Gill microcirculation of the air-breathing climbing perch, Anabas testudineus (Bloch):relationships with the accessory respiratory organs and systemic circulation

The general macrocirculation and branchial microcirculation of the air-breathing climbing perch, Anabas testudineus, was examined by light and scanning electron microscopy of vascular corrosion replicas. The ventral aorta arises from the heart as a short vessel that immediately bifurcates into a dorsal and a ventral branch. The ventral branch distributes blood to gill arches 1 and 2, the dorsal branch to arches 3 and 4. The vascular organization of arches 1 and 2 is similar to that described for aquatic breathing teleosts. The respiratory lamellae are well developed but lack a continuous inner marginal channel. The filaments contain an extensive nutritive and interlamellar network; the latter traverses the filament between, but in register with, the inner lamellar margins. Numerous small, tortuous vessels arise from the efferent filamental and branchial arteries and anastomose with each other to form the nutrient supply for the filament, adductor muscles, and arch supportive tissues. The efferent branchial arteries of arches 1 and 2 supply the accessory air-breathing organs. Arches 3 and 4 are modified to serve primarily as large-bore shunts between the dorsal branch of the ventral aorta and the dorsal aorta. In many filaments from arches 3 and 4, the respiratory lamellae are condensed and have only 1-3 large channels. In some instances in arch 4, shunt vessels arise from the afferent branchial artery and connect directly with the efferent filamental artery. The filamental nutrient and interlamellar systems are poorly developed or absent. The respiratory and systemic pathways in Anabas are arranged in parallel. Blood flows from the ventral branch of the ventral aorta, through gill arches 1 and 2, into the accessory respiratory organs, and then returns to the heart. Blood, after entering the dorsal branch of the ventral aorta, passes through gill arches 3 and 4 and proceeds to the systemic circulation. This arrangement optimizes oxygen delivery to the tissues and minimizes intravascular pressure in the branchial and air-breathing organs. The efficiency of this system is limited by the mixing of respiratory and systemic venous blood at the heart.     

Fine structure of the respiratory organs of the Climbing perch, Anabas testudineus (Pisces: Anabantidae)

An electron microscopic study has been made of the three respiratory organs of climbing perch. The gill structure is similar to that of the other telcosts but the thickness of the water/blood barrier is much greater, being as great as 20 μm in some specimens. The increased thickness is due to a multilayered epithelium which is thinner (3.5–7 μm) over the marginal channel of the secondary lamellae. The other two main layers, basement membrane and pillar cell flange, are relatively thin (about 1 μm).
The pillar cells have a typical structure, but in certain regions they are contiguous with one another and line well-defined blood channels. Some of the columns of basement membrane material in such regions may be common to adjacent pillar cells.
The air-breathing organs are (a) the lining of the suprabranchial chambers , and (b) the labyrinthine plates attached to the dorsal region of branchial arches. Electron microscopy showed that their structure is well adapted for gas exchange, the air/blood barriers being only 0.12–0.3 μm, comprising an epithelial layer, basement membrane, and thin capillary endothelium. The many parallel blood channels of the respiratory islets of both organs are separated by pillar-like structures which differ from the pillar cells of the secondary lamellae. Thus the hypothesis that the air-breathing organs represent modified gills is not supported by this study.
The fine structure of the non-respiratory region of the air-breathing organs is similar to that of the skin, and includes chemoreceptor-like cells. Evidence concerning the possible homology of pillar cells with plain muscle cells is discussed.     

Autonomic nervous control of the circulatory system in the air-breathing fish Channa argus

