WITHDRAWAL-INDUCED ANTIHYPERTENSIVE EFFECT OF VASOPRESSIN - ROLE OF THE L-ARGININE NITRIC-OXIDE PATHWAY

The role of the endothelium-derived relaxing factor (EDRF), nitric oxi de (NO), in the withdrawal-induced antihypertensive phenomenon (WAP) o f arginine vasopressin (AVP) was studied in conscious unrestrained rat s implanted with femoral arterial catheters for the measurement of art erial blood pressure. Cessation of a 3-h intravenous infusion of AVP ( 20 ng.kg(-1).min(-1)) was followed by a large and long-lasting fall in mean arterial blood pressure below preinfusion control values in spon taneously hypertensive rats (SHR; -47.5 +/- 6.4 mmHg; 1 mmHg = 133.3 P a) but not in normotensive Wistar-Kyoto (WKY) and Sprague-Dawley (SD) control rats. Chronic treatment of normotensive SD rats with the NO sy nthesis inhibitor, N-omega-nitro-L-arginine (L-NNA; 0.5 g.L-1 in drink ing water for 2 weeks), resulted in sustained hypertension. Similar to the SHR model, a large fall in blood pressure (-37.8 +/- 7.7 mmHg) wa s observed in this model of hypertension following cessation of the AV P infusion. In SHR, inhibition of NO synthesis with L-NNA (0.05 g.L-1 in drinking water for 2 weeks) failed to attenuate the fall in blood p ressure following withdrawal of AVP. Chronic treatment with the NO pre cursor, L-arginine (L-Arg; 1.25 g.L-1 in drinking water for 2 weeks), did not affect the amplitude or the time course of the WAP in SHR. The results indicate that the L-Arg/NO pathway is not essential to the ex pression of the WAP to AVP in the SHR.