NG-hydroxy-L-arginine prevents the haemodynamic effects of nitric oxide synthesis inhibition in the anaesthetized rat.

1. We have investigated the effects of L-hydroxy-L-arginine (L-HOArg), an intermediate in the biosynthesis of nitric oxide (NO) from L-arginine (L-Arg), on the haemodynamic effects (systemic blood pressure and renal blood flow) of the NO synthesis inhibitor NG-nitro-L-arginine methyl ester (L-NAME) in the anaesthetized rat. 2. L-Arg or L-HOArg (3 mg kg-1 min-1), but not D-arginine (D-Arg) or NG-hydroxy-D-arginine (D-HOArg), elicited a slight but significant increase in total renal blood flow (RBF) of 11 +/- 2% and 11 +/- 1%. Since mean arterial blood pressure (MAP) did not change this dose of L-Arg or L-HOArg resulted in a reduced renal vascular resistance (RVR) of the same magnitude. 3. Bolus injections of L-NAME, at 0.3 or 1 mg kg-1 i.v., produced a significant fall in RBF of 11 +/- 2% and 32 +/- 5% and an increase in MAP of 7 +/- 3 mmHg and 22 +/- 5 mmHg, respectively. Consequently, RVR was elevated by 21 +/- 5% and 52 +/- 10%. 4. L-Arg or L-HOArg (3 mg kg-1 min-1) reduced the L-NAME-induced (0.3 or 1 mg kg-1) falls in RBF and increases in RVR by more than 65%. Neither D-Arg nor D-HOArg (3 mg kg-1 min-1) had any significant effect on the changes in RBF or RVR induced by L-NAME. 5. L-Arg or L-HOArg (3 mg kg-' min-') attenuated the pressor effect of L-NAME (3 mg kg-') by 73% and 64%, respectively, while neither the D-isomer of arginine nor hydroxyarginine had any effect.6. These results demonstrate that L-HOArg antagonizes the haemodynamic effects of NO-biosynthesis inhibition in vivo, thus supporting the hypothesis that L-HOArg is an intermediate in the formation of NO from L-Arg.