Abnormal distal tubular sodium reabsorption during dopamine infusion in patients with essential hypertension evaluated by the lithium clearance methods

The effects of low-dose dopamine infusion on renal hemodynamics, tubular function estimated by the lithium clearance technique and plasma levels of angiotensin II (Ang II), aldosterone (Aldo), atrial natriuretic peptide (ANP) and arginine vasopressin (AVP) were studied in 11 patients with essential hypertension (HT) and in 10 healthy control subjects (CS). Antihypertensive treatment was terminated at least 2 weeks prior to examination. Dopamine (2 micrograms/kg/min) was infused for two hours. Before dopamine infusion all measured parameters, but blood pressure, did not deviate significantly between the two groups, including 24 h urinary sodium excretion prior to investigation (HT: 166.9 mmol/24 h vs. CS: 183.4 mmol/24 h, medians). Dopamine infusion resulted in an exaggerated natriuresis in the HT group when compared with the CS group; sodium excretion: (HT: from 260 to 759 mumol/min vs. CS: from 255 to 432 mumol/min, p < 0.01) and fractional sodium excretion: (HT: from 1.6 to 4.2% vs. CS: from 1.6 to 2.4%, p < 0.01 median values). Distal fractional sodium reabsorption was significantly lower in the HT patients (HT: from 94.0 to 88.5% vs. CS: from 94.0 to 91.6%, p < 0.01). ANP decreased significantly only in the HT group (HT: from 4.8 to 3.5 pmol/l vs. CS: from 3.2 to 3.4 pmol/l, p < 0.01). Renal hemodynamics, blood pressure, urinary output, Ang II, Aldo, and AVP were changed to the same degree or unchanged in both groups. It is concluded that the exaggerated natriuretic response seen in patients with essential hypertension during low-dose dopamine infusion probably is due to a enhanced dopamine sensitivity mainly in the distal parts of the nephron.