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To investigate the renal, haemodynamic and neurohormonal responses to low-dose infusions of atrial natriuretic factor (ANF) in hypertensive humans.Ten patients with mild-to-moderate essential hypertension received incremental infusions of 3 and 6 ng/kg per min ANF or vehicle alone whilst on a constant dietary sodium intake. A 90-min basal clearance period was followed by two 2-h infusion periods, with urine collection in the last 90 min of each period. In each of the three clearance periods, glomerular filtration rate (GFR), renal tubular function, and the activity of the renin–angiotensin and sympathetic nervous systems were determined.The renal sites of ANF action were established by simultaneous measurements of 51Cr-ethylenediaminetetraacetate lithium and sodium clearances. Plasma concentrations of neurohormones were measured by radioimmunoassays.Plasma ANF concentrations increased by 1.6- and 2.5-fold during the lower and higher ANF infusion rates, respectively. Plasma cyclic guanosine monophosphate concentrations increased in parallel. ANF caused no changes in supine systolic and diastolic blood pressure or in heart rate. In contrast, haematocrit values increased progressively across the study. The renal effects of ANF administration were characterized by an unaltered GFR and significant increases in the renal clearances of lithium (a marker of end-proximal fluid delivery) and sodium when compared with vehicle infusions, whereas urine flow did not change. Estimated values of fractional proximal and distal tubular sodium reabsorption decreased significantly. Plasma concentration of active renin decreased during ANF infusions, but no significant changes in plasma levels of renin substrate, angiotensin I, angiotensin II or aldosterone were observed. A subtle activation of the sympathetic nervous system was indicated by a moderate increase in plasma noradrenaline during the ANF infusions.These results indicate that even small increases in plasma ANF, as can be found during physiological conditions, induce natriuresis in patients with essential hypertension by enhancing fluid delivery from the proximal tubules, in addition to impairing distal fractional sodium reabsorption. With minor exceptions, the ANF infusions caused qualitatively and quantitatively similar renal, haemodynamic and endocrine effects in the hypertensive patients as in a previously studied group of normotensive subjects.