Chronic Bilateral Renal Denervation in Experimental Diabetic Nephropathy

Abstract

Chronic bilateral renal denervation (BRD) has been shown to reduce systemic blood pressure and improve insulin sensitivity and renal function in both human and experimental animal models. However, the effect of chronic BRD on the progression of diabetic nephropathy has not been explored. The aim of this study was to investigate the effects of chronic BRD on the progression of diabetic renal injury and dysfunction in a renin over-expressing hypertensive transgenic (mRen-2)27 rat (TGR).

Following streptozotocin induction of diabetes, BRD or sham surgeries were conducted repeatedly (at the 3rd, 6th and 9th week following induction) in both diabetic and normoglycaemic (NG) TGRs. Blood glucose was successfully controlled in the diabetic groups at 25-30 mM using subcutaneous insulin administration. BRD resulted in a progressive decrease in systolic blood pressure (SBP) from first denervation to termination (at 12-weeks post-diabetic induction) in both NG and diabetic rats. In contrast, SBP was unchanged in sham-operated NG and diabetic groups over the same period. Absolute urinary albumin excretion was significantly increased in diabetic compared to NG sham operated TGRs; the effect of BRD in diabetic TGR resulted in a significantly reduced rates of albumin excretion compared to the intact innervated group. The presence of albuminuria indicative of glomerular podocyte injury was confirmed by the significant loss of podocin positive staining in the glomeruli of innervated diabetic rats but not in the BRD-treated diabetic animals. Plasma urate levels in diabetic innervated rats were also high compared to NG innervated rats; BRD intervention in diabetic TGRs resulted in a significantly reduced plasma urate level compared to the innervated TGRs. Microscopic examination confirmed the presence of glomerular basement membrane thickening and mesangial expansion in the diabetic kidneys. This morphological appearance was markedly reduced by BRD intervention. Significantly higher expressions of type I and type IV collagen and α-smooth muscle actin (SMA) were observed in innervated diabetic kidneys compared to innervated NG groups. BRD intervention in the diabetic TGRs significantly reduced type I and type IV collagen and α-SMA expression at termination when compared to the sham operated diabetic animal data. Expression of transforming growth factor (TGF)-β1 was observed in the glomeruli, proximal tubules and interstitial space of both NG and diabetic innervated animals. This expression was markedly reduced in both renal cortical and medullary interstitium and glomeruli following BRD.

To address the effects of renal innervation and diabetic onset on renal haemodynamics and tubular functions, a separate group of TGRs underwent the same experimental protocol of diabetic induction and BRD surgeries were studied using clearance techniques at termination. Renal haemodynamics was not changed by chronic BRD or diabetes at the end of the 12 week study period. However, renal tubular handling of sodium and water were altered as shown by decreased levels of urine flow rate, absolute and fractional excretions of sodium observed in the diabetic TGRs compared to NG controls. In addition, an acute renin angiotensin system (RAS) inhibition protocol using captopril infusion resulted in a natriuretic and diuretic response in the chronic denervated diabetic TGRs; whereas it produced an anti-natriuretic response in the chronic innervated diabetic group. In contrast, this difference was not observed in NG TGRs where both chronic innervated and denervated NG TGRs produced anti-natriuretic and anti-diuretic responses upon acute RAS inhibition.

These results indicate that chronic BRD intervention after the onset of diabetes decreased SBP in the hypertensive TGR model. In addition, this intervention strategy reduced levels of renal pathological markers occurring consequent to diabetic induction including: albuminuria, plasma uric acid, pro-fibrotic TGF-β1, fibrotic collagens I and IV and α-SMA in the injured kidney. Glomerulosclerosis and podocyte injury were also reduced by BRD. Functionally, chronic BRD resulted in an enhanced diuretic and natriuretic response in diabetic TGRs upon acute RAS inhibition which suggested that chronic BRD may also change angiotensin II receptor expression in diabetic kidneys.

In conclusion, this study has shown that renal denervation has clear renoprotective effects in an animal model of hypertension with hyperglycaemia, representative of the histological and protein expression changes observed in patients with diabetic nephropathy.