The control of the cardiovascular system with particular emphasis on the regulation of blood distribution in the gills and air-breathing organ was studied in the air-breathing teleost Channa argus. Perfused head preparations were used in addition to experiments with isolated strip preparations of arteries and heart chambers. The distribution of adrenergic nerves was investigated using Falck-Hillarp fluorescence histochemistry. This preliminary study shows an adrenergic control system composed of chromaffin cells and adrenergic nerves similar to that found in other teleosts investigated, although the systemic arteries (coeliac artery, dorsal aorta and the vasculature of the air-breathing organ) appear to lack an adrenergic innervation. The reactions of isolated artery strip preparations to acetylcholine and adrenaline resemble those seen in other teleosts, and there is a prominent inhibitory effect of L-isoprenaline suggestive of arterial beta-adrenoceptors. The general vascular resistance of the gill apparatus-air-breathing organ increases in response to acetylcholine or adrenaline, and there is a redistribution of perfusion flow from the air-breathing organ circuit (anterior venous outflow from the first and second pair of gills and the air-breathing organ) to the general systemic circuit (dorsal aortic outflow from the third and fourth pair of gills). Stimulation of the vagal branch entering the air-breathing organ mimics the effects of acetylcholine or adrenaline. This innervation is probably non-adrenergic since no adrenergic nerve fibres could be demonstrated in the vasculature of the air-breathing organ using the histochemical technique. An adrenergic control of the vasculature of the air-breathing organ is not likely, since the concentration of adrenaline needed to affect the vasculature is not reached in the plasma even during "stress".     

Circulatory Anatomy in Bimodally Breathing Fish

SYNOPSIS. The development of air-breathing organs in bimodallybreathing fish has necessitated a degree of vascular remodellingin order to enhance gas exchange and support other homeostaticactivities. Macrocirculatory changes include several plumbingschemes that allow perfusion of the gills, air-breathing organ,and systemic circulations in a variety of in-parallel and in-seriesarrangements. The incorporation of structural adaptations designedto minimize admixture of oxygenated and deoxygenated blood intransit through the heart as well as vascular shunts furtherincreases the efficiency of the gas exchange process. A numberof anatomical modifications in capillary architecture and endothelialcell structure are found in air-breathing fish and appear tobe unique to these vertebrates. The physiological significanceof the microcirculatory adaptations remains, to a large extent,speculative.     

Autonomic control of the heart in the Asian swamp eel (Monopterus albus)

The Asian swamp eel (Monopterus albus) is an air-breathing teleost with very reduced gills that uses the buccal cavity for air-breathing. Here we characterise the cardiovascular changes associated with the intermittent breathing pattern in M. albus and we study the autonomic control of the heart during water- and air-breathing. The shift from water- to air-breathing was associated with a rise in heart rate from 27.7 ± 1.6 to 41.4 ± 2.6 min(-1) and an increase in cardiac output from 23.1 ± 3.0 to 58.7 ± 6.5 mLmin(-1)kg(-1), while mean systemic blood pressure did not change (39.0 ± 3.5 and 46.4 ± 1.3 cmH(2)O). The autonomic control of the heart during water- and air-breathing was revealed by infusion of the β-adrenergic antagonist propranolol and muscarinic antagonist atropine (3 mgkg(-1)) in eels instrumented with an arterial catheter. Inhibition of the sympathetic and parasympathetic innervations of the heart revealed a strong vagal tone on the heart of water-breathing eels and that the tachycardia during air-breathing is primarily mediated by withdrawal of cholinergic tone.     

Understanding the gastrointestinal physiology and responses to feeding in air-breathing Anabantiform fishes

The Mekong Delta is host to a large number of freshwater species, including a unique group of facultative air-breathing Anabantiforms. Of these, the striped snakehead (Channa striata), the climbing perch (Anabas testudineus), the giant gourami (Osphronemus goramy) and the snakeskin gourami (Trichogaster pectoralis) are major contributors to aquaculture production in Vietnam. The gastrointestinal responses to feeding in these four species are detailed here. Relative intestinal length was lowest in the snakehead, indicating carnivory, and 5.5-fold greater in the snakeskin, indicating herbivory; climbing perch and giant gourami were intermediate, indicating omnivory. N-waste excretion (ammonia-N + urea-N) was greatest in the carnivorous snakehead and least in the herbivorous snakeskin, whereas the opposite trend was observed for net K⁺ excretion. Similarly, the more carnivorous species had a greater stomach acidity than the more herbivorous species. Measurements of acid–base flux to water indicated that the greatest postprandial alkaline tide occurred in the snakehead and a potential acidic tide in the snakeskin. Additional findings of interest were high levels of both PCO₂ (up to 40 mmHg) and HCO₃⁻ (up to 33 mM) in the intestinal chyme of all four of these air-breathing species. Using in vitro gut sac preparations of the climbing perch, it was shown that the intestinal net absorption of fluid, Na⁺ and HCO₃⁻ was upregulated by feeding but not net Cl⁻ uptake, glucose uptake or K⁺ secretion. Upregulated net absorption of HCO₃⁻ suggests that the high chyme (HCO₃⁻) does not result from secretion by the intestinal epithelium. The possibility of ventilatory control of PCO₂ to regulate postprandial acid–base balance in these air-breathing fish is discussed.     

