Diabetes is known to induce endothelial dysfunction and thus an impairment of endothelium-dependent vasodilatation of micro- and macrocirculation in different animal species as well as in humans. It has been shown that even transient increases in plasma glucose concentrations impair vascular function. One of the proposed mechanisms involves reduced bioavailability of nitric oxide (NO) due to increased scavenging by reactive oxygen species (ROS). Moreover, hyperglycemia is reported to induce endothelial nitric oxide synthase (eNOS) uncoupling, leading to increased production of superoxide instead of NO by eNOS and thus aggravating endothelial dysfunction. eNOS uncoupling is among others the result of enhanced oxydation of tetrahydrobiopterin (BH4), an essential cofactor of eNOS, by ROS that cause cofactor insufficiency. The aim of our study was to assess the effect of acute hyperglycemia on endothelium-dependent, NO-mediated vasodilatation in rat aorta. Further, we wanted to investigate whether the potential impairment of NO-mediated vasodilatation could be reversed either by the application of sepiapterin, a precursor of BH4, or by preatretment of aortic rings by antioxidants.

In isolated, precontracted, endothelium-intact rat aortic rings, incubated with diclofenac (0.01 mM) 30 minutes as well as two hours exposure to high glucose concentration (30 mM) caused a rightward shift in the concentration-relaxation curve to acetylcholine (ACh), as compared to 5 mM -glucose solution or 30 mM-mannitol solution. The effect of hyperglycemia was more pronounced after 30 minutes (pD2=7.16±0.07 for 5mM glucose and 6.61±0.14 for 30 mM glucose solution, p<=0.01, one-way ANOVA) than after two hours exposure to hyperglycemia (pD2=7.46±0.12 for 5mM glucose and 7.11±0.13 for 30 mM glucose solution, p<=0.05). The effect of hyperglycemia was significantly attenuated when rings were preincubated with sepiapterin (10 mM) for one hour. On the contrary, incubation of rings with the antioxidant vitamin C (100 mM) did not affect the glucose-induced impairment of NO-mediated relaxation. However, coincubation of rings with vitamin C and EDTA (26 mM) did abolish the effect of high glucose.

The results show that acute hyperglycemia diminishes the endothelium-dependent, NO- mediated relaxation in rat aorta. There is indirect evidence that diminished relaxation might be the consequence of eNOS uncoupling and increased superoxide production as sepiapterin, precursor of BH4, as well as vitamin C complemented by EDTA did restore the reduced vasodilatation. We may implicate that administration of antioxidants and/or precursors of BH4 might reverse the detrimental effects of hyperglycemia on endothelial function.