Although systemic renin activity is suppressed in diabetes, renin-angiotensin system (RAS) inhibitors exert renoprotective effects. Aliskiren, a renin inhibitor, is renoprotective in diabetic animal models. While it is possible to measure tissue RAS activity, no reports of in vivo images exist. We investigated the feasibility of using a fluorescence resonance energy transfer system to image enzymatic activity in order to evaluate the effects of aliskiren on diabetic nephropathy.Design and Method:
First, we induced diabetes in C57BL/6J mice using streptozotocin, then treated them with either aliskiren (25 mg/kg/day) or the angiotensin type 1 receptor blocker valsartan (15 mg/kg/day) for 4 weeks. Finally, we utilized renin fluorescence resonance energy transfer substrate to assess renin activity.Results:
Renin activity was much higher in the kidneys of diabetic mice compared to those of the non-diabetic control mice. While aliskiren inhibited this activity, valsartan did not. We noted that production of reactive oxygen species intensified and the bioavailability of nitric oxide diminished in the glomeruli of diabetic mice. Aliskiren and valsartan significantly ameliorated these effects. They suppressed glomerular production of reactive oxygen species and urinary albumin excretion. In fact, urinary albumin excretion in diabetic mice treated with aliskiren or valsartan was lower than that in untreated diabetic mice. Furthermore, aliskiren and valsartan significantly reduced glomerular permeability by maintaining the glomerular endothelial surface layer.Conclusions:
We found that injecting renin fluorescence resonance energy transfer substrate intravenously enabled us to visualize renin activity in the glomeruli of living animals. Our imaging technique helped reveal that although both aliskiren and valsartan significantly inhibited urinary albumin excretion in diabetic mice, only aliskiren significantly inhibited renin bioactivity. Fluorescence resonance energy transfer could provide a new tool for evaluating tissue and plasma enzymatic activity.