Contribution of hyperglycemia on diabetic complications in obese type 2 diabetic SDT fatty rats: effects of SGLT inhibitor phlorizin

Abstract

The spontaneously diabetic torii (SDT) fatty rat is a new model of type 2 diabetes showing overt obesity, hyperglycemia and hyperlipidemia. With early onset of diabetes mellitus, diabetic microvascular complications, including nephropathy, peripheral neuropathy and retinopathy, are observed at young ages. In the present study, blood glucose levels of female SDT fatty rats were controlled with phlorizin, a non-selective SGLT inhibitor, to examine whether and how these complications are caused by hyperglycemia. Phlorizin treatment adequately controlled plasma glucose levels during the experiment. At 29 weeks of age, urinary albumin excretion considerably increased in SDT fatty rats. Glomerulosclerosis and tubular pathological findings also indicate diabetic nephropathy. These renal parameters tended to decrease with phlorizin; however, effects were partial. Sciatic nerve conduction velocities were significantly delayed in SDT fatty rats compared with Sprague-Dawley (SD) rats. Intraepidermal nerve fiber density, an indicator of subclinical small nerve fiber neuropathy, significantly decreased in SDT fatty rats. Retinal dysfunction (prolongation of peak latency for oscillatory potential in electroretinograms) and histopathological eye abnormalities, including retinal folding and mature cataracts were also observed. Both nerve and eye disorders were prevented with phlorizin. These findings indicate that severe hyperglycemia mainly causes diabetic complications in SDT fatty rats. However, other factors, such as hyperlipidemia and hypertension, may affect diabetic nephropathy. These characteristics of diabetic complications will become helpful in evaluating new drugs for diabetic complications using SDT fatty rats.

Fig. 1.

Hypoglycemic effect of phlorizin in SDT fatty rats. A single dose of phlorizin clearly decreased blood glucose levels. Six hours after administration, blood glucose reached normal levels. At 24 hours, the effect of phlorizin partially remained. White circles; SD rats, black circles; SDT fatty rats (vehicle), gray circles; SDT fatty rats (100 mg/kg phlorizin). Each value represents the mean ± SD (n=4). ^**P<0.01, ^*P<0.05 vs. age-matched SD rats. ^†P<0.05 vs. vehicle treated SDT fatty rats (unpaired t-test).

Fig. 2.

Effect of phlorizin on biochemical parameters of SDT fatty rats. Effects of phlorizin on (A) body weight, (B) food consumption (at the end of experiment; 29 weeks of age), (C) Urinary glucose, (D) plasma glucose levels, (E) blood HbA1c levels, (F) plasma insulin levels, (G) plasma triglyceride levels, (H) plasma free fatty acid levels and (I) plasma total cholesterol levels. Phlorizin treatment improved hyperglycemia and delayed insulin level deceases. Hyperlipidemia was not clearly prevented with phlorizin. White circles; SD rats, black circles; SDT fatty rats (vehicle), gray circles; SDT fatty rats (100 mg/kg phlorizin). Each value represents the mean ± SD (n=8). ^**P<0.01, ^*P<0.05 vs. age-matched SD rats. ^††P<0.01, ^†P<0.05 vs. vehicle treated SDT fatty rats (unpaired t-test).

Fig. 3.

Effect of phlorizin on diabetic nephropathy of SDT fatty rats. Urinary albumin excretion (A), creatinine clearance (B) and urinary 8-OHdG (C) increased in SDT fatty rats. Increases in all these renal parameters were partially prevented with phlorizin treatment. Typical microphotographs of the kidneys of SD rats (D) and SDT fatty rats (E). Tubular dilation and Armanni-Ebstein changes were found in SDT fatty rats. Phlorizin treatment completely prevented these tubular abnormalities (F). Bars=50 µm. Each value represents the mean ± SD (n=8). ^**P<0.01 vs. age-matched SD rats, ^†P<0.05 vs. vehicle treated SDT fatty rats (unpaired t-test).

Fig. 4.

Effect of phlorizin on diabetic peripheral neuropathy of SDT fatty rats. The effects of phlorizin on peripheral nerve dysfunction (sciatic MNCV and SNCV) and IENFD in female SDT fatty rats were evaluated. Impairments of MNCV (A) and SNCV (B) improved significantly with phlorizin treatment compared with vehicle-treated SDT fatty rats. Reductions in IENFD were also prevented with phlorizin (C). Typical confocal microscopic pictures of anti-PGP9.5 immunostained hind paw skin from SD rats (D), vehicle-treated SDT fatty rats (E), and phlorizin-treated SDT fatty rats (F). Bars=20 µm. Each value represents the mean ± SD (n=6–8). ^**P<0.01, ^*P<0.05 vs. age-matched SD rats, ^†P<0.05 vs. vehicle treated SDT fatty rats (unpaired t-test).

Fig. 5.

Effect of phlorizin on diabetic retinopathy and cataracts of SDT fatty rats. Delayed OPs in ERG were observed in SDT fatty rats (A). Cataracts also progressed in SDT fatty rats (B). These eye disorders were prevented with phlorizin treatment. Compared to SD rats (C), retinal folding and thickening were observed in SDT fatty rats (D) and were improved with phlorizin (E). Bars=100 µm. Normal rats showed clear lens (F), but mature cataracts were found in SDT fatty rats (G). Cataracts did not progress in phlorizin treated SDT fatty rats (H). Bars=50 µm. Each value represents the mean ± SD (n=8). ^**P<0.01 vs. age-matched SD rats, ^†P<0.05 vs. vehicle treated SDT fatty rats (unpaired t-test). ^##P<0.01 vs. age-matched SD rats, ^‡‡P<0.01 vs. vehicle treated SDT fatty rats (Wilcoxon rank-sum test).