NOS distribution and NO control of cardiac performance in Fish and Amphibian Hearts

Nitric oxide (NO), generated endogenously by a family of NO synthases (NOS) in the heart, has important autocrine–paracrine effects on cardiac function, modulating the inotropic state, excitation–contraction coupling, diastolic function, heart rate and β-adrenergic responsiveness. Fish and amphibian hearts share common structural and functional aspects with higher vertebrates, while differing in relevant ultrastructural, myoarchitectural, vascular and pumping features. This synopsis deals with cardiac NOS expression and localization in phylogenetically and eco-physiologically different teleost species, as well as in lungfish and frog, thus documenting the long evolutionary history of cardiac NO. In particular, the role of NO in the mechanical performance of teleost and frog hearts, both in the absence (i.e., unstimulated heart preparations) and in the presence of physical (i.e., load changes) and chemical (inotropic agonists) stimuli, is analysed. Using teleost and amphibian hearts as natural models in which the coronary system is absent, or scarcely present, the importance of an endocardial endothelium (EE) NO-mediated intracavitary control of mechanical performance is emphasized. This highlights the ancient autocrine–paracrine role of the cardiac NOS/NO system during the evolution of the poikilotherm vertebrate heart.