Impairment of endothelium‐dependent but not of endothelium‐independent dilatation in guinea‐pig aorta rings incubated in the presence of elevated glucose

Purine compounds such as ATP and adenosine, respectively endothelium‐dependent and‐independent vasodilators, are largely involved in the control of vascular tone and vascular reactivity to contracting stimuli. We investigated the relaxing activity of ATP and adenosine in guinea‐pig aorta rings exposed for 6 h to elevated glucose concentration (50 mM), in order to mimic hyperglycaemic conditions. Guinea‐pigs were reserpine‐treated (2 mg kg−1, i.p., 48 and 24 h before death). Rings of aortae incubated in 50 mM glucose, contracted submaximally by 1 μM noradrenaline, lost endothelium‐dependent relaxation in response to acetylcholine (10 nM to 10 μM). Aortae incubated with 50 mM mannose, as a hyperosmotic control, relaxed to acetylcholine normally. Rings of aortae incubated in 50 mM glucose, contracted submaximally by 3 mM 4‐aminopyridine, lost endothelium‐dependent relaxation in response to ATP (30 μM) whereas endothelium‐independent relaxation in response to adenosine (0.3 mM) was well preserved. The relaxation induced by A23187 or sodium nitroprusside (10 nM to 0.1 μM) did not differ between rings exposed to control (5.5 mM) or elevated glucose (50 mM) and contracted submaximally by 3 mM 4‐aminopyridine. When incubated with aortic tissue in the presence of elevated glucose, the cyclo‐oxygenase inhibitors, indomethacin (10 μM) and mefenamic acid (30 μM), or the scavenger of superoxide anions, superoxide dismutase (150 u ml−1), prevented the impairment of ATP‐mediated relaxation. The present results indicate that endothelium‐dependent, receptor‐induced relaxation in response to acetylcholine and ATP is impaired in guinea‐pig aorta rings exposed to elevated glucose. The endothelial dysfunction caused by glucose might be located at a step between receptor activation and intracellular calcium increase, and might be related to an increased metabolism of arachidonic acid coupled to an increased production, or to a reduced inactivation of superoxide anions.