ANTI-CARDIAC HYPERTROPHIC ACTION OF DES-ASPARTATE-ANGIOTENSIN I IN RATS AND RAT CARDIOMYOCYTES

Abstract

The role of des-aspartate-angiotensin I in the development of cardiac hypertrophy and the possible mechanisms of the peptide on the attenuation of the development of cardiac hypertrophy were studied. The results summarized below represent major original findings which contribute to the understanding of the renin angiotensin system (RAS) and its important member, des-aspartate-angiotensin I, in cardiac hypertrophy. 1. Des-aspartate-angiotensin I dose-dependently attenuated the hypertrophic index. The attenuation decreased with higher doses of the nonapeptide. Indomethacin (30.4 µmoles) significantly blunted the maximum attenuation. An anti-hypertensive dose of 10mg/kg losartan produced comparable effect to that of des-aspartate-angiotensin I. However, the action of losartan was not affected by indomethacin. Ineffective high but not low doses of des-aspartate-angiotensin I, when co-administered with hyppuryl-histidyl-leucine (HHL), showed significant activity. 2. In neonatal cultured rat cardiac myocytes, angiotensin II induced protein synthesis by about 25% as compared with control. Des-aspartate-angiotensin I dose-dependently inhibited angiotensin II-induced increase in protein synthesis in cardiac myocytes. Losartan also inhibited angiotensin II-induced increase in protein synthesis in cardiac myocytes. However, PD123319 failed to antagonize angiotensin II-induced protein synthesis. The data indicate that angiotensin II induces protein synthesis via AT1 receptor. Similar to our findings from animal model, indomethacin reversed attenuating effect of des-aspartate-angiotensin I on angiotensin II-induced increase in protein synthesis. On the other hand, indomethacin failed to inhibit attenuating effect of losartan. The results implies that the different mechanisms are possible for the role of des-aspartate-angiotensin I and losartan on the regression of cardiac hypertrophy. 3. The data of receptor binding assay demonstrated that there existed high affinity binding sites for angiotensin II in rat cardiac ventricular membranes. Guanine nucleotides (GppNHp, 1 mM) inhibited 50% of specific binding of [125I] angiotensin II to rat cardiac ventricular membranes. The other 50% of binding sites were not affected. Both of the binding sites showed similar affinity to angiotensin II. Guanine nucleotides (GppNHp, 1 mM) failed to affect specific binding of [125I] Sar1-I1e8 angiotensin II. Competition experiments showed that rat cardiac myocytes possess AT1 exclusively. As expected, des-aspartate-angiotensin I inhibited specific [125I] Sar1 –I1e8 angiotensin II binding to rat ventricular membranes. The potency of des-aspartate-angiotensin I was comparable to those of angiotensin III. 4. Exogenous angiotensin I was found to be converted to des-aspartate- angiotensin I instead of angiotensin II in the presence of amastatin, bestatin and EDTA. The results indicate the presence of a novel aminopeptidase (aminopeptidase X). The findings were in agreement with previous reports in other tissues. The conversion rate of angiotensin I to des-aspartate-angiotensin I in the homogenates of rat cardiac ventricles is nmoles/mg protein/minute. Interestingly, the activities of this novel enzyme decreased with the age or development of cardiac hypertrophy. The results provide further evidence to support our hypothesis that des-asparte-angiotensin I was involved in the regulation of cardiac hypertrophy.