Renal haemodynamic actions of pressor doses of lysine vasopressin in the rat.

1. Dose-response effects of lysine vasopressin on renal haemodynamics were studied in conscious rats infused with 2.5% (w/v) dextrose solution at 5.8 ml/h. 2. Lysine vasopressin was maximally antidiuretic in the absence of a significant pressor effect at a dose of 2.5 pmol h-1 100 g body weight-1. Doses of vasopressin greater than this induced a dose-dependent increase in arterial blood pressure. 3. The clearance of p-aminohippurate (PAH) was not significantly changed by vasopressin, even at pressor doses. Rats pre-treated with indomethacin to inhibit prostaglandin synthesis showed a decrease in PAH clearance during the infusion of vasopressin at a dose of 30 pmol h-1 100 g body weight-1, and this suggests that the renal vasoconstrictor actions of vasopressin are attenuated by dilator prostaglandins. 4. Inulin clearance was unchanged by non-pressor doses of vasopressin but was decreased in a dose-dependent manner by pressor doses. A maximal effect was induced by a dose of 30 pmol h-1 100 g body weight-1 which decreased inulin clearance from 3.23 +/- 0.76 (mean +/- S.E. of mean) to 1.60 +/- 0.37 ml/min (P less than 0.02). A change in inulin clearance (from 3.42 +/- 0.46 to 2.17 +/- 0.33 ml/min, P less than 0.01) was also observed in rats pre-treated with indomethacin and infused with vasopressin at the same dose. The magnitude of the change was not significantly different from that observed in rats which were not treated with indomethacin. 5. Control rats infused with dextrose showed a slight but significant increase in sodium excretion during the course of the experiment. A similar natriuresis was observed in rats infused with non-pressor doses of vasopressin but was considerably enhanced in rats infused with pressor doses of the peptide. The antidiuresis induced by vasopressin remained maximal in rats infused with pressor doses. 6. Potassium and osmolal outputs were unchanged by non-pressor doses of vasopressin but significantly increased during administration of pressor doses. 7. It is concluded that pressor doses of lysine vasopressin do not alter total renal perfusion in conscious rats when the prostaglandin system is intact. Glomerular filtration is, however, decreased in a dose-dependent manner by these amounts but the mechanism is unclear.