The role of NO-synthetic pathway in structural remodeling of rat myocardium

Abstract

Nitric oxide (NO), endogenously synthesized by nitric oxide synthases (NOSs), generally acts to fine tune and optimise cardiac pump function. Several experimental studies have shown that low (submicromolar) doses of NO exert small positive inotropic effects, while at higher, but still physiological levels, NO enhances cardiomyocyte relaxation and diastolic function. It has been known that reduced NO bioavailability plays an important role in the development of heart failure, but the mechanisms are still uncovered. This thesis deals with structural remodeling of right ventricle (RV) myocardium of rats, induced by chronic modulation of NO-producing system and cold acclimation. In order to examine the possible effects of NO on myocardial structure and molecular basis of its remodeling at room temperature (22±1 ºC) or cold (4±1 ºC) acclimation, adult male rats (Mill Hill hybrid hooded, 2-month-old), were treated with L-arginine or Nω-nitro-L-arginine methyl ester (L-NAME) for 45 days, respectively. Our results demonstrated that cold acclimation per se does not significantly alter the volume densities of myocardial tissue components, but leads to a trend in cardiomyocyte hypertrophy followed by a downward trend in capillarity. Also, cold-acclimated control animals showed a significant decrease of proliferating cell nuclear antigen expression in comparison with room temperature-kept control animals. In all, these results do not indicate the cold-induced RV hypertrophy, at least at histological level. Results demonstrated that the structural alterations observed after the L-arginine treatment corresponded to physiologic cardiac hypertrophy, since no pathological changes in the myocardial structure (increased collagen deposition and/or myofibril distortion) after chronic L-arginine treatment were seen. Namely, L-arginine treatment in RT-acclimated rats led to cardiomyocyte hypertrophy which was followed by a simultaneous increase in capillarity and interstitial connective tissue in the myocardium, maintaining the relative ratio of tissue components unaltered...

Azot oksid (NO), endogeno sintetisan NO sintazama (NOS, engl. NO synthases) ima važnu ulogu u optimizaciji adaptivnog odgovora srca. Poznato je da NO pri fiziološkim koncentracijama ima pozitivno inotropno dejstvo, kao i da je hronično redukovan fiziološki nivo NO spregnut sa izmenjenom funkcijom srca i da stoga može doprineti razvoju srčane insuficijencije. Polazeći od ovih osnova, cilj teze je bio da razjasni mehanizme remodeliranja miokarda desne komore srca pacova u odgovoru na nisku temperaturu, kao i da se ispita uloga NO, modulacijom NO-sintaznog puta. U tu svrhu pacovi Mill Hill soja, starosti 2 meseca izlagani su niskoj temperaturi (4±1 ºC) u vremenu od 45 dana i poređeni sa životinjama aklimiranim na sobnu temperaturu (22±1 ºC). Obe grupe životinja, aklimirane na sobnu i nisku temperaturu dodatno su podeljene u tri podgrupe: 1. netretirane; 2. životinje tretirane supstratom NO sintaza NOS – L-arginin•HCl (2.25%) u pijaćoj vodi; 3. životinje pojene inhibitorom NOSs, L-NAME•HCl (0.01%). U poređenju sa kontrolnim, životinje aklimirane na hladnoću imaju značajno nižu ekspresiju transkripcionog faktora proliferacije ćelija. Mada se beleži trend hipertrofije kardiomiocita i smanjenje kapilarnosti, volumenska gustina komponenti miokarda desne komore se ne menja kod pacova aklimiranih na hladnoću, u poređenju sa životinjama aklimiranim na sobnu temperaturu. Zajedno, rezultati ne pokazuju značajnu hipertrofiju desne komore prouzrokovane hladnoćom, barem ne na histološkom nivou. Rezultati pokazuju da strukturne promene nakon L-arginin tretmana odgovaraju fiziološkoj srčanoj hipertrofiji. Naime, L-arginin tretman pacova aklimiranih na sobnu temperaturu, u poređenju sa kontrolama vodi hipertrofiji kardiomiocita, praćeno istovremenim povećanjem kapilarnosti i vezivnog tkiva. Međutim, relativni odnos komponenti miokarda, količina kolagena i strukturna organizacija miofibrila se ne menja nakon L-arginin tretmana, u poređenju sa kontrolnim životinjama...