lvI/mal ofCardiol'llscl/lar Pharmacology'" 32:300-307 @ 1998 Lippincott-Raven Publishers. Philadelphia Dietary L-Arginine Normalizes Endothelin-Induced Vascular Contractions in Cholesterol-Fed Rabbits Laddawal Phivthong-ngam, Stefanie M. Bode-Boger, Rainer H. Boger, Michael Bohme, *Ralf P. Brandes, *AndreasMiigge, and Jiirgen C. Frolich Institute of Clinical Pharmacology and *Departmellfof Cardiology. Medical School. Hannovel; Germany Summary: The endothelium regulates vascular function by re- leasing the vasodilator autacoid nitric oxide (NO) and the vaso- constrictor peptide endothelin-I (ET-I). Impaired activity of NO as well as excessive activity of ET-I have been demon- strated in hypercholesterolemia and atherosclerosis. Because dietary L-arginine can restore NO function and improve abnor- mal endothelium-dependent relaxation in hypercholesterolemic rabbits, we examined the effects of dietary supplementation with L-arginine in cholesterol-fed rabbits on endothelium-de- pendent vascular relaxation and ET-I-induced vascular con- traction, as well as the systemic synthesis of ET-l. Rabbits were initially fed a diet enriched with I% cholesterol for 4 weeks, followed by 0.5% cholesterol alone or supplemented with 2% L-arginine in drinking water during the next 12 weeks. Cholesterol feeding impaired endothelium-dependent relax- ation of rabbit aortic rings ex vivo and increased urinary im- munoreactive ET-I excretion, along with decreased urinary ni- trate excretion, an index of NO production. L-Arginine par- tially restored endothelium-dependent relaxation in parallel to increased urinary nitrate excretion and decreased urinary im- munoreactive ET-I excretion. Selective inhibition of ET-A re- ceptors with BQ 123 partially restored endothelium-dependent relaxation in hypercholesterolemic rabbits but had no effect on arterial rings from rabbits supplemented with L-arginine or from control animals. The contractile vascular response of aor- tic rings to exogenous ET-I was increased in rabbits fed a high- cholesterol diet; this enhanced contractility to ET-l was com- pletely reversed by L-arginine. These data suggest that L-arginine restores endothelial function and normalizes the synthesis and vasoconstrictor response to ET-I in hypercholes- terolemia. Key Words: Nitric oxide-Endothelin-I-Hyper- cholesterolemia. Hypercholesterolemia and atherosclerosis lead to an impaired endothelium-dependent vasorelaxation both in humans and in animal models (1-4). Bioassay studies have suggested that the mechanism underlying this defect is related to a reduced activity of endothelium-derived ni- tric oxide (NO; 3-6). NO, which has been characterized as the active principle of the endothelium-derived relax- ing factor (EDRF; 7), is formed from the amino acid pre- cursor L-arginine by the enzyme NO synthase (NOS) in vascular endothelial cells (8). It induces smooth-muscle cell relaxation by activating the soluble guanylyl cyclase to form the intracellular second messenger cyclic guano- sine monophosphate (cGMP; 9,10). Endothelial cells not only synthesize the vasodilator NO, but they also produce the vasoconstrictor endothelin (ET). a 21-amino-acid peptide (II). The ET family com- prises three related peptides; ET-l, ET-2, and ET-3 (12). ET-l is the major isoform produced in endothelial cells by the action of ET-converting enzyme (ECE) and exerts potent vasoconstrictor effects by binding to ET-A recep- tors on vascular smooth-muscle cells (11,13). ET-l is present in plasma and urine. It has been reported that plasma levels and tissue immunoreactivity of this peptide are increased in animal models as well as in hypercho- lesterolemic and atherosclerotic patients (14-16). In these diseases, ET-l concentrations are positively corre- lated with the extent of arterial lesion formation (15). Experimental evidence suggests that there is a mutual interaction between the L-arginine/NO and ET-l path- ways. The vasoconstrictor and pressor actions of ET-I are enhanced in the presence of an NOS inhibitor (l7,18). Thrombin-stimulated ET-l formation by isolated porcine aorta is inhibited by EDRF/NO (19). Li.ischeret al. (20) demonstrated that EDRF released by stimulating human arteries with acetylcholine (ACh) or bradykinin reverses ET-l-induced contractions. A disturbed balance between these endothelial autacoids may also underlie the im- paired endothelium-dependent vascular relaxation in hy- Received August 13, 1997; accepted February II, 1998. Address correspondence and reprint requests to Dr. R. H. Boger at Institute of Clinical Pharmacology, Hannover Medical School, Carl- Neuberg-Str. I. 30625 Hannover, Germany. Present address of Dr. Phivthong-ngam: Department of Pharmacol- ogy. Faculty of Medicine. Srinakharinwirot University, Bangkok 10 110, Thailand. 300