Increased mRNA expression of cardiac renin-angiotensin system and collagen synthesis in spontaneously hypertensive rats

Activated Factor X Signaling Pathway via Protease-Activated Receptor 2 Is a Novel Therapeutic Target for Preventing Atrial Fibrillation

BACKGROUND

Activated factor X (FXa), which contributes to chronic inflammation via protease-activated receptor 2 (PAR2), might play an important role in atrial fibrillation (AF) arrhythmogenesis. This study aimed to assess whether PAR2 signaling contributes to AF arrhythmogenesis and whether rivaroxaban ameliorates atrial inflammation and prevents AF.Methods and Results:In Study 1, PAR2 deficient (PAR2-/-) and wild-type mice were infused with angiotensin II (Ang II) or a vehicle via an osmotic minipump for 2 weeks. In Study 2, spontaneously hypertensive rats (SHRs) were treated with rivaroxaban, warfarin, or vehicle for 2 weeks after 8 h of right atrial rapid pacing. The AF inducibility and atrial remodeling in both studies were examined. Ang II-treated PAR2-/- mice had a lower incidence of AF and less mRNA expression of collagen1 and collagen3 in the atrium compared to wild-type mice treated with Ang II. Rivaroxaban significantly reduced AF inducibility compared with warfarin or vehicle. In SHRs treated with a vehicle, rapid atrial pacing promoted gene expression of inflammatory and fibrosis-related biomarkers in the atrium. Rivaroxaban, but not warfarin, significantly reduced expression levels of these genes.

CONCLUSIONS

The FXa-PAR2 signaling pathway might contribute to AF arrhythmogenesis associated with atrial inflammation. A direct FXa inhibitor, rivaroxaban, could prevent atrial inflammation and reduce AF inducibility, probably by inhibiting the pro-inflammatory activation.

Training-Induced Deactivation of the AT1 Receptor Pathway Drives Autonomic Control and Heart Remodeling During the Transition From the Pre- to Hypertensive Phase in Spontaneously Hypertensive Rats

BACKGROUND

The effects of hypertension and exercise training (T) on the sequential interplay between renin-angiotensin system (RAS), autonomic control and heart remodeling during the development of hypertension in spontaneously hypertensive rats (SHR), was evaluated.Methods and Results:Time course changes of these parameters were recorded in 4-week-old SHR submitted to a T or sedentary (S) protocol. Wistar Kyoto rats served as controls. Hemodynamic recordings were obtained in conscious rats at experimental weeks 0, 1, 2, 4, and 8. The left ventricle (LV) was collected to evaluate RAS gene and protein expression, cardiomyocytes' hypertrophy and collagen accumulation. Pre-hypertensive SHR exhibited augmented AT₁R gene expression; at 5 weeks, they presented with elevated pressure, increased LV angiotensinogen and ACE mRNA expression, followed by sympathoexcitation (from the 8^thweek onwards). Marked AT₁R protein content, myocytes's hypertrophy, collagen deposition and increased pressure variability were observed in 12-week-old sedentary SHR. In addition to attenuating all these effects, T activated Mas receptor expression augmented parasympathetic modulation of the heart, and delayed the onset and reduced the magnitude, but did not block the development of genetic hypertension.

CONCLUSIONS

The close temporal relationship between changes in the LV ACE-Ang II-AT₁R axis, autonomic control and cardiac remodeling at both the establishment of hypertension and during exercise training reveals the essential role played by the AT₁R pathway in driving cardiac remodeling and autonomic modulation during the transition from the pre- to hypertensive phase.