Comparative morphological examination of vertebral bodies of teleost fish using high-resolution micro-CT scans

Vertebral bodies of teleost fish are formed by the sclerotomal bone covering the chordacentrum. The internal part of the sclerotomal bone is composed of an amphicoelous hourglass shaped autocentrum, which is common in most fish species. In contrast, the external shape of the sclerotomal bone varies extensively among species. There are multiple hypotheses regarding the composition and formation of the external structure. However, as they are based on studies of few extant or extinct species, their applicability to other species remains to be clarified. To understand the morphology, formation, and composition of vertebral bodies in teleosts, we performed a comparative analysis using micro-CT scans of 32 species from 10 orders of Teleostei and investigated the detailed morphology of the sclerotomal bone, especially its plate-like ridge and trabeculae. We discovered two structural characteristics that are shared among most of the examined species. One was the sheet-like trabeculae that extend radially from the center of the vertebral body with a constant thickness. The other was the presence of hollow spaces on the internal parts of the lateral ridge and trabeculae. The combination of different arrangements of sheet-like trabeculae and internal hollow spaces formed different shapes of the lateral structure of the vertebral body. The properties of these two characteristics suggest that the external part of the sclerotomal bone grows outward by deposition at the bone tip, and that, concurrently, bone absorption occurs in the internal part of the sclerotomal bone. The vertebral arches were also formed by the sheet-like trabeculae, indicating that both, the vertebral body and the arches, are formed by the same component. The micro-CT scanning data were uploaded to a public database so they can be used for future studies on fish vertebrae.     

Reproduction and spawning behavior of the climbing perch Anabas testudineus (Perciformes, Anabantidae) in an aquarium

Results of study of the spawning and reproductive behavior of the climbing perch Anabas testudineus in an aquarium are presented. Main specific features of reproduction of this species are revealed. In the performed experiments, climbing perches did not build a nest, did not prepare a spawning substrate, and did not show any forms of parental care. The pre-spawning and spawning behavior of the climbing perch is described, and fecundity and gonadosomatic index are determined. It is shown that intermittent spawning is typical of these fish. Mating system of the climbing perch is either polygamy or promiscuity. Sexual dimorphism in the climbing perch was not found. The eggs of the climbing perch have positive buoyancy and belong to the really pelagic type, which is rare for freshwater fish. Infanticide is a common phenomenon for the climbing perch. Breeding of the given species under aquarium conditions can be exercised without artificial hormonal stimulation, at different sex ratio, and under conditions of stocking density considerably exceeding standard aquacultural recommendations. The main specific features of reproduction of the climbing perch are discussed in relation to evolution of parental care in labyrinth fishes.     

Vasculature of the head and respiratory organs in an obligate air-breathing fish,the swamp eel Monopterus (=Amphipnous) cuchia

Methyl methacrylate vascular corrosion replicas were used to examine the macrocirculation in the head region and the microcirculation of respiratory vessels in the air-breathing swamp eel Monopterus cuchia. Fixed respiratory tissue was also examined by SEM to verify capillary orientation. The respiratory and systemic circulations are only partially separated, presumably resulting in supply of mixed oxygenated and venous blood to the tissues. A long ventral aorta gives rise directly to the coronary and hypobranchial arteries. Two large shunt vessels connect the ventral aorta to the dorsal aorta, whereas the remaining ventral aortic flow goes to the respiratory islets and gills. Only two pairs of vestigial gill arches remain, equivalent to the second and third arches, yet five pairs of aortic arches were identified. Most aortic arches supply the respiratory islets. Respiratory islet capillaries are tightly coiled spirals with only a fraction of their total length in contact with the respiratory epithelium. Valve-like endothelial cells delimit the capillary spirals and are unlike endothelial cells in other vertebrates. The gills are highly modified in that the lamellae are reduced to a single-channel capillary with a characteristic three-dimensional zig-zag pathway. There are no arterio-arterial lamellar shunts, although the afferent branchial artery supplying the gill arches also supplies respiratory islets distally. A modified interlamellar filamental vasculature is present in gill tissue but absent or greatly reduced in the respiratory islets. The macro- and micro-circulatory systems of M. cuchia have been considerably modified presumably to accommodate aerial respiration. Some of these modifications involve retention of primitive vessel types, whereas others, especially in the microcirculation, incorporate new architectural designs some of whose functions are not readily apparent.     

