Potentiation of L-NAME-induced systemic and renal vasoconstrictor responses by alpha1-adrenoceptor antagonism

Acute inhibition of nitric oxide synthase results in systemic and renal vasoconstriction, which might be attributable to unopposed activity of the sympathetic nervous (SNS) and the renin-angiotensin system (RAS). We studied the effects of N-nitro-L-arginine methyl ester (L-NAME) during alpha1-adrenoceptor blockade and concomitant angiotensin II type 1 (AT1)-receptor blockade in hypertensive individuals pretreated with hydrochlorothiazide (Hct; 25 mg once daily).

Thirteen individuals (47 +/- 9 years) were studied during administration of placebo, and after pretreatment with Hct + doxazosin (Dox; 8 mg twice daily for 9 days), with Hct + Dox + losartan (Los; 50 mg twice daily for 9 days), or (n = 5) with doxazosin or Dox + Los without hydrochlorothiazide. Mean arterial pressure (MAP) and cardiac output were derived from the finger blood pressure signal recorded by Finapres. Systemic vascular resistance (SVR) was calculated as MAP/cardiac output. Five renal clearance studies of 40 min were performed. Renal vascular resistance (RVR) was calculated as MAP divided by renal blood flow (RBF). L-NAME (12.5 microg/kg per min intravenously) was given during the third clearance period.

MAP was 113 +/- 11 mmHg at baseline and decreased to 99 +/- 10 mmHg during the administration of Hct + Dox and to 92 +/- 10 mmHg during Hct + Dox + Los. This decrease in MAP was caused by a decrease in SVR (P = 0.0009). Pretreatment with Hct + Dox or Hct + Dox + Los had no effect on glomerular filtration rate or RBF. Infusion of L-NAME during the administration of Hct + Dox resulted in an augmented (P < 0.0001) increase in MAP (18%), SVR (61%) and RVR (70%) compared with those observed with placebo (8, 30 and 49%, respectively). This augmentation was abolished by losartan.

L-NAME-induced systemic and renal vasoconstrictor responses are potentiated during alpha1-adrenoceptor blockade. This potentiation was abolished by AT1-receptor antagonism. In man, unopposed activity of the SNS or SNS and RAS is not involved in the systemic and renal vasoconstriction induced by L-NAME.