The effect of resveratrol on angiotensin II levels and the rate of transcription of its receptors in the rat cardiac hypertrophy model

This study investigated the effect of resveratrol on serum and cardiac levels of angiotensin II and transcription of its main receptors following pressure overload induced-hypertrophy. Rats were divided into untreated (Hyp) and resveratrol treated hypertrophied groups (H + R). Intact animals served as the control (Ctl). Cardiac hypertrophy was induced by abdominal aortic banding. Blood pressure (BP) was recorded via left carotid artery cannula. Fibrosis was confirmed by Masson trichrome staining. Angiotensin II level was measured using an ELIZA test. Gene expression was assessed by a real time PCR (RT-PCR) technique. We observed that in the H + R group BP and heart weight/body weight were decreased significantly (p < 0.001, p < 0.05, respectively vs Hyp). The cardiac levels of angiotensin II and AT1a mRNA were increased in the Hyp group (p < 0.01 vs Ctl). In the H + R group the AT1a mRNA level was decreased significantly (p < 0.05 vs Hyp). It could be concluded that resveratrol protects the heart against hypertrophy progression in part by affecting cardiac AT1a transcription.

Macroarray analysis in the hypertrophic left ventricle of renin-dependent hypertensive rats: identification of target genes for renin

Introduction The aim of this work was to identify new renin target genes in left ventricular hypertrophy during hypertension.

Materials and methods We compared left ventricle gene expression from four transgenic TGR(mRen2)27 (TG+/-) rats and four non-transgenic littermates (TG-/-) using cDNA macroarray. Hybridisation signals were quantified with a phosphorimager, and normalised to an external scale. Data analysis was performed with Statistical Analysis for Microarrays (SAM 1.21) software. The mRNA levels of candidate genes were determined by semi-quantitative RT-PCR in three different hypertensive rats: TG+/-, spontaneously hypertensive (SHR) and genetically Lyon hypertensive (LH) rats, compared to their respective controls (TG-/-, Wistar-Kyoto, Lyon low blood pressure rats).

Results Out of 1,200 genes present on the macroarray, 233 were reliably measured and only three were overexpressed (Biglycan, β1-adenosine monophosphate-activated protein kinase [AMPK] and amyloid precursor like protein 2 [APLP2]) and 19 were underexpressed in the left ventricle of TG+/compared with TG-/-. APLP2 is a member of the amyloid precursor protein (APP) family. APLP2 and APP mRNA levels were increased in TGR(mRen2)27 but significantly decreased in LH rats, while only APP was increased in SHR rats.

Conclusions We report new genes associated with renin-dependent left ventricular hypertrophy. Moreover, this work shows for the first time that the APP family gene expression could be altered in response to high renin activity and this effect is independent of cardiac remodelling and hypertension.

Persistent phenotypic shift in cardiac fibroblasts: impact of transient renin angiotensin system inhibition

Fibrotic cardiac remodeling ultimately leads to heart failure - a debilitating and costly condition. Select antihypertensive agents have been effective in reducing or slowing the development of cardiac fibrosis. Moreover, some experimental studies have shown that the reduction in fibrosis induced by these agents persists long after stopping treatment. What has not been as well investigated is whether this transient treatment results in a protection against future fibrotic cardiac remodeling. In the present review, previously published studies are re-examined to assess whether the relative percent increase in collagen deposition over an off-treatment period is attenuated, relative to control, following transient antihypertensive treatment in young or adult rats. Present findings suggest that transient inhibition of the renin angiotensin system (RAS) not only produces a sustained reduction in cardiac fibrosis, but also results in a degree of protection against future collagen deposition. In addition, prior transient RAS inhibition appears to alter the cardiac fibroblast phenotype such that these cells show a muted response to myocardial injury - namely reduced proliferation, chemokine release, and collagen deposition. This review puts forth several potential mechanisms underlying this long-term cardiac protection that is afforded by transient RAS inhibition. Specifically, fibroblast phenotypic change, cardiac fibroblast apoptosis, sustained suppression of the RAS, persistent reduction in left ventricular hypertrophy, and persistent reduction in arterial pressure are each discussed. Identifying the mechanisms ultimately responsible for this change in cardiac fibroblast response to injury, hypertension, and aging may reveal novel targets for therapy.