A scanning electron microscope study of the gills of the air-breathing catfish, Clarias batrachus L.

Using the scanning electron microscope, the gills of the air-breathing catfish, Clarias batrachus , have been studied. The overall morphology of the gills are similar to other teleosts. In contrast to water-breathing species, however, microridges are absent from the surfaces of the secondary lamellae and only short microvilli are present. Long, convoluted microridges are present on the epithelial cells of the gill filaments. The possible roles of these structures in relation to water flow are discussed.     

A novel technique for estimating the compact myocardium in fishes reveals surprising results for an athletic air-breathing fish, the Pacific tarpon

A method for quickly assessing the relative proportion of compact myocardium in the ventricle of teleosts is introduced and used in juvenile Pacific tarpon Megalops cyprinoides , a member of the only air-breathing elopomorph teleost genus. The proportion of compact myocardium increased with body mass, reaching up to 60% of the ventricular mass. The finding for tarpon was a surprising discovery since recent literature has suggested that air breathing evolved primarily as means of supplying oxygen to the fish heart during activity. The present data, which represent the first quantitative assessment of the compact myocardium for any air-breathing fish, suggest that myocardial oxygen supply in the tarpon is supplemented by the coronary circulation associated with compact myocardium during exercise, while air breathing is important during aquatic hypoxia. Compact myocardium was also measured as a point of reference in an extant representative from a more ancient fish lineage than the elopomorphs, the water-breathing spiny dogfish Squalus acanthias and found to be only 9% of ventricular mass. In conclusion, the presence of a coronary circulation in extant elasmobranchs may mean that the coronary circulation evolved well before air breathing in fishes and, for tarpon at least, the coronary oxygen supply to the ventricular myocardium has not necessarily been superseded by air breathing.     

Electron microscopy of cardiac tissue in a primitive vertebrate Myxine glutinosa

Hearts of the Atlantic hagfish, Myxine glutinosa were studied with the electron microscope after prefixation in phosphate buffered glutaraldehyde or buffered formalin and subsequent postifxation in phosphate buffered osmium tetroxide. Epicardial, myocardial and endocardial layers are identified; however the hearts of Myxine lack an extensive capillary system comparable to the coronary vessels of other vertebrate heart tissues. Instead, blood is supplied to cells via an elaborate system of channels which extend between numerous trabeculae that make up the cardiac wall of this organism. Fine structural features of special interest include the presence of numerous dense granules (chromaffin granules) within myofibers and also specific granular cells which lack the contractile elements that are characteristic of both skeletal and cardiac myofibers. Another prominent feature noted includes an elaborate system of tubular invaginations within the subjacent sarcoplasm. These elements appear to be specific for the myofibers. They are continuous with the plasma membrane and project into the peripheral sarcoplasmic matrix. Crystalline inclusions are also observed in the sarcoplasm of the myofibers. These are compared with similar inclusions in other cellular components. The Golgi complex is very extensive in the myofibers of Myxine, and granules of varying sizes and densities often appear in the vicinity of the Golgi saccules. The observations suggest that the numerous vesicles around the Golgi Complex represent intermediate stages in the formation of the chromaffin granules. The structure and function of the extensive tubular invaginations are compared with the transverse tubules reported in several mammalian heart tissues.     

