Influence of Glucose Fluctuation on Peripheral Nerve Damage in Streptozotocin-Induced Diabetic Rats

Abstract

Background: It is unclear whether glycemic variability (GV) is a risk factor for diabetic peripheral neuropathy (DPN), and whether control of GV is beneficial for DPN. The purpose of this study was to investigate the effect of GV on peripheral nerve damage by inducing glucose fluctuation in streptozotocin-induced diabetic rats.

Methods: Rats were divided into four groups: normal (normal glucose group [NOR]), diabetes without treatment (sustained severe hyperglycemia group; diabetes mellitus [DM]), diabetes+once daily insulin glargine (stable hyperglycemia group; DM+LAN), and diabetes+once daily insulin glargine with twice daily insulin glulisine (unstable glucose fluctuation group; DM+Lantus [LAN]+Apidra [API]). We measured anti-oxidant enzyme levels and behavioral responses against tactile, thermal, and pressure stimuli in the plasma of rats. We also performed a quantitative comparison of cutaneous and sciatic nerves according to glucose fluctuation.

Results: At week 24, intraepidermal nerve fiber density was less reduced in the insulin-administered groups compared to the DM group (P<0.05); however, a significant difference was not observed between the DM+LAN and DM+LAN+API groups irrespective of glucose fluctuation (P>0.05; 16.2±1.6, 12.4±2.0, 14.3±0.9, and 13.9±0.6 for NOR, DM, DM+LAN, and DM+LAN+API, respectively). The DM group exhibited significantly decreased glutathione levels compared to the insulin-administered groups (2.64±0.10 μmol/mL, DM+LAN; 1.93±0.0 μmol/mL, DM+LAN+API vs. 1.25±0.04 μmol/mL, DM; P<0.05).

Conclusion: Our study suggests that glucose control itself is more important than glucose fluctuation in the prevention of peripheral nerve damage, and intra-day glucose fluctuation has a limited effect on the progression of peripheral neuropathy in rats with diabetes.

Keywords: Diabetes mellitus; Diabetic neuropathies; Insulin; Peripheral nerves.

Fig. 1.

(A) Body weight change, (B) mean blood glucose levels on the first day of week 24, and (C) glycosylated hemoglobin (HbA1c) levels in the experimental groups. Values are presented as mean±standard error of mean. NOR, normal; DM, diabetes mellitus; DM+LAN, DM treated with insulin glargine; DM+LAN+API, DM treated with insulin glargine and glulisine (n=8–10 in each group). ^aP<0.05 vs. normal, ^bP<0.05 vs. DM.

Fig. 2.

Comparison of glycemic variability indices of the experimental groups: (A) 8-point glucose monitoring on the first day of week 24; (B) mean standard deviation (SD) of blood glucose on the first day of week 24; (C) absolute change in HbA1c (0 to 24th week); (D) mean SD of HbA1c (0 to 24th week); and (E) % coefficient of variation (CV) of HbA1c (0 to 24th week). Values are presented as mean±standard error of mean. NOR, normal; DM, diabetes mellitus; DM+LAN, DM treated with insulin glargine; DM+LAN+API, DM treated with insulin glargine and glulisine (n=8–10 in each group). ^aP<0.05 vs. normal, ^bP<0.05 vs. DM, ^cP<0.05 vs. DM+LAN.

Fig. 3.

The level of antioxidant enzymes of the experimental groups at week 24. (A) Superoxide dismutase, (B) catalase activities, and (C) glutathione level in blood. Values are presented as mean±standard error of mean. NOR, normal; DM, diabetes mellitus; DM+LAN, DM treated with insulin glargine; DM+LAN+API, DM treated with insulin glargine and glulisine (n=8–10 in each group). ^aP<0.05 vs. normal, ^bP<0.05 vs. DM.

Fig. 4.

The threshold of responses with diverse sensory tests in the experimental groups at 24 weeks. (A) Von Frey filament response, (B) the responses for hot plate, (C) tail flick test, and (D) Randall-Sellito test. Values are presented as mean±standard error of mean. NOR, normal; DM, diabetes mellitus; DM+LAN, DM treated with insulin glargine; DM+LAN+API, DM treated with insulin glargine and glulisine (n=8–10 in each group). ^aP<0.05 vs. normal, ^bP<0.05 vs. DM, ^cP<0.05 vs. DM+LAN.

Fig. 5.

Quantitative comparison of cutaneous nerves with (A) the mean intraepidermal nerve fiber density and (B) immunohistochemistry of cutaneous small nerve fibers of the dorsum (×100). Arrows indicate immunostained small nerve fibers. Bar indicates 100 μm. Values are presented as mean±standard error of mean. NOR, normal; DM, diabetes mellitus; DM+LAN, insulin glargine treated DM; DM+LAN+API, insulin glargine with glulisine treated DM (n=8–10 in each group).

Fig. 6.

Quantitative comparison of sciatic nerve, including (A) the diameter of myelin sheath, (B) the diameter of axon, and (C) immunohistochemistry of the sciatic nerve of the experimental groups (×1,000). Bar indicates 20 μm. Values are presented as mean±standard error of mean. NOR, normal; DM, diabetes mellitus; DM+LAN, DM treated with insulin glargine; DM+LAN+API, DM treated with insulin glargine and glulisine (n=8–10 in each group). ^aP<0.05 vs. normal, ^bP<0.05 vs. DM.