Coronary capillary remodeling in non-insulin-dependent diabetic rats: amelioration by inhibition of angiotensin converting enzyme and its potential clinical implications

A new insight of mechanisms, diagnosis and treatment of diabetic cardiomyopathy

Diabetes mellitus is one of the most common chronic diseases across the world. Cardiovascular complication is the major morbidity and mortality among the diabetic patients. Diabetic cardiomyopathy, a new entity independent of coronary artery disease or hypertension, has been increasingly recognized by clinicians and epidemiologists. Cardiac dysfunction is the major characteristic of diabetic cardiomyopathy. For a better understanding of diabetic cardiomyopathy and necessary treatment strategy, several pathological mechanisms such as impaired calcium handling and increased oxidative stress, have been proposed through clinical and experimental observations. In this review, we will discuss the development of cardiac dysfunction, the mechanisms underlying diabetic cardiomyopathy, diagnostic methods, and treatment options.

Diazoxide inhibits aortic endothelial cell apoptosis in diabetic rats via activation of ERK

Endothelial cell (EC) survival is critical in the maintenance of endothelial function as well as in the regulation of angiogenesis and vessel integrity since endothelial dysfunction is the initial lesion of atherosclerosis. The goal of this study was to examine the effect of diazoxide, a mitochondrial ATP-sensitive K(+)(mito K(ATP)) channel opener, on aorta ECs apoptosis and its potential mechanism in Otsuka Long-Evans Tokushima Fatty (OLETF) rats at prediabetic stage. Diazoxide (25 mg kg(-1) day(-1)) was administered intraperitoneally from age 8 weeks to age 30 weeks. Thoracic aorta and cultured thoracic aortic ECs were used. The thickening of thoracic aortic wall and apoptosis of ECs were markedly increased in OLETF rats early from the age of 16 weeks, at the impaired glucose tolerance stage, compared with Long-Evans Tokushima Otsuka rats, in conjunction with intimal hyperplasia and perivascular fibrosis. In contrast, diazoxide treatment inhibited these changes. Further study strongly demonstrated that extracellular signal-regulated kinases (ERKs) are key regulatory proteins in protecting ECs from apoptosis. Diazoxide could significantly enhance phosphorylation of ERK via opening mito K(ATP) channels. This role was reversed by both 5-hydroxydecanoate, selectively closing mito K(ATP) channels, and PD-98509, MEK inhibitors. The present studies demonstrate that diazoxide prevents the onset and development of macrovascular disease in OLETF rats by inhibiting apoptosis directly via phosphorylated ERK increase in aorta ECs. Our findings establish the basis for the therapeutic potential of diazoxide in atherosclerotic disease.

Alterations in ion channel physiology in diabetic cardiomyopathy

Diabetes mellitus is one of the most common chronic illnesses worldwide. This article focuses on a subgroup of diabetic patients with a specific cardiac complication of this disease--diabetic cardiomyopathy. This article initially gives some general background on diabetic cardiomyopathy and ion channels. Next the focus is on how diabetic cardiomyopathy alters calcium homeostasis in cardiac myocytes and highlights the specific alterations in ion channel function that are characteristic of this type of cardiomyopathy. Finally, the importance of the renin-angiotensin system in diabetic cardiomyopathy is reviewed.

Histochemical characteristics of soleus muscle in angiotensin-converting enzyme gene knockout mice

We examined the histochemical characteristics of soleus muscle in the angiotensin-converting enzyme (ACE) gene (Ace in mice, ACE in humans) knockout mice. Serial sections of soleus muscle of wild-type (Ace+/+, n=20) and heterozygous mutant (Ace+/-, n=24) mice were stained for myosin adenosine triphosphatase activity to identify different muscle fiber types. Capillaries were visualized by amylase-periodic acid-Schiff staining. ACE activity in the serum and gastrocnemius muscle was higher in male mice than in female mice. Female and male Ace+/- mice had markedly lower ACE activity in the serum and the gastrocnemius muscle than did female and male Ace+/+ mice, respectively. In both male and female mice, the composition of fiber types (type I and IIa) did not differ significantly between Ace+/+ and Ace+/- mice. There was no significant gender difference in capillary density. Ace+/- mice had significantly more capillaries around type IIa fibers (5.44 +/- 0.18 vs. 5.01 +/- 0.13, p<0.05) than Ace+/+ mice. The differences in the number of capillaries around type I fibers and in the number of capillaries around per fiber (capillary:fiber ratio) between Ace+/- and Ace+/+ mice were not significant (p<0.1). There was no significant difference in the mean cross-sectional area occupied by one capillary and the number of capillaries per fiber area between Ace+/+ and Ace+/- mice. In conclusion, knockout of the Ace gene in mice increased capillary density, as expressed by the mean number of capillaries around type IIa fibers. This finding suggests a possible mechanism for the cardioprotective effects of ACE inhibitors.