An orally active angiotensin-(1-7) inclusion compound and exercise training produce similar cardiovascular effects in spontaneously hypertensive rats

Low angiotensin-(1-7) (Ang-(1-7)) concentration is observed in some cardiovascular diseases and exercise training seems to restore its concentration in the heart. Recently, a novel formulation of an orally active Ang-(1-7) included in hydroxy-propyl-beta-cyclodextrin (HPB-CD) was developed and chronically administered in experimental models of cardiovascular diseases. The present study examined whether chronic administration of HPB-CD/Ang-(1-7) produces beneficial cardiovascular effects in spontaneously hypertensive rats (SHR), as well as to compare the results obtained with those produced by exercise training. Male SHR (15-week old) were divided in control (tap water) or treated with HPB-CD/Ang-(1-7) (corresponding to 30μgkg(-1)day(-1) of Ang-(1-7)) by gavage, concomitantly or not to exercise training (treadmill, 10 weeks). After chronic treatment, hemodynamic, morphometric and molecular analysis in the heart were performed. Chronic HPB-CD/Ang-(1-7) decreased arterial blood pressure (BP) and heart rate in SHR. The inclusion compound significantly improved left ventricular (LV) end-diastolic pressure, restored the maximum and minimum derivatives (dP/dT) and decreased cardiac hypertrophy index in SHR. Chronic treatment improved autonomic control by attenuating sympathetic modulation on heart and vessels and the SAP variability, as well as increasing parasympathetic modulation and HR variability. Overall results were similar to those obtained with exercise training. These results show that chronic treatment with the HPB-CD/Ang-(1-7) inclusion compound produced beneficial effects in SHR resembling the ones produced by exercise training. This observation reinforces the potential cardiovascular therapeutic effect of this novel peptide formulation.

Uptake and metabolism of the novel peptide angiotensin-(1-12) by neonatal cardiac myocytes

Background

Angiotensin-(1–12) [Ang-(1–12)] functions as an endogenous substrate for the productions of Ang II and Ang-(1–7) by a non-renin dependent mechanism. This study evaluated whether Ang-(1–12) is incorporated by neonatal cardiac myocytes and the enzymatic pathways of ¹²⁵I-Ang-(1–12) metabolism in the cardiac myocyte medium from WKY and SHR rats.

Methodology/Principal Findings

The degradation of ¹²⁵I-Ang-(1–12) (1 nmol/L) in the cultured medium of these cardiac myocytes was evaluated in the presence and absence of inhibitors for angiotensin converting enzymes 1 and 2, neprilysin and chymase. In both strains uptake of ¹²⁵I-Ang-(1–12) by myocytes occurred in a time-dependent fashion. Uptake of intact Ang-(1–12) was significantly greater in cardiac myocytes of SHR as compared to WKY. In the absence of renin angiotensin system (RAS) enzymes inhibitors the hydrolysis of labeled Ang-(1–12) and the subsequent generation of smaller Ang peptides from Ang-(1–12) was significantly greater in SHR compared to WKY controls. ¹²⁵I-Ang-(1–12) degradation into smaller Ang peptides fragments was significantly inhibited (90% in WKY and 71% in SHR) in the presence of all RAS enzymes inhibitors. Further analysis of peptide fractions generated through the incubation of Ang-(1–12) in the myocyte medium demonstrated a predominant hydrolytic effect of angiotensin converting enzyme and neprilysin in WKY and an additional role for chymase in SHR.

Conclusions/Significance

These studies demonstrate that neonatal myocytes sequester angiotensin-(1–12) and revealed the enzymes involved in the conversion of the dodecapeptide substrate to biologically active angiotensin peptides.

