To further evaluate the role of the local regulatory TGF system in the glomerular dysregulation of early diabetes mellitus we performed studies in diabetic Ins2- Akita mice, a mouse model of type I diabetes due to a mutation in the Insulin2 gene. The effect of TGF in diabetes was assessed by breeding the Ins- mutation into an adenosine 1 receptor (A1AR) null background since this genetic variation has been shown to induce a congenital deficiency of TGF regulation . TGF responses in Ins2-A1AR-- double mutants were in fact abolished while they were attenuated in Ins2- mice. To determine GFR in conscious mice we used a two compartment analysis of FITC labeled inulin plasma clearance that required only a total of 15 microl of blood. GFR in conscious wild type mice at 14, 24, and 33 weeks averaged 240 12 microlmin, 347 24 microlmin, and 294 16 microlmin, while in Ins2- it was 315 24 microlmin, 457 28 microlmin(p<0.05 vs. WT), and 402 35 microlmin. Remarkably and unexpectedly, there was a marked increase of GFR in double mutants at all ages averaging 532 28 microlmin, 604 33 microlmin, and 704 76 microlmin(p<0.01 vs. both WT and Ins2- at all ages). Histological evidence of glomerular injury was more pronounced in double than single mutants, and urinary albumin excretion was significantly higher in double mutant than single mutant or WT mice at all ages. The marked elevation of GFR in diabetic mice without A1AR indicates that an A1AR-dependent mechanism, presumably TGF, prevents a vasodilatory or less vasoconstrictive diabetic environment from causing an excessive elevation of GFR. Thus, in this experimental model TGF cannot be the cause of diabetic hyperfiltration since it is chronically non-functional. Rather, TGF appears to function as a mechanism that limits the degree of diabetic hyperfiltration and thereby protects against diabetic nephropathy. ? ? It has been suggested that adenosine may play a role in insulin-dependent glucose uptake into cells, but the adenosine receptor subtypes involved in this effect are unclear. We have therefore explored the role of adenosine 1 receptors in glucose homeostasis. Fasting glucose levels were elevated in A1AR-- compared to wild type at both 8 and 20 weeks of age as was fasting plasma insulin. Following an i.v. glucose load the area under the plasma glucose curve was significantly greater in A1AR-- vs. wild type mice. Insulin sensitivity expressed as modified Belfiore-index was significantly lower in the A1AR deficient mice in all age groups. A high fat diet caused a further deterioration of insulin sensitivity in both A1AR-- and wild type mice. Our results show that adenosine signaling via A1AR contributes to insulin-controlled glucose homeostasis.? ? Islet-antigen 2 (IA-2) is a major autoantigen in diabetes mellitus with about 60% of the diabetic population showing antibodies against this protein. IA-2 and its closely related isoform IA-2beta are constituents of the membrane of dense core vesicles in the brain, adrenal gland, and pancreas. Evidence from IA-2IA2beta deficient mice suggests a role in the exocytosis. We have performed studies to examine whether IA-2 may play a role in the release of renin from the dense core vesicles in renal juxtaglomerular cells. We confirmed the presence of IA-2 in the kidney by RT-PCR and immunoblotting, and localized the IA-2 expressing cells to the glomerular vascular pole consistent with expression in granular cells. A functional role of IA-2 in renin secretion was shown by the observation that IA-2IA2beta double null mutant mice as well as IA-2 deficient mice have a significant reduction in renal renin mRNA and plasma renin that is not accompanied by a reduction in plasma aldosterone. The regulation of renin expression and renin secretion by variations in salt intake were found to be unaffected by the absence of IA-2. Thus, IA-2 appears to be a novel determinant of renin expression and release consonant with its role in dense core vesicle-containing cell function in general.
| Hashimoto, Seiji; Yamada, Kanji; Kawata, Tetsuya et al. (2009) Abnormal autoregulation and tubuloglomerular feedback in prediabetic and diabetic OLETF rats. Am J Physiol Renal Physiol 296:F598-604 |
| Faulhaber-Walter, Robert; Chen, Limeng; Oppermann, Mona et al. (2008) Lack of A1 adenosine receptors augments diabetic hyperfiltration and glomerular injury. J Am Soc Nephrol 19:722-30 |
