Free radicals in diabetic endothelial cell dysfunction

Glutathione reverses systemic hemodynamic changes induced by acute hyperglycemia in healthy subjects

The present study aimed at evaluating whether acute elevations of plasma glucose concentrations could influence systemic hemodynamic parameters and baroreflex activity in humans. Plasma glucose concentrations were acutely raised to 15 mmol/l in 12 healthy male volunteers. During hyperglycemia, there were significant increments of systolic [from 118 +/- 9 to 138 +/- 12 (SD) mmHg, P < 0.01] and diastolic (from 71 +/- 5 to 85 +/- 6 mmHg, P < 0.001) blood pressure, as well as heart rate (from 76 +/- 7 to 85 +/- 10 beats/min, P < 0.01) and plasma catecholamine levels. Both maximal and steady-state mean arterial pressure after squatting were higher during hyperglycemia (27 +/- 8 and 16 +/- 6 mmHg, respectively) compared with levels obtained during euglycemia (18 +/- 5 and 2 +/- 0.6 mmHg, respectively, P < 0.001). The infusion of the somatostatin analogue octreotide (25 micrograms as i.v. bolus followed by a 0.5 microgram/min infusion), to avoid the possible confounding vascular actions of insulin, did not influence the hemodynamic effects of hyperglycemia except for a lesser increase of both heart rate and plasma catecholamines. Glutathione (600 mg as an iv bolus followed by a 5 mg/min infusion) completely prevented the vascular effects of hyperglycemia. This study shows that acute hyperglycemia, similar to that observed in poorly controlled diabetic patients, produces relevant systemic hemodynamic changes and also alters baroreflex activity via a glutathione-sensitive presumably free radical-mediated pathway.

Impaired vasoreactivity to nitric oxide in experimental diabetic neuropathy

Nerve blood flow (NBF) is reduced in experimental diabetic neuropathy (EDN), but the mechanism of its reduction is uncertain. We tested the hypothesis that reduced NBF might be due to alterations of nitric oxide synthase (NOS) and endothelin of microvascular endothelial cells of sciatic nerve. We evaluated epineurial arteriolar vasoreactivity in response to superfused test agents. NBF was measured using microelectrode H2 polarography. Vasoconstrictor responses to endothelin-1 (ET-1; 10(-6), 10(-7), 10(-8), 10(-9), 10(-10) M) showed dose-response curves with similar EC50 values, indicating no change in potency. We applied the NOS inhibitor NG-nitro-L-arginine and observed reduced inhibition of NBF in EDN, correctable with insulin treatment and also with infused L-arginine. We conclude that vasoreactivity is disturbed in EDN, and is due to a combination of an impairment of NOS activity with reduced NO and increased endothelin effect (normal receptor sensitivity and increased plasma values) in EDN. Hyperglycemia is likely to be the mechanism of NOS inhibition since insulin treatment reversed this abnormality.