Renal Urea Transport Alterations with Prolonged Renal Urea Transport Alterations with Prolonged Hyperglycaemia

Abstract

PurposeTo explore whether hyperpolarized urea could serve as a possible imaging bio‐marker of the renal function via the intra‐renal osmolality gradient in diabetes.MethodsThirteen 250 g female wistar rats were randomly grouped in a diabetic group (four weeks of untreated type‐1 diabetes) of six animals and a healthy control group of seven animals. Diabetes was induced by an i.v. injection of freshly prepared streptozotocin (STZ; 55 mg/kg body weight). Rats were considered diabetic when the blood glucose levels exceeded 15 mmol/L at 48 h after injection of STZ. For the MR examinations, the rats were anesthetized and a tail vein catheterisation was performed. The MR examination was performed with a 9.4 T pre‐clinical MR system (Agilent, UK) equipped with a dual tuned 13C/1H volume rat coil (Doety scientic, US). Urea was polarized in a 5T SPINLab (GE Healthcare, Brøndby, DK) to a reproducible polarization of more than 40% [1]. The hyperpolarized sample was rapidly dissolved and a final solution of 1.5 ml hyperpolarized urea was injected intravenously to the animal over a period of 15 sec. A 1H gradient echo coronal and axial scan was acquired (TR/TE 100 ms/2.6 ms, flip 20°, FOV 60 mm2, matrix 128 × 128). The scan was performed as an anatomical 1H scout (Figure 1A). 3D 13C balanced steady state free precession (BSSFP) imaging sequences were performed (flip 15°, TR/TE 5.4 ms/2.7 ms, sw 20kHz, FOV 60 mm3, matrix 48×48×48 covering both kidneys) and initiated 35 s after the start of hyperpolarized [13C,15N]urea injection (Figure 1B).At the time of euthanasia, both kidneys were rapidly dissected. Cortex and inner medulla (IM) were collected separately and analyses for different urea transporters were done with qPCR.ResultsThe rats administered with STZ, all developed sustained hyperglycaemia within 48 hours. 3D balanced steady state 13C MRI images of hyperpolarized urea showed a cortico‐medullary urea gradient (Figure 1) with an accumulation of urea in the medullary and pelvic segments. A significant higher urea‐to‐total‐signal in the cortical region (p=0.008) concomitant with a significant decreased urea‐to‐total‐signal in the pelvic region (p=0.03) were found in the diabetic animals (Figure 2A). This resulted in a reduced intra‐renal urea‐to‐cortical distribution in the medulla (p=0.04) and pelvic (p=0.002) regions in the diabetics when compared to controls (Figure 2B). A significant intra‐renal increased urea accumulation from the cortex to the pelvis was found in both the diabetic and the control kidney, (p<0.0001) (Figure 2B). The transcription levels of the urea transporters were all significantly altered in the diabetic rat kidney compared to the control animals (Figure 3).ConclusionThis study illustrated the potential of using hyperpolarized urea in estimating renal function, by showing a decreased intra‐renal distribution after prolonged hyperglycemia. This was consistent with the findings of a gradually decreasing function found in the diabetic kidney. The transcription of the urea transporters was generally up‐regulated, supporting the increased oxygen consumption. In contrast, the lower urea gradient in the diabetic kidney, compared to the healthy controls, verified the reduced renal function after four weeks of diabetes.Support or Funding InformationThe Danish Research Council supported the study.