1. In six healthy subjects the role of renal prostaglandins (PG) in modulating the actions of the renin-angiotensin and renal adrenergic nervous systems on renal function was investigated.

2. During high dietary sodium intake (350 mmol/day) for 4 days no changes in urinary excretion of PGE2, PGF, noradrenaline or adrenaline were noted, whereas plasma renin activity (PRA) and urinary aldosterone excretion were suppressed.

3. After 4 days of low sodium intake (35 mmol/day) urinary excretion of PGE2, aldosterone and noradrenaline, as well as PRA, had significantly increased.

4. Inhibition of PG synthesis with indomethacin (2 mg/kg body weight) had no effects on renal function on day 5 of high sodium intake. Despite suppression of PRA and urinary aldosterone, indomethacin significantly reduced p-aminohippurate (PAH) clearance, glomerular filtration rate (GFR) and urinary sodium excretion on day 5 of low sodium intake, when urinary noradrenaline excretion remained high.

5. The results point to the crucial role of the renal adrenergic nervous system in controlling renal vascular resistance and sodium conservation in healthy subjects during low sodium intake, which is unmasked when renal PG synthesis is blocked by indomethacin. Enhanced renal PG synthesis during sodium restriction therefore not only attenuates the vascular and tubular effects of the renin-angiotensin system but, more importantly, also those of the highly stimulated renal adrenergic nervous system.

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