Therapeutic effects of vitamin D analogs on cardiac hypertrophy in spontaneously hypertensive rats

Vitamin D inhibits renin expression and blocks the compensatory induction of renin associated with the use of renin-angiotensin system inhibitors. Here we test the therapeutic effects of two commonly used vitamin D analogs and their combination with losartan on the development of left ventricular hypertrophy. One-month-old male spontaneously hypertensive rats were treated with vehicle, losartan, paricalcitol, doxercalciferol, a combination of losartan and paricalcitol, or a combination of losartan and doxercalciferol for 2 months. Blood pressure was markedly reduced by losartan, but not by paricalcitol or doxercalciferol alone. Echocardiograpy demonstrated a 65 to 80% reduction in left ventricular wall thickness with losartan, paricalcitol, or doxercalciferol monotherapy and almost complete prevention of left ventricular hypertrophy with the combination therapies. Attenuation of cardiac and cardiomyocyte hypertrophy, and suppression of atrial and brain natriuretic peptides, were most marked in the combination therapy groups. These changes were well correlated with left ventricular gene and microRNA expression profiles in the different treatment groups. Renal and cardiac renin expression was markedly increased in losartan-treated animals, but nearly normalized with combination therapy. The same vitamin D analogs suppressed plasma renin activity in patients receiving chronic hemodialysis. These data demonstrate that vitamin D analogs have potent antihypertrophic activity in part via suppression of renin in the kidney and heart, and combination of these analogs with losartan achieves much better therapeutic effects because of the blockade of the compensatory renin increase.

Cardiac fibroblasts: at the heart of myocardial remodeling

Cardiac fibroblasts are the most prevalent cell type in the heart and play a key role in regulating normal myocardial function and in the adverse myocardial remodeling that occurs with hypertension, myocardial infarction and heart failure. Many of the functional effects of cardiac fibroblasts are mediated through differentiation to a myofibroblast phenotype that expresses contractile proteins and exhibits increased migratory, proliferative and secretory properties. Cardiac myofibroblasts respond to proinflammatory cytokines (e.g. TNFalpha, IL-1, IL-6, TGF-beta), vasoactive peptides (e.g. angiotensin II, endothelin-1, natriuretic peptides) and hormones (e.g. noradrenaline), the levels of which are increased in the remodeling heart. Their function is also modulated by mechanical stretch and changes in oxygen availability (e.g. ischaemia-reperfusion). Myofibroblast responses to such stimuli include changes in cell proliferation, cell migration, extracellular matrix metabolism and secretion of various bioactive molecules including cytokines, vasoactive peptides and growth factors. Several classes of commonly prescribed therapeutic agents for cardiovascular disease also exert pleiotropic effects on cardiac fibroblasts that may explain some of their beneficial outcomes on the remodeling heart. These include drugs for reducing hypertension (ACE inhibitors, angiotensin receptor blockers, beta-blockers), cholesterol levels (statins, fibrates) and insulin resistance (thiazolidinediones). In this review, we provide insight into the properties of cardiac fibroblasts that underscores their importance in the remodeling heart, including their origin, electrophysiological properties, role in matrix metabolism, functional responses to environmental stimuli and ability to secrete bioactive molecules. We also review the evidence suggesting that certain cardiovascular drugs can reduce myocardial remodeling specifically via modulatory effects on cardiac fibroblasts.

Increased expression of IL-6 and LIF in the hypertrophied left ventricle of TGR(mRen2)27 and SHR rats

Cytokines from the interleukin-6 (IL-6) family have been reported to play an important synergistic role with angiotensin II in the development of pathological cardiac hypertrophy. Whether their expression pattern changes in vivo, in an angiotensin I-dependent hypertrophied myocardium has not been reported. In this study, we addressed that issue using two animal models of angiotensin II-dependent cardiac hypertrophy. Heterozygous transgenic TGR(mRen2)27 (TGR) with an overactive cardiac renin angiotensin system and the closely related spontaneously hypertensive rats (SHR) were compared to their respective control rats. The mRNA levels of IL-6, leukemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF) and cardiotrophin-1 (CT-1) as well as their receptor subunits, glycoprotein 130 (gp130), IL-6 receptor (IL-6R), LIFR, and CNTFR, were measured by semi-quantitative RT-PCR. The protein levels of IL-6, LIF and CT-1 were investigated by western blot. TGR and SHR both displayed significant over expression of mRNA and protein levels for IL-6 and LIF. In TGR, the increased level of LIF was accompanied by a decrease in mRNA levels for LIFR and CNTFR. In SHR, a higher level of mRNA IL-6R was observed. By contrast, the mRNA and protein levels for CT-1 and the mRNA level for gp130 did not vary in these two models. These findings suggest that IL-6 and LIF, but not CT-1, contribute to angiotensin II-dependent left ventricular hypertrophy in the two hypertensive rat models, TGR(mRen2)27 and SHR.