Coronary capillary network remodeling and hypofibrinolysis in aged obese diabetic rats: implications for increased myocardial vulnerability to ischemia

Despite the known abnormalities of cardiac function in patients with overt non-insulin dependent diabetes mellitus (NIDDM) the temporal changes of coronary capillary network remodeling leading to potential microcirculatory dysfunction have not been elucidated. To this end, left ventricular subendocardial capillary network of Otsuka Long-Evans Tokushima Fatty (OLETF) rats, characterized by hypertension, obesity, hyperglycemia, hyperinsulinemia and mild NIDDM, and control Long-Evans Tokushima (LETO) rats were investigated. Total capillary density in OLETF was significantly higher than that in LETO at 20 weeks, suggesting compensatory improvement of O2 transport at early stages of NIDDM. The increase in capillary density in OLETF was lost at 40 and 60 weeks due to the decreases of intermediate capillary portions and venular capillary portions. Although capillary domain area (area innervated by single capillary) in OLETF was lower than that in LETO at 20 weeks, the values were similar between OLETF and LETO at 40 and 60 weeks, suggesting that adaptive improvement in the capacity for 02 transport with a high perfusion was lost in late stages of NIDDM. Activity of plasma plasminogen activator inhibitor-1 (PAI-1), the major physiologic inhibitor of proteo(fibrino)lysis, in OLETF was higher than that in LETO at 40 and 60 weeks, suggesting that increase of PAI-1 may downregulate compensatory adaptive capillary network remodeling by inhibiting proteolysis and angiogenesis in the cardiac interstitium. Loss of adaptive myocardial microcirculation may therefore contribute to increased vulnerability in ischemic injury and to cardiac dysfunction in NIDDM.

Cardiovascular protective role of a low-dose antihypertensive combination in obese Zucker rats

OBJECTIVE

Obesity, non-insulin-dependent diabetes mellitus (NIDDM) and hypertension are leading causes associated with increased cardiovascular morbidity and mortality. In modern times, the combined first line antihypertensive therapy with at least two drugs with a different mechanism of action to achieve a better blood pressure control, is increasing in acceptance worldwide. The aim of the present study was to determine possible beneficial effects of the low-dose combination (LDC) of an angiotensin-converting enzyme (ACE) inhibitor, perindopril (PER), and the diuretic indapamide (IND), regarding myocardial and vessels protection in an animal model of hypertension, obesity and NIDDM, such as the obese Zucker rat (OZR), and control lean Zucker rats (LZR).

DESIGN

Ten-week-old male OZR (fa/fa) and LZR (Fa/fa) were used in this study. OZR group (G1, n=8), OZR with LDC group (G2, n=8); LZR group (G3, n=8) and LZR with LDC group (G4, n=8). During 6 months, G2 and G4 received a daily dose of 1 mg/kg combination of 0.76 mg/kg PER + 0.24 mg/kg IND, (ratio of doses 0.32), by gavage, and G1 and G3 received an equal volume of vehicle throughout the experiment. In order to evaluate cardiac dimensions and left ventricular mass (LVM) transthoracic echocardiograms were performed, at baseline and at the end of the experiment. Urine and blood samples for biochemical determination were obtained. After 6 months of treatment all rats were sacrificed, hearts were harvested for light microscopy (LM), high-resolution light microscopy (HRLM), immunohistochemistry including monoclonal antibodies against transforming growth factor beta (TGFbeta1) and anti-collagen type I (COL I) and type III (COL III) and electron microscopy (EM) studies.

RESULTS

At the end of the study OZR treated with LDC presented: (1) lower blood pressure (128.9 +/- 4 versus 150.3 +/- 3.6 mmHg, P< 0.05); (2) smaller cardiac dimensions (P< 0.01); (3) lower LVM/100 g body weight (0.17 +/- 0.02 versus 0.30 +/- 0.05, P< 0.01); (4) higher fractional shortening (34.5 +/- 3.2 versus 23.3 +/- 5.9%, P< 0.01) than OZR untreated. Moreover, OZR that received LDC showed higher: (1) myocyte density (48 +/- 1.5 versus 20 +/- 2.5 myocytes/area, P< 0.01); (2) capillary density (30.5 +/- 3.1 versus 9.5 +/- 1.6 capillaries/area, P<0.01); (3) myofilament thickness (12.05 +/- 0.27 versus 9.83 +/- 0.39 nm, P<0.01); and lower amounts of: (1) TGFbeta1 in myocytes (P< 0.01), interstitium (P< 0.01) and vessel wall (P< 0.05); (2) COL I and COL III (P< 0.01), and COL I /COL III ratio (P< 0.01), compared with untreated OZR. Finally, OZR-treated with LDC showed not only unsubstantial modification in carbohydrate and lipid metabolism when compared with untreated OZR, but also an improvement in insulin/glucose ratio (P< 0.05).

CONCLUSION

These results suggest that LDC of PER + IND can control cardiovascular damage in OZR providing an additional help in the metabolic scenario likewise.

