['Nitric oxide (NO) reduces endothelin-1 (ET-1) production and blunts ET-1 dependent vasoconstriction. The direct effects of smooth muscle ET(A) receptor stimulation on NO-mediated relaxation are unknown. We hypothesized that endothelium-derived ET-1 regulates vascular tone by reducing smooth muscle sensitivity to NO, possibly through activation of protein kinase C (PKC).', 'Rings of rabbit middle cerebral artery were mounted on microvessel myographs to measure isometric tension. Dose-response curves to acetylcholine (ACh) and sodium nitroprusside (SNP; an NO donor) were obtained with or without ET-1 receptor blockade. Experiments were performed in the presence of indomethacin (10 micromol/L). Results are expressed as mean+/-SEM.', 'In depolarized conditions (40 mmol/L KCl physiological solution), ACh-induced relaxation was entirely NO-dependent, as indicated by its suppression by N(omega)-nitro-L-arginine (P<0.05). Arterial sensitivity (pD(2)) to ACh (6.32+/-0.11, n=6) was increased (P<0.05) to 6.77+/-0.10 (n=6) by BQ123 (ET(A) receptor antagonist, 5 micromol/L) but not by BQ788 (ET(B) receptor antagonist, 5 micromol/L; 6.08+/-0.22, n=5). Consistent with this finding, blockade of ET(A) receptors increased (P<0.05) vascular sensitivity to SNP (6.95+/-0.10, n=8), whereas BQ788 had no influence on arterial sensitivity to SNP (6.17+/-0.07, n=7) compared with control (6.43+/-0.13, n=11). In denuded arteries, the sensitivity to SNP (7.10+/-0.08, n=8) was reduced by exogenous ET-1 (6.51+/-0.35, n=7, P<0.05). Chelerythrine, a PKC inhibitor, did not alter smooth muscle sensitivity to NO, whereas phorbol 12-myristate 13-acetate, a PKC activator, strongly increased it.'] Does endothelium-derived endothelin-1 reduce cerebral artery sensitivity to nitric oxide by a protein kinase C-independent pathway?