Intact renal afferent arteriolar autoregulatory responsiveness in db/db mice

Abstract

The db/db mouse is a genetic model of type 2 diabetes that exhibits progressive renal disease. Obesity, hyperglycemia, and albuminuria (822 Ϯ 365 vs. 28 Ϯ 8 g/day) are evident in 18-wk-old db/db compared with db/m (lean littermate control) mice. Our goal was to determine the blood pressure (BP) phenotype of the db/db mouse. Mean arterial BP measured in conscious mice by radiotelemetry was not different between db/db (n ϭ 9) and db/m (n ϭ 12) mice, averaging 113 Ϯ 3 and 112 Ϯ 2 mmHg, respectively. The circadian BP profile of db/db mice was shifted to the left and exhibited a significant reduction in amplitude compared with db/m mice. Heart rate (487 Ϯ 9 vs. 542 Ϯ 7 beats/min; P Ͻ 0.05) and locomotor activity were significantly reduced in db/db compared with db/m mice. We tested the hypothesis that intact afferent arteriole (AA) responsiveness to increases in renal artery pressure (RAP) and angiotensin (ANG) II sensitivity contributes to normal BP in this diabetic model. AA diameters of in vitro blood-perfused juxtamedullary nephrons of db/db mice (15.7 Ϯ 0.5 m; n ϭ 38) were significantly larger than those of db/m mice (12.5 Ϯ 0.4 m; n ϭ 37). AA responses to increases in RAP and ANG II were not different between kidneys of db/db and db/m mice. Significant AA vasoconstriction to 1 nM ANG II was observed in kidneys of db/db mice (Ϫ11 Ϯ 4%), while 10 nM ANG II decreased AA diameter in both groups [db/db, Ϫ20 Ϯ 4%, (n ϭ 12); db/m, Ϫ26 Ϯ 4% (n ϭ 12)]. In summary, AA responses to increases in renal perfusion pressure and ANG II remain intact in db/db mice. Diabetic renal disease occurs in db/db mice independently of elevated BP. radiotelemetry; juxtamedullary nephron; angiotensin II; hyperglycemia TYPE 2 DIABETES MELLITUS AFFECTS 24 million Americans and 220 million people worldwide (1, 4) and is the sixth leading cause of death in the US. Obesity has been identified as the principal risk factor associated with the rising prevalence of type 2 diabetes (7). According to the American Diabetes Association, ϳ73% of adults with diabetes are hypertensive; therefore, over one-quarter of patients are normotensive. Diabetic nephropathy is the most common cause of end-stage renal disease in the Western world. Diabetic nephropathy is characterized by progressive albuminuria, declining glomerular filtration rate (GFR), and increased risk for cardiovascular disease.The db/db mouse was first identified at the Jackson Laboratory in 1966 (16). The diabetes mutation (db) is inherited as an autosomal recessive trait and resembles the metabolic disturbances in diabetes mellitus in humans. Thirty years later, it was discovered that the db mutation is a G-to-T point mutation in the gene encoding the leptin receptor. This mutation causes abnormal splicing and defective signaling of the adipocytederived hormone leptin (6, 19). The lack of leptin receptor signaling in the hypothalamus leads to persistent hyperphagia and obesity. The db/db mouse exhibits hyperleptinemia and hyperinsulinemia and develops hyperglycemia in association with insulin...

Major role for ACE-independent intrarenal ANG II formation in type II diabetes

“…Experiments were conducted using the mouse in vitro bloodperfused juxtamedullary nephron technique as we have previously reported in detail (10,28). Briefly, mice were anesthetized with pentobarbital sodium (90 mg/kg ip), the right renal artery was cannulated, the upper third of the cortex was removed, the distal arteries were tied, and the pelvic mucosa and connective tissues were removed, revealing the juxtamedullary nephrons.…”

“…The kidney was placed on the stage of a microscope, and the afferent arteriole was visualized using a video camera, image enhancer, and recorded on a DVD. Kidneys were studied under euglycemic (5 mM ϭ 110 mg/dl glucose) and hyperglycemic (30 mM ϭ 540 mg/dl glucose) incubation conditions (5% BSA perfusion solution, 1% BSA superfusion solution, rat plasma) for control and diabetic mice, respectively (28). Donor blood was collected from anesthetized rats subjected to bilateral nephrectomy and exsanguinated via a carotid arterial cannula.…”

“…Renal arterial perfusion pressure and vessel diameters were sampled at 1 Hz and converted to a digital form using analog-to-digital data-acquisition and -analysis software as we have previously described (28,42). Afferent arteriolar luminal diameters were measured manually and continuously throughout the protocol at a single site along the length of the afferent arteriole using a digital image-shearing monitor.…”

“…The EC 50 was determined for afferent arteriole dose-responses to ANG I and ANG II (ANG II data obtained from Ref. 28) by measuring the concentration of the peptide that produced a halfmaximal effect. Statistical analyses were performed on the raw data by one-way repeated-measures or two-way ANOVA followed by Dunnett's or Bonferroni's test.…”

“…The impact of altered ACE and ACE2 protein expression on intrarenal ANG II formation has not been determined in this model. We recently reported that the renal afferent arteriole vasoconstrictor responses to ANG II remain intact in db/db mice (28). However, the functional consequence of reductions in ACE enzyme activity on the intrarenal formation of ANG II from ANG I on the renal microvasculature of type II diabetes has not been previously investigated.…”