Myocardial bradykinin following acute angiotensin-converting enzyme inhibition, AT1 receptor blockade, or combined inhibition in congestive heart failure

Angiotensin II Exerts Positive Feedback on the Intrarenal Renin-Angiotensin System by an Angiotensin Converting Enzyme-Dependent Mechanism1

BACKGROUND

Plasma angiotensin II (ANG II) is not increased significantly in renovascular hypertension (RVH), but tissue ANG II levels are elevated in both kidneys of renovascular rats. Because the contralateral, non-ischemic kidney is critical for maintenance of hypertension in RVH, this study sought to understand the mechanism by which intrarenal ANG II levels are augmented in the non-ischemic kidney. This study tested the hypothesis that an incremental increase in plasma ANG II induces the intrarenal renin-angiotensin system (RAS) in the non-ischemic kidney by an angiotensin converting enzyme (ACE) dependent mechanism.

METHODS

To simulate the incremental increase in plasma ANG II induced by the ischemic kidney in RVH, an ANG II infusion model was used. This model used a chronic infusion of ANG II (40 ng/min) or vehicle by osmotic minipump into uninephrectomized rats. Parallel groups were treated with the ACE inhibitor Enalaprilat (200 mg/kg/day). Intrarenal ACE activity was measured by radioenzymatic assay. ANG II levels were quantified by radioimmunoassay.

RESULTS

Hypertension was evident in ANG II-infused rats, compared to control rats (155 +/- 4 versus 112 +/- 1 mmHg; P < 0.001). Concurrent treatment with Enalaprilat reversed the hypertension induced by ANG II infusion (98 +/- 3 versus 155 +/- 4 mmHg; P < 0.001). ANG II up-regulated intrarenal ACE activity in the non-ischemic kidney (59.2 +/- 11.9 versus 25.2 +/- 6.8 units/mg protein; P < 0.01). Enalaprilat significantly decreased renal ACE activity in ANG II-treated rats, compared to ANG II alone (11.4 +/- 1.0 versus 59.2 +/- 11.9 units/mg protein; P < 0.001). Intrarenal ANG II was increased in ANG II-infused rats, compared to control animals (52.9 +/- 7.1 versus 23.0 +/- 3.2 fmol/mg tissue; P < 0.001), and Enalaprilat prevented ANG II-induced increases in intrarenal ANG II (29.9 +/- 2.6 versus 52.9 +/- 7.1 fmol/mg tissue; P < 0.05).

CONCLUSION

Incremental changes in plasma ANG II induce de novo production of ANG II in the non-ischemic kidney to augment intrarenal ANG II content. ACE inhibition blocks this positive feedback loop, suggesting that ANG II activates the intrarenal RAS by an ACE-dependent mechanism. The impact of ACE inhibition on blood pressure suggests that this feedback loop may be an important mechanism for maintenance of hypertension in RVH.

Valsartan improves mitochondrial function in hearts submitted to acute ischemia

The effect of valsartan, an angiotensin II-type I receptor blocker, on the mitochondrial function, was studied using an ex vivo animal model (hearts from Wistar rats), perfused in a Langendorff system and then submitted to global acute ischemia. Parameters evaluated were: membrane electrical potential (DeltaPsi, using a tetraphenylphosphonium-TPP+-electrode), oxygen consumption by the respiratory chain (Clark-type O2 electrode), phosphorylation lag phase (time necessary to phosphorylate a fixed amount of ADP) and ATP/ADP ratio (adenine nucleotides quantified by high-pressure liquid chromatography-HPLC). Valsartan acts preferentially in the phosphorylation, increasing ATP/ADP ratios (succinate: 1.6+/-0.4 versus 0.5+/-0.1--P<0.05; ascorbate/N,N,N',N'-tetramethyl-P-phenylenodiamine-TMPD: 1.1+/-0.2 versus 0.4+/-0.1--p<0.05 versus ischemia in the absence of valsartan) and decreasing lag phase (glutamate/malate: 50.0+/-9.6 s versus 127.2+/-19.03 s-84.6+/-16.2% versus 215.3+/-32.2%; P=0.01; succinate: 111.8+/-33.1 s versus 275.73+/-45.99 s-168.2+/-49.8% versus 414.9+/-69.2%; P=0.02 or ascorbate/TMPD: 11.0+/-3.9 s versus 62.4+/-11.63 s-34.9+/-12.4% versus 198.1+/-36.9%; P=0.001 versus ischemia in the absence of valsartan). This enables a higher energy production in hearts submitted to acute ischemia, for which having energy becomes critical to preserve mitochondrial function. These mechanisms allow us to better understand valsartan cytoprotection in ischemic cardiomyopathy.

Myocardial Interstitial Matrix Metalloproteinase Activity Is Altered by Mechanical Changes in LV Load

LV myocardial remodeling is a structural hallmark of hypertensive hypertrophy, but molecular mechanisms driving this process are not well understood. The matrix metalloproteinases (MMPs) can cause myocardial remodeling in chronic disease states, but how MMP activity is altered with a mechanical load remains unknown. The present study quantified interstitial MMP activity after a discrete increase in LV load and dissected out the contributory role of the angiotensin II Type 1 receptor (AT1R). Pigs (38 kg) were randomized to undergo (1) increased LV load by insertion of an intra-aortic balloon pump (IABP) triggered at systole for 3 hours, then deactivated (n=11); (2) IABP and AT1R blockade (AT1RB; valsartan, 3 ng/kg/hr; n=6). MMP activity was directly measured in the myocardial interstitium using a validated inline digital fluorogenic microdialysis system. IABP engagement increased LV peak pressure from 92+/-3 to 113+/-5 and 123+/-7 mm Hg in the vehicle and AR1RB group, respectively, and remained elevated throughout the IABP period (P<0.05). With IABP disengagement, segmental shortening (% change from baseline of 0) remained depressed in the vehicle group (-32.2+/-11.8%, P<0.05) but returned to baseline in the AT1RB group (2.3+/-12.5%). MMP activity decreased with IABP in both groups. At IABP disengagement, a surge in MMP activity occurred in the vehicle group that was abrogated with AT1RB (3.03+/-0.85 versus 0.07+/-1.55 MMP units/hr, P<0.05). A transient increase in LV load caused a cyclic variation in interstitial MMP activity that is regulated in part by the AT1R. These temporally dynamic changes in MMP activity likely influence myocardial function and structure with increased LV load.