NO modulation of myocardial performance in fish hearts

In the mammalian heart, intracardiac nitric oxide (NO) regulates in an autocrine–paracrine manner cardiac function in the beat-to-beat response (Starling's law of the heart), short-term response (phasic control, e.g. excitation–contraction coupling, responses to neurotransmitters and endocrines) and long-term response (tonic control by altering gene expression). This trio of NO temporal-dependent actions has a long evolutionary history, as we have documented in the prototypic vertebrate heart, the teleost heart. This heart shares a common structural and functional scenario with higher vertebrate hearts exhibiting, at the same time, differences in myoarchitecture (trabecular vs. compact type), blood supply (lacunary vs. vascular) and pumping performance (sensitivity to filling pressure), thus providing challenging opportunities for revealing aspects of unity and diversity of cardiac NO in vertebrates. Using in vitro working teleost heart preparations we have shown that, under basal conditions, NO through a cGMP-mediated mechanism modulates ventricular performance (negative inotropism) and remarkably increases the sensitivity to filling pressure (i.e. the Frank–Starling response). NO-cGMP mechanism also influences the short-term response elicited by inotropic agents such as acetylcholine and angiotensin II. A role of NO in long-term cardiac adaptation is illustrated by morphologic evidence (e.g. NOS immuno-localization in phylogenetically distant species) which emphasizes the importance of NO in reshaping the angio-myoarchitecture of the fish heart ventricle (i.e. compensation for regional heterogeneity). Finally, by studying the avascular hearts of teleosts and amphibians that lack vascular endothelium, a relevant role of endocardial endothelium-NO signalling in intracavitary regulation of myocardial performance has been firmly established, thus revealing its early evolutionary role in non-mammalian vertebrates.