Angiotensin II binding sites in the heart of Scyliorhinus canicula: an autoradiographic study

The vasopressor action of angiotensin II (ANG II) in ball pythons (Python regius)

Angiotensin II (ANG II) is part of the renin-angiotensin system (RAS) in vertebrates and exert vasoconstriction in all species studied. The present study examines the vasopressor effect of ANG II in the ball python (Python regius), and examines whether ANG II exert its effect through direct angiotensin receptors or through an activation of α-adrenergic receptors. The studies were conducted in snakes with chronic arterial catheters that had recovered from anesthesia. In addition to demonstrating a clear and pronounced dose-dependent rise in arterial blood pressure upon repeated injections of boluses with ANG II (0.001-1 μg/kg), we demonstrate that the pressor response persisted following α-adrenergic blockade using the α-adrenergic antagonist phentolamine (2.5 mg/kg). Unfortunately, it proved impossible to block the ANG receptors using losartan (1, 3 or even 10 mg/kg). The pressor response to ANG II was associated with a significant rise in heart rate at the higher dosages, pointing to a resetting of the barostatic mechanism for heart rate regulation. The responses were similar in fasting and digesting pythons despite the expected rise in baseline values for blood pressure and heart rate of the digesting snakes.

Exploring cardiac plasticity in teleost: the role of humoral modulation

The fish heart represents an established natural model for evaluating basic mechanisms of the coordinated physiological reactions which maintain cardiac steady-state. This is due to its relatively simple design, but also to its multilevel morpho-functional flexibility which allows adequate responses to a variety of intrinsic (body size and shape, swimming performance, etc.), and extrinsic (temperature, salinity, oxygen level, water chemistry, etc.) factors related to the animal life style. Nowadays, although many gaps are still present, a huge literature is available about the mechanisms that fine-tune fish cardiac performance, particularly in relation to the influence exerted by substances possessing cardio-modulatory properties. Based on these premises, this review will provide an overview of the existing current knowledge regarding the humoral control of cardiac performance in fish. The role of both classic (i.e. catecholamines, angiotensin II and natriuretic peptides), and emerging cardioactive substances (i.e. the chromogranin-A-derived peptides vasostatins, catestatin and serpinin) will be illustrated and discussed. Moreover, an example of cardiomodulation elicited by peptides (e.g., nesfatin-1) associated to the regulation of feeding and metabolism will be provided. The picture will hopefully emphasize the complex circuits that sustain fish cardiac performance, also highliting the power of the teleost heart as an experimental model to deciphering mechanisms that could be difficult to explore in more elaborated cardiac morpho-functional designs.

Immunohistochemical localization of renin-containing cells in two elasmobranch species

Renin immunoreactivity was localized at the light and electron microscopic level in two elasmobranch fish species, the Atlantic stingray, Dasyatis sabina, and river ray, Potamotrygon humerosa. At the light microscopic level, the peroxidase-anti-peroxidase method showed a positive immunoreactivity in modified smooth muscle cells in kidney afferent arterioles as well as in arterioles of several organs: rectal gland, inter-renal gland, conus arteriosus, and gill. Electron microscopic renin-positive immunogold localization was confined to the contents of membrane bound granules in the modified smooth muscle cells of these arterioles. The presence of renin-containing granules in the modified smooth muscle, "granular cells," of the renal glomerular afferent arteriole of these two stingray species adds support to earlier studies which showed the structural components of a complete juxtaglomerular apparatus and some of the biochemical and molecular components of a renin-angiotensin system (RAS) as found in teleost fish, reptiles, birds, and mammals. A notable result, however, was the renin-positive immunoreaction in the arteriolar wall of all other organs studied here. The presence of this "diffuse renin system" in the connective tissue of various organs suggests that in these two stingray species in addition to local organ-specific functions, the RAS may act as a systemic mechanism to regulate blood pressure and blood flow in the body.

Humoral control of cardiac remodeling in fish: role of Angiotensin II

Angiotensin II (AngII), the principal effector of the Renin-Angiotensin-System (RAS), is a multipotent hormone whose biological actions include short-term modulation as well as long-term adjustments. In the eel heart, AngII elicits short-term inotropic and chronotropic effects. However, information regarding the influence of AngII on cardiac remodeling, expressed as morphological and hemodynamic changes, is lacking. To clarify the putative actions of AngII on eel cardiac remodeling, we used freshwater eels (Anguilla anguilla) intraperitoneally injected for 4 weeks with saline or AngII (0.4 or 1.2 nmol g BW(-1)) or AngII (1.2 nmol g BW(-1)) plus the AT₂ receptor antagonist CGP42112. Using an in vitro working heart preparation, the cardiac response (stroke volume changes) to preload and afterload increases has been evaluated. Hearts of all groups showed similar Frank-Starling responses. However, in response to afterload increases, stroke volume rapidly decreased in control hearts, while it was better maintained in AngII-treated counterparts. These effects were abolished by an antagonist of the AT₂ receptor, whose cardiac expression was revealed by western blotting analysis. We also found by immunolocalization and immunoblotting that AngII influences both expression and localization of molecules which regulate cell growth [such as c-kit, heat shock protein 90 (Hsp-90), endothelial Nitric Oxide Synthase "(eNOS)-like" isoform] and apoptosis [i.e. apoptosis repressor with CARD domain (ARC)], thus playing a role in cardiac long-term adjustments. These results point to a role of AngII in eel heart remodeling, providing new insights regarding the modulation of cardiac plasticity in fish.

