In patients with diabetic kidney disease (DKD), plasma renin activity is usually decreased, but there is limited information on urinary renin and its origin. Urinary renin was evaluated in samples from patients with longstanding type I diabetes mellitus and mice with streptozotocin-induced diabetes mellitus. Renin-reporter mouse model (Ren1d-Cre;mT/ mG) was made diabetic with streptozotocin to examine whether the distribution of cells of the renin lineage was altered in a chronic diabetic environment. Active renin was increased in urine samples from patients with DKD (n=36), compared with those without DKD (n=38; 3.2 versus 1.3 pg/mg creatinine; P<0.001). In mice with streptozotocin-induced diabetes mellitus, urine renin was also increased compared with nondiabetic controls. By immunohistochemistry, in mice with streptozotocin-induced diabetes mellitus, juxtaglomerular apparatus and proximal tubular renin staining were reduced, whereas collecting tubule staining, by contrast, was increased. To examine the role of filtration and tubular reabsorption on urinary renin, mice were either infused with either mouse or human recombinant renin and lysine (a blocker of proximal tubular protein reabsorption). Infusion of either form of renin together with lysine markedly increased urinary renin such that it was no longer different between nondiabetic and diabetic mice. Megalin mRNA was reduced in the kidney cortex of streptozotocin-treated mice (0.70±0.09 versus 1.01±0.04 in controls, P=0.01) consistent with impaired tubular reabsorption. In Ren1d-Cre;mT/mG with streptozotocin-induced diabetes mellitus, the distribution of renin lineage cells within the kidney was similar to nondiabetic renin-reporter mice. No evidence for migration of cells of renin linage to the collecting duct in diabetic mice could be found. Renin mRNA in microdissected collecting ducts from streptozotocin-treated mice, moreover, was not significantly different than in controls, whereas in kidney cortex, largely reflecting juxtaglomerular apparatus renin, it was significantly reduced. In conclusion, in urine from patients with type 1 diabetes mellitus and DKD and from mice with streptozotocin-induced diabetes mellitus, renin is elevated. This cannot be attributed to production from cells of the renin lineage migrating to the collecting duct in a chronic hyperglycemic environment. Rather, the elevated levels of urinary renin found in DKD are best attributed to altered glomerular filteration and impaired proximal tubular reabsorption.

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Increased expression of renin in the kidney collecting tubule of rodents made diabetic by streptozotocin (STZ) has been well demonstrated but whether this site is the main source of urinary renin is unknown. We wanted to examine the origin and significance of urinary renin in diabetic kidney disease (DKD). Total and active renin was evaluated in urines from people with longstanding type 1 diabetes of more than 25 years, with (n=36) or without DKD (n=38) (eGFR 101 vs. 39 mL/min/1.73m 2 ; p<0.001). Mice given STZ (n=15) or vehicle (n=8) 20 weeks prior to study were also studied. In people with DKD, total renin was markedly increased compared to people without DKD (82 vs. 49 pg/mg Cr; p=0.023). Active renin was also significantly increased in people with DKD compared to people without DKD (3.2 vs. 1.3 pg/mg Cr; p<0.001). In mice with STZ-induced DKD a significant increase in renin was found compared to controls (1093±319 vs. 64±18 pg/mg Cr; p=0.0001). To examine the role of filtration and tubular reabsorption on urinary renin, human active renin was measured in urines from non-diabetic mice infused with human recombinant renin (hrRenin) (n=8), a combination of lysine and hrRenin (n=5) and non-infused controls (n=15). Urines of mice infused with a combination of lysine (a blocker of proximal tubular protein reabsorption) and hrRenin had markedly higher urinary human active renin than those of controls (179±129 vs. 1.6±0.4 pg/mg Cr; p=0.001). The values were also markedly higher than those of mice infused with hrRenin only (4.4±1.1 pg/mg Cr; p=0.003). The effect of lysine was also evaluated in regard to endogenous mouse renin. Urinary mouse renin in mice infused with lysine (n=5) was markedly increased compared to non-infused controls (n=18) (22360±8673 vs. 346±82 pg/mg Cr; p=0.001). In conclusion, in humans with DKD, urine concentrations of both total and active renin are increased. In mice with STZ-induced DKD, urine total renin is also markedly increased. The data further demonstrate that 1) renin is both filterable and reabsorbable in normal mice and 2) the increase of urinary renin in DKD can be attributed largely to impaired reabsorption mainly in the proximal tubule.