Renal ischemia/reperfusion injury (I/R-I) is the leading cause of acute kidney injury (AKI) in several disease states, including hypovolemic shock, sepsis and surgery. Imbalance in energy metabolism and mitochondrial function is a hallmark in I/R-I which can be caused by mechanisms like oxidative stress, apoptosis and inflammation¹, I/R-I 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, recently lactate dehydrogenase (LDH) has been proposed as renal injury marker², this has shown promising results, although with current techniques a biopsy is needed to measure LDH activity. Recently, hyperpolarized [1-13C]pyruvate magnetic resonance imaging has been used to quantify in vivo metabolic changes in pyruvate dehydrogenase (PDH) and lactate dehydrogenase (LDH) activity in individual diabetic rat kidneys³. Here we apply in situ [1-13C]pyruvate measurements to investigate renal injury and metabolism.Sixteen Wistar Rats were included in the study. Rats were subjected to unilateral renal ischemia for 60 min followed by reperfusion for 24 hours (n=8). Sham operated rats were prepared in parallel (n=8). A midline incision in the abdomen was made and the left renal artery was carefully dissected. I/R-I was induced by clamping the left artery with a non-traumatic clamp for 60 min. Sham rats were exposed to the same surgical procedure, except for renal I/R-I. Temperature and respiration was monitored during the surgical procedure. A tail vein catheter was inserted for injection of hyperpolarized [1-13C]pyruvate. Temperature, arterial oxygen saturation and respiration rate were monitored throughout the experiment. Each animal received one injection of 1.1 mL hyperpolarized [1-13C]pyruvate over 10 s. The experiments were performed in a 3 T clinical MR system (GE Healthcare) equipped with a dual tuned 13C/1H volume rat coil. A slice-selective 13C IDEAL spiral sequence was used for hyperpolarized [1-13C]pyruvate imaging acquiring images every 5 s initiated 20 s after the start of injection. Flip angle=10º, 11 IDEAL echoes and one initial spectrum per IDEAL encoding, TR/TE/ΔTE=100 ms/0.9 ms/0.9 ms, FOV=80x80 mm2, 5 x 5 mm real resolution and an axial slice thickness of 15 mm covering both kidneys. After MRI scanning, kidneys were stored for further biochemical analysesI/R-I resulted in statistically significantly increased plasma creatinine, BUN and decreased creatinine clearance, indicating AKI. Additionally kidney injury molecule 1 (KIM-1) was significantly increased in rats subjected to I/R-I, indicating cellular injury. Hyperpolarized [1-13C]pyruvate showed a reduced LDH activity in the post-ischemic kidneys (figure 1), calculated from pyruvate-to lactate conversion. This was verified by activity measurements of LDH in terminal kidneys. Using a bilateral I/R-I model increased LDH activity was also observed in urine samples. Like LDH activity a reduced ALT and PDH activity was also found in the I/R-I model, calculated from pyruvate-bicarbonate conversion, and pyruvate-to-alanine conversion (figure 1). An increased LDH mRNA transcription, together with a reduced NADH/NAD+ ratio indicates a preference for anaerobic glycolysis in the I/R-I kidney. This is supported by MR results when comparing lactate-to-bicarbonate formation, which individually describes the anaerobic and aerobic glycolysisIn conclusion, a decreased LDH activity is observed in the I/R-I kidney, and this is correlated with tissue injury. Although the general LDH activity in the I/R-I kidney is lowered, a weighted preference towards anaerobic metabolism was evident in the I/R-I kidney.