Role of the renal sympathetic nerve in renal glucose metabolism during the development of type 2 diabetes in rats

Abstract

Aims/hypothesis: Recent clinical studies have shown that renal sympathetic denervation (RDX) improves glucose metabolism in patients with resistant hypertension. We aimed to elucidate the potential contribution of the renal sympathetic nervous system to glucose metabolism during the development of type 2 diabetes.

Methods: Uninephrectomised diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) rats underwent RDX at 25 weeks of age and were followed up to 46 weeks of age.

Results: RDX decreased plasma and renal tissue noradrenaline (norepinephrine) levels and BP. RDX also improved glucose metabolism and insulin sensitivity, which was associated with increased in vivo glucose uptake by peripheral tissues. Furthermore, RDX suppressed overexpression of sodium-glucose cotransporter 2 (Sglt2 [also known as Slc5a2]) in renal tissues, which was followed by an augmentation of glycosuria in type 2 diabetic OLETF rats. Similar improvements in glucose metabolism after RDX were observed in young OLETF rats at the prediabetic stage (21 weeks of age) without changing BP.

Conclusions/interpretation: Here, we propose the new concept of a connection between renal glucose metabolism and the renal sympathetic nervous system during the development of type 2 diabetes. Our data demonstrate that RDX exerts beneficial effects on glucose metabolism by an increase in tissue glucose uptake and glycosuria induced by Sglt2 suppression. These data have provided a new insight not only into the treatment of hypertensive type 2 diabetic patients, but also the pathophysiology of insulin resistance manifested by sympathetic hyperactivity.

Keywords: Blood pressure; Glucose metabolism; Insulin resistance; Renal sympathetic denervation (RDX); Sglt2; Slc5a2; Sodium-glucose cotransporter 2; Type 2 diabetes.

Fig. 1

(a) OLETF rats show significantly higher kidney tissue NE levels compared with LETO rats. Kidney tissue NE levels in OLETF rats subjected to RDX are almost undetectable. (b, c) OLETF rats show higher body weight (b) and daily average food intake (c) compared with LETO rats, which were not affected by RDX. (d, e) OLETF rats show markedly elevated fasting blood glucose (d) and HbA_1c (e) levels compared with LETO rats, which are suppressed by RDX. (To convert values for HbA_1c in DCCT % into mmol/mol, subtract 2.15 and multiply by 10.929.) (f–h) OLETF rats show markedly increased SBP (f), MAP (g) and DBP (h) compared with LETO rats. RDX suppresses SBP, MAP and DBP in OLETF rats. However, heart rate was not different among the groups (i). ^† p < 0.05, ^†† p < 0.01, ^††† p < 0.005 LETO vs OLETF; ^‡ p < 0.05, ^‡‡ p < 0.01, ^‡‡‡ p < 0.005 OLETF vs OLETF+RDX; ^§ p < 0.05 LETO vs LETO+RDX; ^§§§ p < 0.005 LETO vs LETO+RDX. White bars and white diamonds, LETO group; black bars and black circles, OLETF group; light grey bars and black diamonds, LETO+RDX; dark grey bars and white circles, OLETF+RDX

Fig. 2

(a–f) Blood glucose levels and their respective AUCs at 30 (a, b), 35 (c, d) and 45 (e, f) weeks of age during OGTT. OLETF rats show higher glucose levels and AUCs, which are suppressed by RDX. (g–l) Plasma insulin levels and their AUC at 30 (g, h), 35 (i, j) and 45 (k, l) weeks of age. OLETF rats show higher plasma insulin levels and AUC, which were suppressed by RDX. ^††† p < 0.005 LETO vs OLETF; ^‡ p < 0.05, ^‡‡ p < 0.01, OLETF vs OLETF+RDX; ^§ p < 0.05, LETO vs LETO+RDX. White bars and white diamonds, LETO group; black bars and black circles, OLETF group; light grey bars and black diamonds, LETO+RDX; dark grey bars and white circles, OLETF+RDX

Fig. 3

(a) OLETF rats show lower GIR during the hyperinsulinaemic–euglycaemic clamp study, which is increased by RDX. (b) During rate constant net tissue uptake of 2-[³H]DG measurement, OLETF rats show lower glucose uptake in BAT, WAT, soleus muscles and liver tissues compared with LETO rats. RDX improves glucose uptake by these tissues in OLETF rats. (c, d) OLETF rats show elevated plasma NE levels (c) and urinary NE excretion (d), which are suppressed by RDX. (e) OLETF rats show higher urinary glucose excretion, which is further increased by RDX. ^† p < 0.05, ^†† p < 0.01, ^††† p < 0.005 LETO vs OLETF; ^‡ p < 0.05, ^‡‡ p < 0.01, ^‡‡‡ p < 0.005 OLETF vs OLETF+RDX; ^§ p < 0.05 LETO vs LETO+RDX. White bars, LETO group; black bars, OLETF group; light grey bars, LETO+RDX; dark grey bars, OLETF+RDX

Fig. 4

(a) Glut1 mRNA levels are similar among the treatment groups. (b, c) Sglt1 (b) and Glut2 (c) mRNA expression is upregulated. (d) Sglt2 mRNA level is markedly upregulated in OLETF rats, which is suppressed by RDX. (e) Immunofluorescence micrographs of staining with anti-SGLT2 antibody (red fluorescence) (original magnification, ×200). The staining intensity is weaker in OLETF+RDX than OLETF. All mRNA data are expressed as the relative difference in expression compared with LETO rats after normalisation for β-actin expression. ^† p < 0.05, ^††† p < 0.005 LETO vs OLETF; ^‡ p < 0.05, ^‡‡ p < 0.01 OLETF vs. OLETF+RDX. White bars, LETO group; black bars, OLETF group; light grey bars, LETO + RDX; dark grey bars, OLETF+RDX

Fig. 5

(a) OLETF rats show overt proteinuria, which is reduced by RDX. (b) Plasma creatinine concentration was slightly elevated in OLETF rats and is attenuated by RDX. (c) Representative micrographs of PAS-stained renal sections. (d) PAS-positive area within the total glomerular area. RDX partially reduced PAS-positive area. ^† p < 0.05, ^††† p < 0.005, ^†††† p < 0.001 LETO vs OLETF; ^‡ p < 0.05, ^‡‡ p < 0.01 OLETF vs OLETF+RDX. Scale bar, 50 μm. White bars, LETO group; black bars, OLETF group; light grey bars, LETO+RDX; dark grey bars, OLETF+RDX

Fig. 6

(a) Treatment with high glucose for 12 h upregulated SGLT2 mRNA expression, which was further enhanced by exposure to NE in HK2 human kidney proximal tubule epithelial cells. (b) GLUT2 mRNA expression was also upregulated by high glucose treatment, and was further enhanced by exposure to NE. (c, d) In contrast, SGLT1 (c) and GLUT1 (d) mRNA expression remained unaltered after high glucose and/or high glucose plus NE treatment. Data are expressed as the relative difference in expression compared with 5 mmol/l glucose after normalisation for β-actin expression. *p < 0.05, **p < 0.01 vs 5 mmol/l glucose; ^¶¶ p < 0.01 vs 5 mmol/l glucose + NE; ^† p < 0.05 vs 15 mmol/l glucose. ^‡ p < 0.05, as indicated

Fig. 7

(a) Schematic diagram summarising the proposed mechanisms by which renal sympathetic nerves contribute to glucose metabolism during the development of type 2 diabetes. (b) Effects of RDX on renal glucose metabolism. CNS, central nervous system, SNS; sympathetic nervous system