Renal ischemia and reperfusion assessment with 3D hyperpolarized 13C urea
Per Mose Nielsen1, Esben Søvsø Szocska Hansen2, Thomas Nørlinger3, Rikke Nørregaard3, Lotte Bonde Bertelsen4, Hans Stødkilde Jørgensen5, and Christoffer Laustsen3

1Clinical medicine, Aarhus university, Aarhus C, Denmark, 2MR Research Centre, institute of clinical medicine, Aarhus C, Denmark, 3Institute of clinical medicine, Aarhus C, Denmark, 4institute of clinical medicine, Aarhus C, Denmark, 5MR Research Centre, Institute of clinical medicine, Aarhus C, Denmark

Synopsis

Renal homeostasis is determined by the active transport of water and waste products, and the key to this is the active intra-renal transport of ions, creating the needed osmotic gradient. The maintenance of this cortico-medullary ion gradient is believed to be a measurement of kidney function. We investigated 3D hyperpolarized 13C,15N-urea for mapping of this intra-renal gradient in a unilateral ischemic reperfusion renal rat model. A 3D balanced steady state sequence with an isotropic 1.25 mm resolution was used to image renal ischemic reperfusion injury. We revealed a significant reduction in the intra-renal ion gradient in the ischemic kidney.

purpose

Renal ischemia reperfusion injury (IRI) is a major cause of acute kidney injury (AKI), which can lead to a rapid (within 48h) reduction in the kidney function1. The kidneys main function is to maintain body water/salt balance and to secrete waster products produced in the body. To preserve this function, a medullary osmotic gradient is needed to maintain the ability to regulate the concentration of final excreted urine. Renal tubules are the main site of water-salt exchange, and here dysregulation of essential transport proteins like aquaporins, sodium, and urea transporters is seen in response to IRI2. IRI is often diagnosed by measuring blood urea nitrogen (BUN) and creatinine clearance. Albeit sensitive, the residual function of the contralateral kidney (in unilateral disease) reduces the specificity of these methods. Thus, a particularly interesting alternative in renal investigations is hyperpolarized non-metabolic bio probes3. A particularly appealing bio-probe for renal investigations is urea. We therefore suggest that monitoring of the renal functional filtrations state via the renal cortico-medullary urea gradient may contribute to a more detailed and sensitive quantitative measure of renal status in patients experiencing AKI.

Methods

Twelve female Wistar Rats were included in the study. Rats were subjected to unilateral renal ischemia for 60 min followed by reperfusion for 24 hours, the contralateral healthy kidney served as control. A tail vein catheter was inserted for injection of hyperpolarized [13C,15N2]urea before the MRI scanning. A 23Na MR examination was performed with a gradient echo sequence, (TR/TE/flip angle/ spectral width/ matrix / FOV) 50 ms/2.0 ms/90°/10 kHz/ 32 x 32 x 8 / 60 x 60 x 60, hereafter a 3D fully balanced steady state imaging sequence with hard pulse excitation and refocusing pulses was employed for 3D imaging of the kidney 13C,15N urea distribution. The experimental parameters where (TR/TE/flip angle/spectral width/matrix/FOV) 5.4 ms/2.7 ms/15°/20 kHz/48 x 48 x 48/60 x 60 x 60. The experiments were performed in a pre-clinical 9.4T MRI (Agilent) equipped with a triple-tuned Doety 1H/13C/23Na coil. After MRI scanning, kidneys were excised and stored for further biochemical analyses.

