Increased nitric oxide activity compensates for increased oxidative stress to maintain endothelial function in rat aorta in early type 1 diabetes

Abstract

Hyperglycaemia and oxidative stress are known to acutely cause endothelial dysfunction in vitro, but in the initial stages of diabetes, endothelium-dependent relaxation is preserved. The aim of this study was to investigate how endothelium-dependent relaxation is maintained in the early stages of type 1 diabetes. Diabetes was induced in Sprague-Dawley rats with a single injection of streptozotocin (48mg/kg, i.v.), and after 6weeks, endothelium-dependent and endothelium-independent relaxations were examined in the thoracic aorta in vitro. Lucigenin-enhanced chemiluminescence was used to measure superoxide generation from the aorta. Diabetes increased superoxide generation by the aorta (2,180 ± 363 vs 986 ± 163AU/mg dry tissue weight). Acetylcholine (ACh)-induced relaxation was similar in aortae from control (pEC(50) 7.36 ± 0.09, R (max) 95 ± 3%) and diabetic rats (pEC(50) 7.33 ± 0.10, R (max) 88 ± 5%). The ACh-induced relaxation was abolished by the combined presence of the nitric oxide synthase inhibitor N-nitro-L-arginine (L-NNA, 100μM) and an inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10μM) in control rats, but under the same conditions, the diabetic aortic rings showed significant relaxation to ACh (pEC(50) 6.75 ± 0.15, R (max) 25 ± 4%, p < 0.05). In diabetic aortae, the addition of haemoglobin, which inactivates nitric oxide, to L-NNA + ODQ abolished the response to ACh. The addition of the potassium channel blockers, apamin and TRAM-34, to L-NNA + ODQ also abolished the relaxation response to ACh. Diabetes significantly elevated plasma total nitrite/nitrate and increased expression of endothelial nitric oxide synthase (eNOS) and calmodulin in aortae. These data indicate that after 6weeks of diabetes, despite increased oxidant stress, endothelium-dependent relaxation is maintained due to the increased eNOS expression resulting in increased NO synthesis. In diabetic arteries, NO acts both through and independently of cGMP pathways to cause relaxation.