Endothelium-derived endothelin-1 reduces cerebral artery sensitivity to nitric oxide by a protein kinase C-independent pathway

Background and Purpose-Nitric oxide (NO) reduces endothelin-1 (ET-1) produc tion and blunts ET-1 dependent vasoconstriction. The direct effects of smoo th muscle ETA receptor stimulation on NO-mediated relaxation are unknown. W e hypothesized that endothelium-derived ET-1 regulates vascular tone by red ucing smooth muscle sensitivity to NO, possibly through activation of prote in kinase C (PKC). Methods-Rings of rabbit middle cerebral artery were mounted on microvessel myographs to measure isometric tension. Dose-response curves to acetylcholi ne (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 mu mol/L). Results are expressed as mean +/- SEM. Results-In depolarized conditions (40 mmol/L KCl physiological solution), A Ch-induced relaxation was entirely NO-dependent, as indicated by its suppre ssion by N-omega-nitro-L-arginine (P <0.05). Arterial sensitivity (pD(2)) t o ACh (6.32 +/-0.11, n=6) was increased (P <0.05) to 6.77 +/-0.10 (n=6) by BQ123 (ETA receptor antagonist, 5 mu mol/L) but not by BQ788 (ETB receptor antagonist, 5 mu mol/L; 6.08 +/- 0.22, n=5). Consistent with this finding, blockade of ETA 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 d enuded 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-myrista te 13-acetate, a PKC activator, strongly increased it. Conclusions-Blockade of ETA but not ETB receptors sensitizes vascular smoot h muscle to exogenous and endothelium-derived NO. This suggests that ET-1 r egulates smooth muscle sensitivity to NO by a PKC-independent pathway. This represents an alternative pathway by which NO and ET-1 interact to regulat e vascular tone.