Vasorelaxation by Red Blood Cells and Impairment in Diabetes

Abstract

Abstract-Vascular dysfunction in diabetes is attributed to lack of bioavailable nitric oxide (NO) and is postulated as a primary cause of small vessel complications as a result of poor glycemic control. Although it has been proposed that NO is bound by red blood cells (RBCs) and can induce relaxation of blood vessels distal to its site of production in the normal circulation, the effect of RBC glycation on NO binding and relaxation of hypoxic vessels is unknown. We confirm RBC-induced vessel relaxation is inversely related to tissue oxygenation and is proportional to RBC S-nitrosohemoglobin (HbSNO) content (but not nitrosylhemoglobin content Key Words: nitric oxide Ⅲ hemoglobin Ⅲ glycosylation Ⅲ relaxation T he endothelial dysfunction associated with diabetes has been attributed to a lack of bioavailable nitric oxide (NO). [1][2][3] The mechanisms proposed to explain this deficiency include reduced production of NO by NO synthase (NOS) 4,5 and inactivation of NO by reactive oxygen species (ROS) produced either by glycated proteins or directly from vascular endothelium. 6,7 However, these only incompletely explain reduced relaxant responses of microvessels to agonists such as bradykinin in the presence of glycohemoglobin (HbA1c).NO released into the vascular lumen is metabolized (1) by oxyhemoglobin to form methemoglobin and nitrate, (2) by reaction with oxygen (O 2 ) to produce nitrite and nitrate (NOx), (3) through binding to deoxyhemoglobin to produce iron nitrosyl hemoglobin (HbNO), and (4) by reaction with thiol-containing proteins in blood (RSNO) that include glutathione (GSNO) and hemoglobin (S-nitrosohemoglobin, HbSNO). 8 -11 It has been proposed that NO bioavailability in vivo may be governed in part by its reformation from these circulating metabolites as well as by the above local factors. 12,13 This NO "reserve" is likely to be critical in tissue regions where blood supply is limiting and where O 2 demand is increased. The conformational change that occurs during oxygenation of HbNO in the lungs is thought to induce transfer of the NO to an adjacent thiol on the cysteine 93 group of the ␤-chain, producing HbSNO. The levels of HbSNO in human red blood cells (RBCs) are reflected in the physiological O 2 gradient as first shown by Funai et al, 14 and the occurrence of transpulmonary gradients in HbSNO at the expense of HbNO has recently been demonstrated in normal subjects. 15 The HbSNO is thought to pass on its NO to other smaller molecular weight proteins in blood and these can donate NO to cause vasodilatation in the peripheral microvasculature. 16 -19,21 The occurrence and physiological relevance of this NO reserve in the normal circulation has been questioned 21,22 with arguments both for and against. 13,[23][24][25][26] We hypothesized that because protein glycation can produce changes in configuration and ligand binding properties, 27-29 increased HbA1c may directly influence NO metabolism in blood. In a previous study in patients with type 1 diabetes, we demonstrated increased concentrations ...

Hypoxia, red blood cells, and nitrite regulate NO-dependent hypoxic vasodilation

“…These data are in contrast to previous studies showing vasodilatory effects of RBCs added to rabbit aortic preparations under hypoxic conditions. 39,43 To directly compare our results with those of these studies, nitrite-free RBCs were added to hypoxic rabbit thoracic aorta preparations. Interestingly, using this vascular preparation, RBCs alone induced an initial and brief 10% relaxation that was followed by vasoconstriction ( Figure 5B).…”

“…Previous studies have shown a vasodilatory effect of RBCs added to rabbit thoracic aorta at low oxygen tensions (approximately 10 mmHg), 39,43 a response that has been interpreted in favor of the hypothesis that SNOHb is a modulator of hypoxic vasodilation. However, deoxygenation of RBCs also results in ATP release, which stimulates vasodilation by activation of P 2Y (purinergic) receptors and eNOS.…”

Neurovascular Coupling in Rat Brain Operates Independent of Hemoglobin Deoxygenation

“…However, hemoglobin deoxygenation-based mechanisms may have a role in preventing tissue from moderate or severe hypoxia during intense metabolic stress. It was shown recently that the blood flow response in the heart and the diameter response of isolated thoracic aorta from the rabbit or mouse during severe hypoxia depend on O 2 -dependent delivery and release of NO from hemoglobin (Datta et al, 2004;Diesen et al, 2008;James et al, 2004).…”

SNO-hemoglobin is not essential for red blood cell–dependent hypoxic vasodilation

“…In the R-state, SNO-Hb remains relatively unreactive but upon deoxygenation, T-state SNO-Hb can rapidly react with thiols [GSH or anion exchanger-1 (AE-1) thiols] and transmit a vasodilatory signal out of the RBC (suppl Fig 1) 8-12 . This hypothesis has been extended recently to suggest that dysfunction in this pathway leads to a variety of systemic vascular and pulmonary diseases and contributes to pathologic effects associated with diabetes, congestive heart failure and with transfusion of banked blood [13][14][15][16][17] . These pioneering studies also began to resolve a long standing question in hemoglobin biology, namely the role of the highly conserved β93Cys residue.…”