Different effects on inhibition of cardiac hypertrophy in spontaneously hypertensive rats by monotherapy and combination therapy of adrenergic receptor antagonists and/or the angiotensin II type 1 receptor blocker under comparable blood pressure reduction

To confirm that alpha1, beta adrenoceptor antagonists and angiotensin II type 1 receptor blockers (ARBs) have different abilities to attenuate progressive cardiac hypertrophy despite their comparable lowering of blood pressure, we compared the effect of these agents alone or in combination on hypertensive cardiac hypertrophy. Eight-week-old spontaneously hypertensive rats (SHR) were divided into 7 groups. Single administration of doxazosin, atenolol, or losartan, or half-dose combinations of these drugs were given orally for 6 weeks. The control group did not receive any drugs. The heart weight-to-body weight ratio (HW/BW), left ventricular mass index (LVMI), plasma brain natriuretic peptide (BNP) and left ventricular BNP mRNA expression were measured after 6-week administration. Blood pressure did not differ among the drug-treated groups, all of which showed lower blood pressure than the control group. The HW/BW and LVMI of the drug-treated groups, except the doxazosin group, were lower than in the control group. Moreover, the LVMI values of the groups receiving losartan were significantly lower than those in the groups without losartan (p < 0.05). Plasma BNP of the drug-treated groups was lower than that in the control group (p < 0.05). The left ventricular BNP mRNA expression of the drug-treated groups, except the doxazosin group, was lower than that in the control group. The atenolol group showed a higher level of BNP mRNA than the groups receiving losartan monotherapy or combination therapies (p < 0.05). In conclusion, the ARB had the strongest attenuating effect on the development of hypertensive cardiac hypertrophy, and the alpha1 and beta adrenergic receptor blockers were more effective in combination than as monotherapies in SHR.

Cardiac hypertrophy in vitamin D receptor knockout mice: role of the systemic and cardiac renin-angiotensin systems

Our recent studies suggest that 1,25-dihydroxyvitamin D3 functions as an endocrine suppressor of renin biosynthesis. Genetic disruption of the vitamin D receptor (VDR) results in overstimulation of the renin-angiotensin system (RAS), leading to high blood pressure and cardiac hypertrophy. Consistent with the higher heart-to-body weight ratio, the size of left ventricular cardiomyocytes in VDR knockout (KO) mice was markedly increased compared with wild-type (WT) mice. As expected, levels of atrial natriuretic peptide (ANP) mRNA and circulating ANP were also increased in VDRKO mice. Treatment of VDRKO mice with captopril reduced cardiac hypertrophy and normalized ANP expression. To investigate the role of the cardiac RAS in the development of cardiac hypertrophy, the expression of renin, angiotensinogen, and AT-1a receptor in the heart was examined by real-time RT-PCR and immunostaining. In VDRKO mice, the cardiac renin mRNA level was significantly increased, and this increase was further amplified by captopril treatment. Consistently, intense immunostaining was detected in the left ventricle of captopril-treated WT and VDRKO mice by use of an anti-renin antibody. Levels of cardiac angiotensinogen and AT-1a receptor mRNAs were unchanged in the mutant mice. These data suggest that the cardiac hypertrophy seen in VDRKO mice is a consequence of activation of both the systemic and cardiac RAS and support the notion that 1,25-dihydroxyvitamin D(3) regulates cardiac functions, at least in part, through the RAS.

Angiotensin converting enzyme inhibition for cardiac hypertrophy in patients with end-stage renal disease: what is the evidence?

