IMPAIRMENT OF NITRIC OXIDE-MEDIATED RELAXATIONS IN ANESTHETIZED AUTOPERFUSED STREPTOZOTOCIN-INDUCED DIABETIC RATS

This work was designed to determine in vivo the influence of the metab olic control of streptozotocin-induced diabetic rats, measured by the levels of haemoglobin glycosylation in blood (HbA(lc)), on developing vascular endothelial dysfunction. For this, the vasoactive responses t o basal and stimulated endothelial nitric oxide (NO) were studied usin g the technique of the anaesthetized autoperfused rat, analyzing the r esponses to acetylcholine (ACh) and N-G-nitro-L-arginine methyl ester (L-NAME) in non-diabetic and diabetic rats with different degrees of m etabolic control (four groups with HbA(lc) levels of 5.5-7.4%, 7.5-9.4 %, 9.5-12%, and >12%, respectively). When administered over a noradren aline-induced vasopressor tone, ACh (0.25, 0.75, 2.5, 7.5 and 25 mu g kg(-1)) induced dose-dependent vasodilatatory responses in all rat gro ups, reducing both mean arterial pressure and perfusion pressure of th e left hindlimb. These responses were similar in non-diabetic and in d iabetic rats with good metabolic control (HbA(lc) 5.5-7.4%), while dia betic rats with levels of HbA(lc) higher than 7.5% showed significantl y lower vasodilatatory responses to ACh. In untreated diabetic rats, t he relaxant responses evoked by the NO donor sodium nitroprusside were also impaired. On the other hand, increasing doses of L-NAME (0.1 to 10 mg kg(-1)) enhanced both mean arterial pressure and left hindlimb p erfusion pressure in diabetic and non-diabetic rats. As with ACh, the responses to L-NAME were significantly reduced in diabetic rats with H bA(lc) levels higher than 7.5%. To determine the mechanism underlying the NO-mediated endothelial dysfunction, the responses to ACh in untre ated diabetic rats (HbA(lc) >12%) were studied in the presence of the NO substrate L-arginine, in the presence of the oxygen-derived free ra dical scavenger superoxide dismutase (SOD), or in the presence of both compounds. Both L-arginine and SOD produced a partial improvement of the ACh-induced vasodilatatory responses, but the effects of these age nts were not additive. In this group of animals, SOD also induced a pa rtial recovery of the L-NAME-evoked vasoconstrictions. In non-diabetic and untreated diabetic rats, the plasma levels of NO derivatives and arginine were measured. No significant differences were obtained in th e amount of nitrites plus nitrates, while plasma levels of arginine we re markedly reduced in the untreated diabetic animals. The results ind icate that the endothelial dysfunction associated to diabetes is close ly related to the level of metabolic control of the disease. Therefore , it is possible to establish a threshold for developing endothelium i mpairment from percentages of HbA(lc) higher than 7.5%. As the respons es to the NO synthase blocker L-NAME were analogously impaired, it is reasonable to suggest that diabetic endothelial dysfunction is related to the interference with mechanisms linked both to stimulated and bas al production of NO. We suggest that this interference is partially du e to a deficit in the substrate availability for NO and to an increase d generation of superoxide anions.