinus – arterial) with 100 µg of clonidine and control level. Data are expressed as the mean value ± SEM.