Dopaminergic Control of Circadian Norepinephrine Levels in Patients with Essential Hypertension

This study examines the influence of dopamine on recumbent circadian mean arterial blood pressure (MAP) levels and secretory patterns of norepinephrine (NE) in essential hypertension. Nine patients with sustained essential hypertension were studied after they had reached metabolic equilibrium of a constant 100-mmol sodium and 80-mmol potassium intake. MAP measurements and plasma NE determinations were made at 30-min intervals over 24 h during a control (no medication) and a bromocriptine (BEC) period (2.5 mg, three times a day for 5 days). In the control period, a clear circadian rhythm in blood pressure was evidenced in these hypertensive patients, with lowest MAP occurring during sleep. NE was secreted in an episodic manner, with most secretory peaks occurring between 0600-1800 h. Sleep was characterized by lower levels of NE than in waking hours and a paucity of secretory peaks. During the control period, recumbent 24-h MAP was strikingly correlated with plasma NE (gamma = 0.67; P less than 0.001). BEC treatment was associated with a depression in MAP throughout the 24-h cycle and a lessening of the relationship between MAP and plasma NE (gamma = 0.35; P less than 0.025). BEC eliminated much of the circadian variation in plasma NE secretion in these hypertensives. Thus, circadian variations in sympathetic nervous system activity and blood pressure in patients with essential hypertension appear to be modulated by a central and/or peripheral dopaminergic mechanism.