Effect Of AngⅡ/AT1R And PI3k/Akt/eNOS Signaling Pathway On The Hypertensive Atrial Structural Remodeling And Impact Of Telmisartan On The Atrial Fibrillation Susceptibility In Spontaneously Hypertensive Rats

Evaluation of the time course of change in the atrial fibrillation susceptibility in spontaneously hypertensive rats Objective: We aimed to develop an in vivo rat model of atrial fibrillation(AF) using a transvenous catheter; and to determine a time point at which the AF susceptibility of hypertensive model is increased compared to the age-matched normotensive one; finally, to preliminarily probe the underlying mechanism of the hypertension-related increase in AF susceptibility. Methods: AF induction and in vivo electrophysiologic study were conducted through a transvenous electrode catheter in male spontaneously hypertensive rats(SHR, n=10) at 12 to 32 weeks of age and age-matched Wistar-Kyoto rats(WKY, n=10). Every 4 weeks, body weight(BW), systolic blood pressure(SBP) and heart rate(HR) were measured; left atrium diameter(LA), interventricular septal thickness(IVST), left ventricular posterior wall thickness(LVPWT), left ventricular end-diastolic diameter(LVDD), left ventricular end-systolic diameter(LVSD), fractional shortening(LVFS) were measured by echocardiographic study; a surface limb lead ECG was recorded and P wave duration was calculated; atrial effective refractory period(AERP), AF inducibility(AFI) and mean AF duration(MAFD) were evaluated. Results: The BW of SHR was lighter than that of WKY at all ages, and SBP was consistently higher in the SHR group. While there was no difference in HR between two groups throught the 20 weeks period. In the echocardiographic study, LA in SHR was larger than that in WKY after 16 weeks; SHR animals displayed remarkable cardiac hypertrophy with increased left ventricular wall thickness at all ages. The PWD was significantly longer in SHR than that in WKY after 20 weeks; the AERP was similar between two groups at basic stimuli of 150 ms, 120 ms and 100ms; the AFI had no statistical difference between two groups; the MAFD was prolonged in SHR after 28 weeks, but did not differ between two groups before that time. Correlation analysis revealed that LA was significantly positive correlated with the PWD(r=0.87,P<0.01); PWD(r=0.76,P<0.01), LA(r=0.77,P<0.01), IVST(r=0.70,P<0.01) and LVPWT(r=0.69,P<0.01) showed significant positive correlation with the MAFD. Conclusion: After 28 weeks, the SHR demonstrated an increased AF susceptibility compared to the WKY. The left ventricular hypertrophy, left atrial enlargement andprolonged PWD, resulted by long-standing hypertension, were associated with the increased AF susceptibility. The atrial remodeling characterized by atrial fibrosis and left atrial enlargement maybe the major cause of the increased AF susceptibility in hypertension.Effect of AngⅡ/AT1 R and PI3K/Akt/e NOS signaling pathway on hypertensive atrial structural remodelingObjective: The activated renin-angiotensin system and the insulin resistance are two common coexisting pathophysiologic disturbances responsible for the increased atrial fibrillation(AF) susceptibility in hypertension. We sought to investigate the infuence of the activated angiotensin Ⅱ/angiotensin Ⅱ type 1 receptor(AngⅡ/AT1R) signaling pathway, the impaired phosphatidylinositol 3-kinase/protein kinase B/ endothelial nitric oxide synthase(PI3K/Akt/e NOS) signaling pathway and the interactions between the two signaling pathway, the underling molecular mechanism of the two pathophysiologic disorders, on hypertensive atrial structual remodeling.Methods: Eighty-eight 28-week-old male spontaneously hypertensive rats(SHR) were randomly assigned to four groups:(1) SHR group(without treatment, n=22);(2) IGF group(receved recombinant human insulin-like growth Factor I(rh IGF-1) 50μg/kg/d, n=22);(3) PFT group(received pifithrin-α(PFT) 2.2 mg/kg, n=22);(4) VAL group(received valsartan 10mg/kg/d, n=22). Over a four-week period, the rats of the IGF group were injected daily with rh IGF-1 50 μg/kg by cauda vein, the rats of PFT group were intraperitoneally injected daily with PFT 2.2 mg/kg, and the rats of the VAL group were received daily intragastrically with 10 mg/kg/d valsartan. In addition, age-matched Wistar-Kyoto rats(WKY group, n=22) were used as normotensive controls. Body weight(BW), systolic blood pressure(SBP) and heart rate(HR) were measured before and after treatment. Four weeks after treatment, echocardiographic study was performed. P wave duration was measured on the surface ECG. Atrial effective refractory period(AERP) and mean AF duration(MAFD) were evaluated through transvenous electrode catheter. A microscopy incorporated with image analysis software was employed to measure the atrial myocardial cell size with hemotoxylin-eosin-staining; to measure the myocardial interstitial collagen volume fraction(CVF) with Masson-staining and to calculate the myocyte apoptosis index(APOI) with TUNEL-staining. The levels of sinaling proteins expressions of the Ang Ⅱ /AT1 R and PI3K/Akt/e NOS signaling pathway were detected by enzyme-linked immunosorbent assay(ELISA) and Western Blot(WB).Results: Compared to the WKY group, the BW of SHR group was