The eel heart: multilevel insights into functional organ plasticity

The remarkable functional homogeneity of the heart as an organ requires a well-coordinated myocardial heterogeneity. An example is represented by the selective sensitivity of the different cardiac cells to physical (i.e. shear stress and/or stretch) or chemical stimuli (e.g. catecholamines, angiotensin II, natriuretic peptides, etc.), and the cell-specific synthesis and release of these substances. The biological significance of the cardiac heterogeneity has recently received great attention in attempts to dissect the complexity of the mechanisms that control the cardiac form and function. A useful approach in this regard is to identify natural models of cardiac plasticity. Among fishes, eels (genus Anguilla), for their adaptive and acclimatory abilities, represent a group of animals so far largely used to explore the structural and ultrastructural myoarchitecture organization, as well as the complex molecular networks involved in the modulation of the heart function, such as those converting environmental signals into physiological responses. However, an overview on the existing current knowledge of eel cardiac form and function is not yet available. In this context, this review will illustrate major features of eel cardiac organization and pumping performance. Aspects of autocrine–paracrine modulation and the influence of factors such as body growth, exercise, hypoxia and temperature will highlight the power of the eel heart as an experimental model useful to decipher how the cardiac morpho-functional heterogeneities may support the uniformity of the whole-organ mechanics.

Molecular classification of an elasmobranch angiotensin receptor: quantification of angiotensin receptor and natriuretic peptide receptor mRNAs in saltwater and freshwater populations of the Atlantic stingray

Among the most conserved osmoregulatory hormone systems in vertebrates are the renin-angiotensin system (RAS) and the natriuretic peptides (NPs). We examined the RAS and NP system in the euryhaline Atlantic stingray, Dasyatis sabina (Lesueur). To determine the relative sensitivity of target organs to these hormonal systems, we isolated cDNA sequences encoding the D. sabina angiotensin receptor (AT) and natriuretic peptide type-B receptor (NPR-B). We then determined the tissue-specific expression of their mRNAs in saltwater D. sabina from local Texas waters and an isolated freshwater population in Lake Monroe, Florida. AT mRNA was most abundant in interrenal tissue from both populations. NPR-B mRNA was most abundant in rectal gland tissue from both populations, and also highly abundant in the kidney of saltwater D. sabina. This study is the first to report the sequence of an elasmobranch angiotensin receptor, and phylogenetic analysis indicates that the D. sabina receptor is more similar to AT(1) vs. AT(2) proteins. This classification is further supported by molecular analysis of AT(1) and AT(2) proteins demonstrating conservation of AT(1)-specific amino acid residues and motifs in D. sabina AT. Molecular classification of the elasmobranch angiotensin receptor as an AT(1)-like protein provides fundamental insight into the evolution of the vertebrate RAS.

Angiotensin II-induced inotropism requires an endocardial endothelium-nitric oxide mechanism in the in-vitro heart of Anguilla anguilla

Using an isolated working heart preparation we show that angiotensin II (ANG II), at concentrations of 10(-10)-10(-7) mol l(-1), elicits negative chronotropism and inotropism in the freshwater eel Anguilla anguilla. The negative inotropism was insensitive to losartan and CGP42112 (AT(1) and AT(2) ANG II receptor antagonists, respectively), and was abrogated by the AT(1) receptor antagonist CV11974, the G protein blocker pertussis toxin (PTx) and the muscarinic antagonist atropine. In contrast, it was not affected by the adrenoceptor antagonists propanolol, sotalol and phentolamine. Using donors (L-arginine) and inhibitors [N(G)-monomethyl-(L)-arginine (L-NMMA), L-N(5)(1-iminoethyl)ornithine ((L)-NIO)] of nitric oxide synthase (NOS), and haemoglobin as NO scavenger, we demonstrate that NO signalling is involved in ANG II-mediated inotropism. Pretreatment with Triton X-100, a detergent that damages the endocardial endothelium (EE), or with 1H-(1,2,4)oxadiazolo-(4,3-a)quinoxalin-1-one (ODQ), a specific inhibitor of soluble guanylate cyclase, or with the cGMP-activated protein kinase (PKG) inhibitor KT5328, abolished ANG II-mediated inotropism. Thus, ANG II-mediated inotropism occurs via an EE-NO-cGMP-PKG mechanism. ANG II did not affect the mechanical performance influenced by preload changes (i.e. the Frank-Starling response), which in the eel heart is modulated by NO. This EE-paracrine-mediated cardio-suppressive action of endoluminal ANG II suggests that the hormone plays an important intracardiac role in the fish heart.