1. The concentrations of prostaglandin E(PGE)- and prostaglandin F(PGF)-like substances in renal venous blood were determined by parallel bioassay of extracts of renal venous effluent before and during adrenergic stimulation of the kidney and were related to simultaneous measurements of renal blood flow and urine flow.

2. When noradrenaline was infused continuously into the renal artery, its initial vasoconstrictor and antidiuretic effects diminished on seven of eight occasions in six dogs. Rapid recovery of renal blood flow and urine flow was invariably associated with increasing concentration in renal venous blood of a substance having the physicochemical, chromatographic and biological properties of a prostaglandin of the E series. In the one instance when rapid early recovery of renal blood flow was not observed the concentration of PGE-like substance was not increased.

3. In contrast, during renal nerve stimulation early rapid recovery of renal blood flow and urine flow did not occur and the concentration of a PGE-like substance in renal venous blood did not increase. The concentration of a PGF-like substance in renal venous effluent did not increase in response to either stimulus.

4. Since PGE2, unlike PGF, is a potent renal vasodilator and diuretic, the intrarenal release of this substance by noradrenaline in concentrations similar to those determined for a PGE-like substance (>0·50 ng/ml assayed as PGE2 equivalents) would account for the changes in renal blood flow and urine flow in these experiments when the renal actions of noradrenaline were attenuated.

5. These results support the proposal that renal prostaglandins function in an intrarenal negative feedback control system which regulates antidiuretic and vasoconstrictor systems.

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