Hypertension attenuates the efficacy of hypoglycemic therapy for preserving coronary flow reserve in patients with type 2 diabetes

Impairment of coronary flow reserve (CFR) in patients with type 2 diabetes has been generally demonstrated; however, there have been few studies investigating CFR in cases of relatively well-controlled diabetes, in distinction to the influence of hypertension. The purpose of the present study was to evaluate the influence of diabetes and hypertension upon CFR in relatively well-controlled patients. This study included 12 healthy controls (C group) and 57 patients with type 2 diabetes (DM) and/or essential hypertension who were divided into three groups as follows: patients with DM (DM group; n = 24), patients with essential hypertension (HT group; n = 15), and patients with both DM and essential hypertension (DM+HT group; n = 18). We excluded patients with evidence of coronary artery disease and/or left ventricular hypertrophy. We performed transthoracic Doppler recording of diastolic coronary flow velocity (CFV) in the left anterior descending coronary artery at rest and after maximal vasodilation by adenosine infusion (140 microg/kg/min for 3 min) CFR was defined as the ratio of hyperemic to averaged basal peak CFV. The CFR (2.92 +/- 0.46) of the DM group was not decreased compared to that of the C group (2.96 +/- 0.58), although the CFR of the HT (2.33 +/- 0.25) and DM+HT (2.35 +/- 0.25) groups were significantly reduced. Left ventricular mass index, relative wall thickness, and diastolic function were worse in the HT and DM+HT groups than in the C and DM groups. Subjects with concentric left ventricular remodeling had a lower CFR than those with normal left ventricular geometry. In conclusion, adequate hyperglycemic control prevented the progression of coronary microcirculatory disturbance, but concomitant hypertension attenuated the effect.

Betaxolol inhibits extracellular signal-regulated kinase and P70S6 kinase activities and gene expressions of platelet-derived growth factor A-chain and transforming growth factor-beta1 in Dahl salt-sensitive hypertensive rats

We evaluated the protective effects of long-term treatment with betaxolol, a specific beta-antagonist, on platelet-derived growth factor (PDGF) A-chain and transforming growth factor (TGF)-beta1 gene expression in the left ventricle of Dahl salt-sensitive hypertensive rats fed a high-salt diet. In addition, we evaluated the relations between these effects and coronary microvascular remodeling, expression of extracellular signal-regulated kinases (ERK) belonging to one subfamily of mitogen-activated protein kinases, and expression of p70S6 kinase belonging to one subfamily of ribosomal S6 kinases. Betaxolol (0.9 mg/kg/day, subdepressor dose) was administered for 5 weeks, from 6 weeks of age to the left ventricular hypertrophy stage at 11 weeks of age. Increased PDGF A-chain and TGF-beta1 mRNA and protein expression were suppressed by betaxolol. Upregulated activities of ERK1/2 and p70S6 kinase phosphorylations were decreased by betaxolol. Betaxolol administration resulted in significant improvements in the wall-to-lumen ratio, perivascular fibrosis and myocardial fibrosis. Thus, we conclude that ERK1/2 and p70S6 kinase activities may play a key role in coronary microvascular remodeling of Dahl salt-sensitive hypertensive rats, and that beneficial effects of betaxolol on cardiovascular remodeling may be at least partially mediated by decreased PDGF A-chain and TGF-beta1 expression in the left ventricle.

Role of intrarenal angiotensin-converting enzyme in nephropathy of type II diabetic rats

To examine the mechanism of nephropathy in Otsuka Long-Evans Tokushima Fatty (OLETF) rats, a recently developed type II diabetic model, we compared the long-term effect of angiotensin-converting enzyme (ACE) inhibitor (imidapril, 1 mg/kg/day), calcium channel blocker (amlodipine, 10 mg/kg/day), and insulin (5-10 U/kg/day) on nephropathy of OLETF rats. Both imidapril and amlodipine, but not insulin, significantly reduced blood pressure of OLETF rats. Imidapril treatment significantly decreased urinary albumin excretions and improved glomerulosclerosis of OLETF rats, while amlodipine failed to improve nephropathy of OLETF rats despite lowering of blood pressure. Insulin treatment, which significantly decreased HbA1c throughout the treatment period, did not ameliorate nephropathy of OLETF rats. Serum ACE activity in OLETF rats was significantly lower than that in genetic control nondiabetic Long-Evans Tokushima Otsuka (LETO) rats. However, glomerular and aortic ACE activities in OLETF rats were significantly higher than those in LETO rats, and were significantly decreased by treatment with imidapril. Furthermore, immunohistochemical analysis of ACE in the kidney using specific antibodies indicated greater ACE immunostaining in the glomeruli and renal vessels of OLETF rats than in those of LETO rats. These observations demonstrate that ACE is involved in the development of nephropathy of OLETF rats and provide evidence that intrarenal ACE rather than circulating ACE may play an important role in nephropathy of this type II diabetic model.