Results and Discussion

All rats in this study showed evidence of severe damage to the IRI kidney 24 hours after the surgery (Figure 1). The total kidney 13C urea signal intensity was significantly decreased with 56% in the IRI kidney compared to the control kidney. This was associated with a decreased urea cortico-medullary gradient of 39% in the outer medulla (OM) and 70% in the inner medulla (IM) (Figure 2). This correlated with similar decreases in sodium gradient (19% in both OM and IM), which has previously been shown also to correlate with IRI4. A general down regulation of urea transporters UT-A1, UT-A3 and UT-B was evident and correlates with several studies indicating a downregulation of urea transporters associated with the loss of cortico-medullary ion gradient5,6

Conclusion

The fact that the intra-renal urea distribution is altered and secondly observable in the injured kidney, demonstrate the potential of hyperpolarized urea examinations in the diseased kidney in human subjects. The recent translation of hyperpolarized MRI to human patients7 and the introduction of high resolution mapping of the intra-renal distribution of urea8, may provide valuable clinical single kidney functional information not obtainable by other means

Acknowledgements

Laboratory tehnician Henrik Vestergaard Nielsen is acknowledged for his expertise and technical support. We thank Jeff Sands for supplying us with primer sequence. The project was funded by The Danish Diabetes Academy supported by the Novo Nordisk Foundation.

References

1. Bonventre JV, Yang L: Cellular pathophysiology of ischemic acute kidney injury. J. Clin. Invest.121: 4210–4221, 2011

2. Fernández-Llama P, Andrews P, Turner R, Saggi S, Dimari J, Kwon TH, Nielsen S, Safirstein R, Knepper MA: Decreased abundance of collecting duct aquaporins in post-ischemic renal failure in rats. J. Am. Soc. Nephrol.10: 1658–1668, 1999

3. Golman K, Ardenkjaer-Larsen JH, Petersson JS, Mansson S, Leunbach I: Molecular imaging with endogenous substances. Proc. Natl. Acad. Sci. U. S. A. 100: 10435–10439, 2003

4. Atthe BK, Babsky AM, Hopewell PN, Phillips CL, Molitoris BA, Bansal N: Early monitoring of acute tubular necrosis in the rat kidney by 23Na-MRI. Am. J. Physiol. Renal Physiol. 297: 1288–98, 2009

5. Fenton RA, Chou C-L, Stewart GS, Smith CP, Knepper MA: Urinary concentrating defect in mice with selective deletion of phloretin-sensitive urea transporters in the renal collecting duct. Proc. Natl. Acad. Sci. U. S. A. 101: 7469–7474, 2004

6. Nielsen S, Terris J, Smith CP, Hediger MA, Ecelbarger CA, Knepper MA: Cellular and subcellular localization of the vasopressin- regulated urea transporter in rat kidney. Proc. Natl. Acad. Sci. 93: 5495–5500, 1996

7. Nelson SJ, Kurhanewicz J, Vigneron DB, Larson PEZ, Harzstark AL, Ferrone M, van Criekinge M, Chang JW, Bok R, Park I, Reed G, Carvajal L, Small EJ, Munster P, Weinberg VK, Ardenkjaer-Larsen JH, Chen AP, Hurd RE, Odegardstuen L-I, Robb FJ, Tropp J, Murray JA: Metabolic Imaging of Patients with Prostate Cancer Using Hyperpolarized [1-13C]Pyruvate. Sci. Transl. Med. 5: 1-22, 2013

8. Reed GD, von Morze C, Bok R, Koelsch BL, Van Criekinge M, Smith KJ, Hong Shang, Larson PEZ, Kurhanewicz J, Vigneron DB: High resolution (13)C MRI with hyperpolarized urea: in vivo T(2) mapping and (15)N labeling effects. IEEE Trans. Med. Imaging 33: 362–371, 2014

Figures

Representative Images showing first anatomic coronal slab of experimental animal, coronal- and axial slab of 13C urea, both indicating the urea gradient in the healthy contralateral kidney (orange arrow), and the abolished gradient in the IRI kidney (green arrow).

Left: Total 13C Urea signal in each kidney. Right: The 13C urea signal specific for the cortex (left), outer medulla (OM) (middle), and inner medulla (IM) (Right) normalized to the cortical signal of 13C urea is plotted. No urea gradient is seen in the IRI kidney.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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