lighter than that of the WKY group and SBP was consistently higher than that of the WKY group throughout the 4 weeks period. Left ventricular hypertrophy(LVH) and arrhythmogenic atrial remodeling including left atrium enlargement(LAE), myocyte hypertrophy, interstitial fibrosis and myocyte apoptosis was found in SHR group, accompanied by prolonged PWD and MAFD. The activities of Ang Ⅱ /AT1 R signaling molecules including AngⅡ, AT1 R, p-ERK1/2 and p22(phox) and gp91(phox), two NADPH oxidase subunits were increased; the activities of PI3K/Akt signaling molecules including p-PI3 K, p-Akt and p-e NOS were decreased. The levels of apoptosis-related molecules p53 and cleaved Caspase-3 expressions and the ratio of Bax/Bcl-2 were increased. Compared to the SHR group, the size of the myocytes, CVF and APOI were decreased by treatment of rh IGF-1, PFT and valsartan. PWD and MAFD were shorter in the groups of IGF, PFT and VAL. The levels of AngⅡ, AT1 R, p-ERK1/2, p22(phox) and gp91(phox) expressions were decreased in the groups of IGF, PFT and VAL. PFT, rh IGF-1 and valsatan could upregulate the expressions of p-PI3 K, p-Akt and p-e NOS. The p53 and cleaved Caspase-3 expressions and the ratio of Bax/Bcl-2 were decreased in these 3 groups. However, only valsartan reduced the SBP and attenuated both LVH and LAE.Conclusion: The activated Ang Ⅱ /AT1 R signaling pathway and impaired PI3K/Akt/e NOS signaling pathway play an important role in the pathogenesis of hypertensive atrial structural remodeling, including myocyte hypertrophy, interstitial fibrosis and myocyte apoptosis in hypertension. There exist a crosstalk between AngⅡ/AT1 R signaling pathway and PI3K/Akt/e NOS signaling pathway in the atrial tissue of hypertensive rats, and p53 may act as a key molecular linking the two signaling pathways.The impact of PPAR-γ partial agonist telmisartan on the structural remodeling and atrial fibrillation susceptibility in spontaneously hypertensive ratsObjective: Telmisartan is an angiotensin receptor blocker(ARB), which act as a partial agonist of peroxisome proliferator-activated receptor-γ(PPAR-γ). PPAR-γ agonists were approved to improve insulin sensitivity. Here, we compared the effect of telmisartan with valsartan, an ARB without PPAR-γ activity, on the atrial fibrillation(AF) susceptibility in hypertensive rats and futher investigate its mechanism.Methods: Sixty-six 28-week-old male spontaneously hypertensive rats(SHR) were randomly assigned to three groups: a SHR group(which received vehicle, n=22), a VAL group(which received valsartan 10mg/kg/d, n=22), and a TEL group(which received telmisartan 10 mg/kg/d, n=22). Over a four-week period, the rats of the SHR group were treated intragastrically with vehicle(saline), the rats of the TEL group received daily intragastrically with 10 mg/kg/d telmisartan, and the rats of the VAL group daily intragastrically with 10mg/kg/d valsartan. Body weight(BW), systolic blood pressure(SBP) and heart rate(HR) were measured before and after treatment. Four weeks after treatment, echocardiographic study was performed and P wave duration was measured on the surface ECG. Atrial effective refractory period(AERP) and mean AF duration(MAFD) were evaluated through transvenous electrode catheter. A microscopy incorporated with image analysis software was employed to measure the atrial myocardial cell size with hemotoxylin-eosin-staining; to measure the myocardial interstitial collagen volume fraction(CVF) with Masson-staining and to calculate the myocyte apoptosis index(APOI) with TUNEL-staining. The levels of sinaling proteins expressions of the AngⅡ/AT1 R and PI3K/Akt/e NOS signaling pathway were detected by enzyme-linked immunosorbent assay(ELISA) and Western Blot(WB).Results: At the end of the experiment, the SBP in the TEL group and VAL group was significantly decreased compared with that in the SHR group. The SBP did not differ between the TEL group and the VAL group throughout the experiment. Both telmisartan and valsartan decreased the left atrium diameter and left ventricular wall thickness, with no significant difference between TEL and VAL treated groups. Compared to the SHR group, the size of the myocytes, CVF and APOI were decreased by treatment of telmisartan and valsartan; and the effect of telmisartan was greater than that of valsartan. Although treatments with both telmisartan and valsartan could shorten PWD and MAFD, the TEL group demonstrted a shorter MAFD than VAL group. Both telmisartan and valsartan down-regulated the expression of AngⅡ/AT1 R signaling molecular(AngⅡ, AT1 R, p-ERK1/2, p22(phox) and gp91(phox)), up-regulated the expression of PI3K/Akt/e NOS signaling molecular(p-PI3 K, p-Akt and p-e NOS), and decreased the p53 and cleaved Caspase-3 expressions and the ratio of Bax/Bcl-2. For all proteins examined, the effects of telmisartan were more pronounced than that of valsartan.Conclusion: Telmisartan provided a better protective effect than valsartan on ameliorating atrial structural remodeling and reducing the AF susceptibility, which may associate with its simultaneously supressing AngⅡ/AT1 R signaling pathway, activating PI3K/Akt/e NOS signaling pathway and reversing the imbalance of AngⅡ/AT1 R and PI3K/Akt/e NOS pathway.