Renin-aldosterone axis in normoalbuminuric insulin-dependent diabetes mellitus patients with glomerular hyperfiltration

Cardiac aldosterone overexpression prevents harmful effects of diabetes in the mouse heart by preserving capillary density

Recent reports showed an unexpected worsening of endothelial function by aldosterone antagonism in diabetic patients, suggesting that aldosterone could interfere with the detrimental consequences of diabetes on microvasculature and thus on cardiac function. To test this hypothesis, diabetes (D) was induced using streptozotocin in transgenic (Tg) male mice overexpressing aldosterone-synthase in the heart and in wild-type (Wt) mice. Eight weeks after streptozotocin injection, impairment of left ventricular systolic function, measured by echocardiography (fractional shortening), was accompanied by a decrease in capillaries/cardiomyocyte ratio (-20%) and VEGFa expression (-40%) in Wt-D mice compared with normoglycemic littermates. Furthermore, Wt-D mice demonstrated an increase in superoxide production (+100%) and protein carbonylation (+33%), hallmarks of oxidative stress. Except for a slight increase in protein carbonylation, all of these diabetes-associated cardiac alterations were undetectable in Tg-D mice. Fibrosis was induced similarly in both diabetic groups. Eplerenone (an aldosterone antagonist) abolished all of the effects of aldosterone-synthase overexpression but had no effect in Wt-D mice. Thus, aldosterone prevents systolic dysfunction through a mineralocorticoid receptor-dependent mechanism that may include preventing VEGFa down-regulation and maintaining capillary density. Understanding how aldosterone prevents VEGFa down-regulation in experimental diabetes could be important to define new strategies targeting the prevention of a decrease in capillary density.

Local renal aldosterone system and its regulation by salt, diabetes, and angiotensin II type 1 receptor

CYP11B2 is the enzyme responsible for aldosterone synthesis mainly in the adrenal gland. In this study, we hypothesized that CYP11B2 gene, protein, and aldosterone are produced locally in kidney and regulated by low salt intake, angiotensin II type 1 (AT1) receptor and insulin-deficient diabetes hyperglycemia. We used real-time RT-PCR, immunohistochemistry staining, and microdialysis techniques to monitor changes in renal CYP11B2 mRNA and protein and aldosterone production in normal, adrenalectomized, or streptozotocin-induced insulin-deficient diabetic hyperglycemic rats. In normal kidney, CYP11B2 mRNA and protein were localized mainly in the renal cortex and upregulated by angiotensin II and low salt intake. The angiotensin II effect was reversed by AT1 receptor blocker valsartan. Immunohistochemistry staining demonstrated presence of CYP11B2 in glomeruli. Although aldosterone was absent in plasma of adrenalectomized rats, it was present in renal interstitium and tissue. Diabetes increased renal cortical and total kidney CYP11B2 mRNA and protein. Lowering blood glucose with insulin decreased total renal CYP11B2 mRNA and protein. Despite lack of significant changes in blood glucose, valsartan treatment caused significant reduction in renal CYP11B2 mRNA and protein. In presence of diabetes, there was an increase in CYP11B2 immunostaining in glomeruli and proximal tubules. This expression was abrogated with insulin or valsartan treatment. These results demonstrate the presence of all components of local renal aldosterone system. This system is physiologically active because it is regulated by angiotensin II and low salt intake. In insulin-deficient diabetes hyperglycemia rat model, glucose, insulin, and AT1 receptor modulate CYP11B2 expression in the kidney.

Low plasma aldosterone despite normal plasma renin activity in uncomplicated type 1 diabetes mellitus: effects of RAAS stimulation

BACKGROUND

Data on levels and responsiveness of PRA and aldosterone in type 1 diabetes mellitus are conflicting. Earlier studies were not standardized with respect to the type of diabetes mellitus, the presence of diabetic complications or sodium intake. Therefore, we studied plasma renin activity and plasma aldosterone in uncomplicated type 1 diabetes mellitus by evaluating the effects of endogenous (sodium restriction) and exogenous (angiotensin I infusion) stimulation.

DESIGN

Twenty-four type 1 diabetic patients and 24 matched healthy subjects were studied after 1 week of liberal sodium diet (200 mmol 24 h-1) and 1 week of low sodium diet (50 mmol 24 h-1). Angiotensin (Ang)I was infused at 4 and 8 ng kg-1 min-1 during both study days.

RESULTS

During liberal and low sodium intake, plasma aldosterone was lower in type 1 diabetic patients compared with healthy subjects both at 08:00 h (P < 0.05) and after a 2-h euglycaemic clamp (P < 0.05), despite similar PRA levels. The correlations between changes in PRA and changes in plasma aldosterone when shifting sodium intake were similar in both groups. During liberal sodium intake, the aldosterone levels after AngI infusion were lower in type 1 diabetic patients, whereas during low sodium they were not different.

CONCLUSIONS

Plasma aldosterone was deceased relative to PRA in uncomplicated type 1 diabetic patients, irrespective sodium intake. The responsiveness to sodium restriction was adequate and sodium restriction was able to overcome the decreased plasma aldosterone response to exogenous AngI, which was observed during liberal sodium in diabetic patients. The lower aldosterone is not secondary to diabetic complications and does not depend on the level of sodium intake.