Blockade of angiotensin signaling improves myocardial function in hypercholesterolemia independent of changes in eicosanoid release.

Abstract

In hypercholesterolemia in the presence or absence of atherosclerosis, cardiovascular dysfunction and altered signaling of angiotensin, nitric oxide, or prostanoids are closely related to enhanced oxidant stress. We analyzed the potentially beneficial effects of the specific angiotensin-converting enzyme inhibitor enalapril and the specific angiotensin receptor blocker losartan on cardiac performance, eicosanoid metabolism, and parameters of oxidant stress in hypercholesterolemic animals. Guinea pigs were fed a 1% cholesterol diet for 8 weeks (Chol) with or without equieffective doses of either enalapril (1.5 mg/kg/d; Ena) or losartan (3 mg/kg/d; Los). Hemodynamics were analyzed in Langendorff hearts. Detection of eicosanoids was by enzyme immunoassay. Estimation of plasma xanthine oxidase (XO) activity was determined by spectrophotometry. In hypercholesterolemic guinea pigs, enhanced oxidant stress (e.g., increased plasma XO activities) was associated with profound myocardial and coronary (e.g., endothelial) dysfunction. Both enalapril and losartan lowered plasma cholesterol levels slightly, but only the angiotensin receptor antagonist effectively suppressed the increased plasma XO activities (from 11.4 +/- 0.7 to 7.6 +/- 2.2 U/L), and at the same time decreased the augmented coronary flow (from 26.0 +/- 1.0 to 23.0 +/- 1.0 mL/min/g tissue) observed in hypercholesterolemic animals. Assessment of left ventricular pressure and contractility (e.g., dp/dtmax) as well as the diastolic relaxation parameter (tau) revealed substantial myocardial dysfunction (systolic and diastolic) in Chol that was more substantially (and comparably) improved during administration of losartan (Los) than during enalapril (Ena). Surprisingly, angiotensin signaling blockade by either antagonist further suppressed the diminished coronary dilator responses to bradykinin (BK; not significant for enalapril) or adenosine (Ado) was demonstrated in Chol Langendorff hearts [delta CPPBK/Ado: from 5.0 +/- 0.5/0.9 +/- 0.1 to 4.4 +/- 1.5/0.4 +/- 0.1 (Ena) or to 1.9 +/- 0.5/0.4 +/- 0.1 (Los) cm2 (area under the curve), respectively]. Finally, as expected from control studies using heart preparations from normocholesterolemic guinea pigs, enhanced cardiac release of eicosanoids, prostacyclin, and thromboxane in Chol (0.48 +/- 0.03 and 0.6 +/- 0.1 ng/min/g) was augmented even further by treatment with enalapril (Ena: 1.6 +/- 0.4 and 1.0 +/- 0.1 ng/min/g), but was significantly reduced to or below control levels in losartan-treated animals (Los: 0.4 +/- 0.1 and 0.2 +/- 0.1 ng/min/g). Blockade of angiotensin signaling via angiotensin-converting enzyme inhibition or receptor antagonism--although differentially acting on enhanced cardiac prostanoid metabolism and oxidant stress--efficiently restored proper systolic and diastolic myocardial performance (losartan was more beneficial than enalapril), probably by counterbalancing altered angiotensin II-->angiotensin receptor signaling in the cardiovascular system of hypercholesterolemic animals. Impaired coronary vasodilator capacity seems to be irreversible after 8 weeks of a high-cholesterol diet, as shown by the unexpected lack of a dilator effect with both enalapril and losartan.