To investigate the effects of dietary sodium on the peripheral dopaminergic mechanism, changes of unconjugated plasma dopamine (DA) and its related humoral factors were studied in 8 patients with essential hypertension (EH) and 8 age-matched normal controls (N) while they were receiving ordinary meals (Na, 130-180 mEq daily) followed by higher sodium (250-300 mEq daily) diets for a week. Plasma and urinary DA, norepinephrine (NE) and epinephrine (E) were measured by the highly sensitive COMT-mediated radioenzymatic procedure, which permits an accurate estimation of plasma DA as low as 5-6 pg/ml.
Under high sodium diets, blood pressure and heart rate were not changed significantly in N and EH subjects. Urinary NE and E tended to decrease, while urinary DA increased significantly in both groups of subjects (p<0.05). There was a significant correlation between urinary sodium and DA (r=0.590, p<0.001), but plasma DA failed to correlate significantly to urinary sodium or DA in all subjects. Plasma NE and E tended to decrease in both N and EH subjects, while plasma DA increased significantly (p<0.05) in EH from 7.2±0.8 pg/ml [mean±SEM] to 9.3±1.0 and slightly in N from 9.1±1.8 to 11.2± 13. Plasma renin activity (PRA) and plasma aldosterone (PAC) were invariably decreased in all subjects, while plasma prolactin (PRL) remained unchanged.
A significant correlation was observed between plasma DA and NE under ordinary meals (r=0.733, p<0.01), but this correlation disappeared under high sodium diets. Plasma DA showed an inverse correlation to PAC (r=-0.351, p<0.05) under both dietary conditions.
Upright posture induced a significant rise (p<0.05) in NE, E, DA, PRA and PAC with ordinary meals, but the responses of NE and PAC were apparently attenuated with high sodium diets.
An intravenous injection of metoclopramide (MCP, 10 mg), a DA receptor antagonist, provoked a slight rise in plasma NE and DA with ordinary meals, of which responses were further enhanced with high sodium diets. MCP induced a definite rise in PAC and PRL in all subjects under both dietary conditions (p<0.01), while plasma E and PRA remained unchanged after MCP challenge.
The results lend support to the view that unconjugated plasma DA could be a useful marker of peripheral dopaminergic activity, which might be a physiological regulator responsible for the suppression of aldosterone secretion and sympathetic nerve activity observed during high sodium intake.
