Type I human diabetics and streptozotocin-induced diabetic mice with higher genetically determined levels of angiotensin-converting enzyme have an increased risk of developing nephropathy. However, previous experiments in mice and computer simulations indicate that modest increases in angiotensin-converting enzyme have minimal effects on blood pressure and angiotensin II levels, although bradykinin decreases significantly, inferring that bradykinin is critical for protecting the kidney in diabetics. Here, we confirm this inference by demonstrating that Akita diabetic mice lacking the bradykinin B2 receptor develop overt albuminuria, excreting the equivalent of >550 mg͞day albumin in humans, which contrasts with the microalbuminuria (equivalent to <150 mg͞day) seen in their simply diabetic littermates. The overt albuminuria is accompanied by a marked increase in glomerular mesangial sclerosis. The importance of bradykinin demonstrated here bears strongly on how current drugs reduce diabetic nephropathy and suggests that B2 receptor-specific agonists merit consideration in this context. maturity onset diabetes of the young ͉ albuminuria ͉ glomerulosclerosis D ifferences in expression of the gene coding for angiotensinconverting enzyme (ACE), such as are observed in humans in relation to the insertion͞deletion polymorphism in the ACE gene (1) and in mice with different numbers of the gene (2), have minimal effects on angiotensin II (ANGII) levels because the steady-state concentrations of the products of ACE are less sensitive to modest changes in the activity of the enzyme than the concentrations of its substrates. Thus, our recent computer simulations (3, 4) and accompanying experimental data indicate that a 50% increase in ACE causes a Ͻ5% increase in steadystate ANGII levels but close to a 20% decrease in bradykinin levels. Hence, our inference that the causative link between genetic differences in ACE expression and diabetic nephropathy (5, 6) is likely to be mediated by the ACE substrate bradykinin. To test this inference, we have combined the following two mutations: a dominant mutation that leads to maturity onset diabetes, Akita (7), resulting from an amino acid change in the insulin 2 gene (Ins2 C96Y ) (8), and a recessive knockout mutation (Bdkrb2 Ϫ ) (9) in the gene coding for the bradykinin B2 receptor, the receptor predominantly expressed in the normal kidney.
Materials and MethodsAnimals. Animals were purchased from The Jackson Laboratory and bred in our mouse facility. The mice having the null allele for the bradykinin B2 receptor (9) were backcrossed to wild-type C57BL͞6J at least six times before mating to mice heterozygous for the Akita mutation (Ins2 ϩ/C96Y ) (7), which already have the C57BL͞6J genetic background. All experiments were conducted in accordance with the guidelines of the University of North Carolina Institutional Animal Care and Use Committee. Blood pressures were measured as described (2). Plasma glucose levels were determined with the glucose-oxidase method (Wako, Richmond, VA)....