Reducing arginase activity via dietary manganese deficiency enhances endothelium-dependent vasorelaxation of rat aorta

L-Arginine is a common substrate for the enzymes arginase and nitric oxide synthase (NOS). Acute inhibition of arginase enzyme activity improves endothelium-dependent vasorelaxation, presumably by increasing availability of substrate for NOS. Arginase is activated by manganese (Mn), and the consumption of a Mn-deficient (Mn-) diet can result in low arginase activity. We hypothesize that endothelium-dependent vasorelaxation is greater in rats fed Mn- versus Mn sufficient (Mn+) diets. Newly weaned rats fed Mn- diets (0.5 pg Mn/g; n = 12) versus Mn+ diets (45 μg Mn/g; n = 12) for 44 ± 3 days had (i) lower liver and kidney Mn and arginase activity (P < 0.05), (ii) higher plasma L-arginine (P < 0.05), (iii) similar plasma and urine nitrate + nitrite, and (iv) similar staining for endothelial nitric oxide synthase in thoracic aorta. Vascular reactivity of thoracic aorta (∼720 μm i.d.) and small coronary arteries (∼110 μm i.d.) was evaluated using wire myographs. Acetylcholine (ACh; 10^-8-10^-4 M) produced greater (P < 0.05) vasorelaxation in thoracic aorta from Mn- rats (e.g., maximal percent relaxation, 79 ± 7%) versus Mn+ rats (e.g., maximal percent relaxation, 54 ± 9%) at 5 of 7 evaluated doses. Tension produced by NOS inhibition using N^G monomethyl-L-arginine (L-NMMA; 10^-3 M) and vasorelaxation evoked by (i) arginase inhibition using difluoromethylornithine (DFMO; 10^-7 M), (ii) ACh (10^-8-10^-4 M) in the presence of DFMO, and (iii) sodium nitroprusside (10^-9-10 ^-4 M) were unaffected by diet. No differences existed between groups concerning these responses in small coronary arteries. These findings support our hypothesis that endothelium-dependent vasorelaxation is greater in aortic segments from rats that consume Mn- versus Mn+ diets; however, responses from small coronary arteries were unaffected.