ENDOTHELIN-1 REGULATES TONE OF ISOLATED SMALL ARTERIES IN THE RAT - EFFECT OF HYPERENDOTHELINEMIA

Chronic elevation of plasma endothelin-1 (ET-1) levels has been report ed in several pathological conditions. To investigate the consequences of increased circulating ET-1 on Vascular responsiveness, Sprague-Daw ley rats (n=16) were chronically instrumented with a minipump deliveri ng ET-1 at a constant dose for 7 days. Plasma ET-1 levels were more th an doubled in treated (0.98+/-0.09 pmol/L; P<.05) versus untreated sha m-operated rats (0.43t0.04 pmol/L), whereas systolic arterial blood pr essure increased (139+/-5 versus 128+/-4 mm Hg in untreated rats; P<.0 5). After rats were killed, segments of middle cerebral (MCA) and mese nteric (MES) arteries were mounted on an isometric myograph. ET-induce d contraction was shifted to the right in ET-1-treated animals and not modified by BQ123 (an ETA receptor antagonist); bosentan (ETA/B recep tor antagonist) prevented ET-1-induced contraction in both groups. Aft er inhibition of nitric oxide synthase with N-omega-nitro-L-arginine ( L-NNA), both phenylephrine and oxymetazoline (an alpha(2)-adrenoceptor agonist) induced MCA contraction. The sensitivity to phenylephrine wa s decreased in ET-1-treated compared with control rats (P<.05). Sensit ivity to phenylephrine-induced contraction was decreased by BQ123 in c ontrol rats only. In contrast, L-NNA revealed greater oxymetazoline-in duced contractions in treated compared with control MCA rings (P<.05); this potentiation was blunted by bosentan but unaffected by BQ123. Re moval of the endothelium revealed a direct constrictor effect of oxyme tazoline that was insensitive to L-NNA alone or combined with bosentan ; however, oxymetazoline induced significantly lower constriction in t reated rat MCA segments. Responses to oxymetazoline were also blunted in treated compared with untreated denuded MES arteries. In conclusion , chronic elevated plasmatic ET-1 decreases smooth muscle cell sensiti vity to contractile agonists both in MCA and MES rings. In cerebral ve ssels, endothelial alpha(2)-adrenoceptor-dependent stimulation induced greater contractile responses in treated rats which were sensitive to bosentan, suggesting that oxymetazoline stimulates ET-1 release from the endothelium. This may represent a compensatory mechanism for the l oss of smooth muscle sensitivity.