Dialysis patients show a high prevalence of cardiovascular complications among which left ventricular hypertrophy is one of the most frequent and is independently predictive of mortality. A recent study indicates that partial regression of left ventricular hypertrophy improves mortality and reduces cardiovascular events in end-stage renal disease (ESRD) patients, suggesting the importance of targeting therapeutic strategies to reduce cardiac hypertrophy and improve the outcome in these patients. The pathogenesis of left ventricular hypertrophy in ESRD patients is multifactorial and includes hypertension, activation of the renin-angiotensin system, increased sympathetic activity, chronic volume overload, chronic anaemia and hyperparathyroidism. In this paper, we review the available experimental and clinical evidence showing the important contribution of the renin-angiotensin system as well as its interaction with the sympathetic nervous system in the pathogenesis of left ventricular hypertrophy in ESRD patients. Furthermore, we summarize the results of currently available clinical studies that examined the effects of angiotensin-converting enzyme inhibition or angiotensin receptor antagonism on left ventricular hypertrophy in ESRD patients, and review evidences that support the use of angiotensin-converting enzyme inhibitors or angiotensin receptor antagonists in the ESRD population.

Augmented diurnal variations of the cardiac renin-angiotensin system in hypertensive rats

There are several controversies concerning the enhanced gene expression of cardiac renin-angiotensin system components in spontaneously hypertensive rats (SHR) compared with their normotensive control Wistar-Kyoto (WKY) rats. We hypothesized that these discrepancies arise from circadian fluctuations in gene expression. We examined the circadian mRNA expression of renin, angiotensinogen, ACE, and angiotensin type 1a (AT1a) and type 2 (AT2) receptors in the hearts of SHR and WKY rats by real-time quantitative reverse transcription-polymerase chain reaction. The cardiac mRNA expression of the renin-angiotensin system components showed circadian oscillations in both SHR and WKY rats. The amplitudes of these circadian fluctuations were greater in the SHR than in the WKY rats. The mRNA levels of the renin-angiotensin system components were also increased in the SHR compared with the WKY rats at many time points (especially during the dark phase). However, the levels of ACE, AT1a receptor, and AT2 receptor mRNA in the SHR and WKY rats were almost the same during the late light phase. In contrast to mRNA expression, ACE activity was similar both at the time of maximum and minimum mRNA expression. The AT1 receptor antagonist candesartan upregulated AT1a receptor mRNA and downregulated ACE mRNA at specific time points only in the SHR group. Our findings of differential diurnal expression of cardiac renin-angiotensin system genes in SHR and WKY rats appear to explain the discrepancies between prior studies. However, the physiological relevance of the differential circadian mRNA expression of the renin-angiotensin system components remains to be elucidated.

The regulatory role of AT1 receptor on activated HSCs in hepatic fibrogenesis: Effects of RAS inhibitors on hepatic fibrosis induced by CCl4

AIM: To assess the effect of ACE inhibitor and AngII type 1 (AT1) receptor antagonist in preventing hepatic fibrosis caused by CCl₄ administration in rats; to investigate whether or not there are expression of AT1 receptors on hepatic stellate cells; and to observe the effect of AngII on proliferation and ECM synthesis of cultured HSCs.

METHODS: Studies were conducted in male Sprague-Dawley rats. Except for the hepatofibrotic model group and the control group, in three treated groups, either enalapril (5 mg/kg), or losartan (10 mg/kg), or enalapril + losartan were given to the fibrotic rats by daily gavage, and saline vehicle was given to model and normal control rats. After 6 wk, liver fibrosis was assessed directly by hepatic morphometric analysis, which has been considered the gold standard for the quantification of fibrosis. The expressions of AT1 receptors and (α-mooth muscle actin, α-SMA) in liver tissue or isolated hepatic stellate cells (HSCs) were detected by immunohistochemical techniques. The effect of AngII on HSC proliferation was determined by MTT method. Effect of AngII on collagen synthesis of HSCs was de termined by ³H-proline incorporation.

