Haemodynamic and renal tubular effects of low doses of endothelin in anaesthetized rats.

1. Renal haemodynamic and tubular transport responses to low-dose infusions (1 and 10 ng kg-1 min-1) of endothelin were investigated in anaesthetized rats. 2. Both doses caused transient increases in mean arterial blood pressure (17 +/- 5 mmHg, P < 0.05 at 1 ng kg-1 min-1) followed by sustained hypotension (-14 +/- 5 mmHg, P < 0.05), reduced renal vascular resistance (-42%, P < 0.05) and increased renal plasma flow (46%, P < 0.05). Glomerular filtration rate was unchanged. 3. Each dose caused profound diuresis and natriuresis. At 1 ng kg-1 min-1 urine flow rate and fractional water excretion increased 5-fold and fractional sodium excretion 10-fold. Fractional potassium excretion and solute-free water clearance were unaltered. 4. End-proximal fluid delivery estimated by lithium clearance doubled (P < 0.05) and fractional proximal and distal sodium reabsorption decreased 10-20% (P < 0.05). Absolute proximal reabsorption also fell with the higher dose. 5. Hypotension and natriuresis persisted for 30 min after terminating infusions. Time-control animals showed no changes in haemodynamics or renal tubular transport. 6. It is concluded that endothelin, at low concentrations, causes renal vasodilatation with concomitant natriuresis due to reduced sodium transport in proximal and distal nephron segments.