Introduction:
Diabetic neuropathy may develop on a background of hyperglycaemia and is associated with increased oxidative stress. Elevated asymmetric dimethylarginine (ADMA) levels are linked to oxidative stress reducing the synthesis of nitric oxide (NO) by uncoupling NO synthase. Oxidative stress induces considerable changes in nerve conduction velocity in diabetic patients. There is strong evidence that α-lipoic acid (ALA) as an antioxidant may improve nerve conduction and relieve neuropathic symptoms. We aimed to investigate the relationship between endothelial dysfunction and NO synthesis in type 2 diabetic patients with peripheral neuropathy after ALA treatment.

Material and methods:
Fifty-four type 2 diabetic patients with neuropathy were included in the study. Serum ADMA concentration, ICAM-1, VCAM-1, oxidised low-density lipoprotein (oxLDL), and TNF-α levels were determined with Enzyme-Linked Immunosorbent Assay (ELISA). Nitric oxide concentration was measured by Griess reaction. Peripheral sensory nerve function was assessed by current perception threshold (CPT) testing. Autonomic function was assessed by Ewing’s five standard cardiovascular reflex tests composite autonomic score (CAS).

Results:
Asymmetric dimethylarginine levels were significantly decreased (0.62 ±0.11 vs. 0.53 ±0.11 µmol/l, p < 0.001), as well as TNF-α concentrations (1.21 ±0.42 pg/ml vs. 1.05 ±0.5 pg/ml, p < 0.05), while NO levels were significantly increased (16.78 ±11.1 vs. 21.58 ±8.84 µmol/l, p < 0.05) after six-months of 600 mg/day ALA treatment. VCAM-1, ICAM-1, and oxLDL levels did not change significantly. The CPT and CAS significantly improved after ALA treatment. The improvement of CPT values was correlated positively with the change of ADMA levels (r = 0.58, p < 0.001). The change in ADMA level was more pronounced in responder patients based on both CPT and CAS.

Conclusions:
Our results suggest that ALA supplementation improves endothelial function characterised by serum levels of ADMA and TNF-α in patients with diabetic neuropathy. Changes in serum ADMA levels may predict the clinical response to ALA treatment.