*2.1. Proteinuric Kidney Injury Increased Renal Sodium*

MRI analysis revealed that puromycin aminonucleoside nephropathy (PAN) injury leads to increased renal sodium content. Both the cortex and medulla in PAN-injured rats had significantly higher sodium than in control rats (Figure 1A). The renal cortex of PAN rats also showed increased water content compared with controls. Although a directionally similar trend occurred in the medulla of PAN-injured rats, the increase did not reach statistical significance (Figure 1B). In companion studies, we measured the sodium concentration in the lymph exiting the kidney, which is thought to reflect the composition within the renal interstitial compartment. These direct measurements revealed that sodium concentration in the renal lymph of PAN rats was significantly higher than in the lymph of control rats (Figure 2A). Sodium concentration in concurrently obtained serum samples was not different between PAN rats and controls (Figure 2B). These results indicate that, in addition to the well-documented proteinuria, hypoalbuminemia, and hyperlipidemia, PAN kidney injury leads to intrarenal sodium and water retention, especially in the renal cortex. Similar to dermal lymphatics of hypertensive animals, which transport excess sodium from the skin [4], our results show for the first time that renal lymphatics are a route for clearing excess sodium from the renal interstitium in the setting of proteinuric kidney injury.

**Figure 1.** Proteinuric kidney injury increased renal sodium and water content. (**A**) Representative sodium 23Na-MRI in uninjured control (Cont) (open circles) and puromycin (PAN)-injured rats (closed squares). The graph shows mean tissue sodium content localized in the cortex and medulla of the kidneys (arrows). (**B**) Representative T2-weighted MRI images and quantitative T2-relaxation time measurements indicative of renal cortical and medullary water content in Cont and PAN-injured rats. Results are expressed as mean ± SEM. n = 5 to 8 rats per group analyzed by unpaired *t* test. \* *p* < 0.05.

**Figure 2.** Proteinuric kidney injury increased sodium concentration in renal lymph. (**A**) Analysis of renal lymph showed higher sodium concentration in PAN-injured (closed squares) vs control rats (open circles). (**B**) Analysis of plasma showed no difference in sodium concentration between PAN-injured vs control rats. Results are expressed as mean ± SEM for 6 to 8 rats per group analyzed by unpaired *t* test. \* *p* < 0.05.
