1. In conscious rats, intracerebroventricular injection of dopamine resulted in a decrease in blood pressure, whereas injection of metoclopramide, the dopamine antagonist, produced an increase in blood pressure. The central depressor effect of dopamine was attenuated by a subpressor pretreatment with intraventricular metoclopramide, but not by phentolamine.

2. Intravenous administration of dopamine increased blood pressure. This increase in blood pressure was almost completely abolished by intravenous phentolamine. Metoclopramide, when injected intravenously, did not induce any change in blood pressure.

3. Plasma renin activity and plasma aldosterone concentration were decreased by intraventricular injection of dopamine, and increased by that of metoclopramide. In contrast, intravenous administration of metoclopramide increased plasma aldosterone concentration without changing plasma renin activity. Plasma concentrations of potassium, sodium and corticosterone were not affected by these treatments.

4. These results suggest that the dopaminergic system in the brain, but not in the systemic circulation, is involved in the regulation of blood pressure. It is also suggested that the central dopaminergic system participates in the regulation of aldosterone secretion by changes in the renin—angiotensin axis, whereas the peripheral dopaminergic modulation of aldosterone secretion appears to occur independently of the renin-angiotensin system.

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