RESULTS: Contrasted to the fibrosis in rats of the model group, groups of rats treated with either enalapril or losartan, or a combination of two drugs showed a limited expansion of the interstitium (4.23 ± 3.70 vs 11.22 ± 4.79, P < 0.05), but no difference was observed among three treated groups (5.38 ± 3.43, 4. 96 ± 2.96, 4.23 ± 2.70, P > 0.05). Expression of AT1 receptors was found in fibrotic interstitium of fibrotic rats, whereas in normal control rats they were limited to vasculature only to a very slight degree. AT1 receptors were also expressed on activated HSCs in the culture. At concentrations from 10^-9 to 10^-5 mol/L, AngII stimulated HSC proliferation in culture in a dose-dependent manner. Increasing AngII concentrations produced corresponding increases in ³H-proline incorporation. Differences among groups were significant.

CONCLUSION: Angiotensin-converting enzyme inhibitors and AT 1 blocker may slow the progression of hepatic fibrosis; activated HSCs express AT1 receptors, and AngII can stimulate the proliferation and collagen synthesis of HSCs in a dose-dependent manner; and activation of RAS may be related to hepatic fibrogenesis induced by CCl₄.

Insulin-stimulated cardiac glucose uptake is impaired in spontaneously hypertensive rats: role of early steps of insulin signalling

OBJECTIVE

Although the heart is one of the target organs of insulin, it is still unknown whether the effect of insulin on cardiac muscle is preserved in essential hypertension, where insulin resistance has been observed in skeletal muscle.

METHODS

We evaluated cardiac glucose uptake and the early steps of insulin signalling in spontaneously hypertensive (SHR, 10-12 weeks old) and in age-matched normotensive Wistar-Kyoto (WKY) rats. Cardiac glucose uptake (micromol/100 g per min) was assessed by 2-[14C]deoxyglucose method. After an overnight fast, 16 WKY rats and 17 SHR underwent a hyperinsulinemic euglycemic clamp. In particular, 2-h intravenous (i.v.) infusion of insulin (10 mU/kg per min) or saline (NaCl 0.9%) was administered, followed by an i.v. bolus injection of 2-[14C]deoxyglucose (100 microCi/kg) to measure cardiac glucose uptake.

RESULTS

During saline infusion, cardiac glucose uptake was significantly higher in SHR compared to WKY rats (85 +/- 18 versus 8 +/- 3 mg/kg per min, P < 0.01). Furthermore, insulin was able to markedly increase cardiac glucose uptake in WKY rats whereas this insulin action was entirely abolished in SHR; thus, the cardiac glucose uptake became similar in the two rat strains (76 +/- 16 versus 82 +/- 16 mg/kg per min, not significant). More importantly, during saline infusion SHR showed a significantly higher phosphorylation of insulin receptor substance-1 (IRS-1) coupled to enhanced association of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) to IRS-1 and to an increased PI 3-kinase activity compared to WKY rats. As expected, insulin exposure evoked an activation of its signalling cascade in WKY rats. In contrast, in SHR, the hormone failed to activate post-receptor molecular events.

CONCLUSIONS

Our data indicate that the heart of SHR shows an overactivity of the proximal steps of insulin signalling which cannot be further increased by the exposure to the hormone. This abnormality may account for the marked increase of basal cardiac glucose uptake and the loss of insulin-stimulated glucose uptake observed in SHR.

Angiotensin II and connective tissue: homeostasis and reciprocal regulation

As a concept traditionally applied to integrative organ physiology, homeostasis likewise applies to self-regulated growth and structure of loose, dense and specialized connective tissues. De novo generation and co-induction of signals, either stimulatory or inhibitory to the formation of these tissues, provide for a reciprocal regulation of their composition; angiotensin (Ang) II is a growth stimulator. Components involved in AngII generation and its biological activity, including angiotensin converting enzyme (ACE) and AngII receptors, are expressed by mesenchymal cells responsible for connective tissue turnover. ACE inhibition or AT1 receptor antagonism attenuate the formation of these connective tissues. The concept of circulatory homeostasis, and the endocrine properties of plasma AngII involved in maintaining same, need each be broadened to encompass auto- and paracrine effects of AngII produced within connective tissues, where it contributes to their homeostatic regulation of structure and composition.