Metabolic and vascular factors in the pathogenesis of diabetic neuropathy.
Reduced nerve perfusion is an important factor in the etiology of diabetic neuropathy. Studies in streptozotocin-induced diabetic rats show that nerve conduction velocity (NCV) and blood flow deficits are corrected by treatment with vasodilator drugs, with angiotensin II and endothelin-1 antagonists being particularly important. The AT1 antagonist ZD7155 also prevents diabetic deficits in regeneration following nerve damage, indicating that hypoperfusion is an important limitation for nerve repair. Metabolic changes include high polyol pathway flux, increased advanced glycosylation, elevated oxidative stress, and impaired omega-6 essential fatty acid metabolism. Aldose reductase inhibitors (ARIs) restore NCV via their effects on perfusion. ARI action probably depends on blocking the conversion of glucose to sorbitol, thus preventing depletion of vasa nervorum glutathione, an important endogenous free radical scavenger. Free radicals cause vascular endothelium damage and reduced nitric oxide vasodilation. Inhibition of advanced glycosylation and autoxidation (autoxidative glycosylation), major sources of free radicals, by aminoguanidine or transition metal chelators, corrects neurovascular dysfunction. Evening primrose oil supplies gamma-linolenic acid (GLA) to improve vasodilator eicosanoid synthesis in diabetes, correcting nerve blood flow and NCV deficits. Interactions between some of these mechanisms have therapeutic implications. Thus, combined ARI and evening primrose oil treatment produced a 10-fold amplification of NCV and blood flow responses. Similarly, GLA effects are markedly enhanced when given in combination with ascorbate as ascorbyl-GLA. Thus, metabolic abnormalities combine to produce deleterious changes in nerve perfusion that make a major contribution to the etiology of diabetic neuropathy. The potential importance of multi-action therapy is stressed.