Vascular organization of the head and respiratory organs of the air-breathing catfish,Heteropneustes fossilis

Light and scanning electron microscopy of vascular replicas from the facultative air-breathing fish Heteropneustes fossilis show modifications in the macrocirculation of the respiratory organs and systemic circulation, whereas, gill microcirculation is similar to that found in typical water-breathing fish. Three and sometimes four ventral aortae arise directly from the bulbus. The most ventral vessel supplies the first pair of arches. Dorsal to this another aorta supplies the second gill arches, and a third, dorsal to, and larger than the other two, supplies the third and fourth arches and the air sacs. Occasionally a small vessel that may be the remnant of a primitive aortic arch arises from the first ventral aorta and proceeds directly to the mandibular region without perfusing gill tissue. The air sac is perfused by a large-diameter extension of the afferent branchial artery of the fourth gill arch and its circulation is in parallel with the gill arches. Blood drains from the air sac into the fourth arch epibranchial artery. A number of arteries also provide direct communication between the efferent air sac artery and the dorsal aorta. All four gill arches are well developed and contain respiratory (lamellar) and nonrespiratory (interlamellar and nutrient) networks common to gills of water-breathing fish. Air sac lamellae are reduced in size. The outer 30% of the air sac lamellar sinusoids are organized into thoroughfare channels; the remaining vasculature, normally embedded in the air sac parenchyma, is discontinuous. A gill-type interlamellar vasculature is lacking in the air sac circulation. Despite the elaborate development of the ventral aortae, there is little other anatomical evidence to suggest that gill and air sac outflow are separated and that dorsal aortic oxygen tensions are maintained when the gills are in a hypoxic environment. Physiological adjustments to hypoxic water conditions probably include temporal regulation of gill and air sac perfusion to be effective, if indeed they are so.     

Two types of endothelial cells in the heart of platyfish (Poeciliidae: Teleostei)

The present study describes two structurally and functionally different endothelial cell types in the heart of platyfish (Xiphophorus maculatus), which reflect adaptions to two quite unlike environments in this organ. The endothelial layers on the wall and valves of the ventricular apertures come in contact with only a small amount of the blood volume and have not evolved any blood cleaning abilities. These endothelial layers mainly protect the underlying tissue against the strain caused by a high blood flow. In contrast, the endothelium on the muscle trabeculae within the heart wall comes in contact with a large part of the blood volume at a low strain and have evolved structural features which make them highly efficient as blood cleaning tissue in this species.     

The anatomical components of the cardiac outflow tract of chondrichthyans and actinopterygians

The outflow tract of the fish heart is the segment interposed between the ventricle and the ventral aorta. It holds the valves that prevent blood backflow from the gill vasculature to the ventricle. The anatomical composition, histological structure and evolutionary changes in the fish cardiac outflow tract have been under discussion for nearly two centuries and are still subject to debate. This paper offers a brief historical review of the main conceptions about the cardiac outflow tract components of chondrichthyans (cartilaginous fish) and actinopterygians (ray‐finned fish) which have been put forward since the beginning of the nineteenth century up to the current day. We focus on the evolutionary origin of the outflow tract components and the changes to which they have been subject in the major extant groups of chondrichthyans and actinopterygians. In addition, an attempt is made to infer the primitive anatomical design of the heart of the gnathostomes (jawed vertebrates). Finally, several areas of further investigation are suggested. Recent work on fish heart morphology has shown that the cardiac outflow tract of chondrichthyans does not consist exclusively of the myocardial conus arteriosus as classically thought. A conus arteriosus and a bulbus arteriosus, devoid of myocardium and mainly composed of elastin and smooth muscle, are usually present in cartilaginous and ray‐finned fish. This is consistent with the suggestion that both components coexisted from the onset of the gnathostome radiation. There is evidence that the conus arteriosus appeared in the agnathans. By contrast, the evolutionary origin of the bulbus is still unclear. It is almost certain that in all fish, both the conus and bulbus develop from the embryonic second heart field. We suggest herein that the primitive anatomical heart of the jawed vertebrates consisted of a sinus venosus containing the pacemaker tissue, an atrium possessing trabeculated myocardium, an atrioventricular region with compact myocardium which supported the atrioventricular valves, a ventricle composed of mixed myocardium, and an outflow tract consisting of a conus arteriosus, with compact myocardium in its wall and valves at its luminal side, and a non‐myocardial bulbus arteriosus that connected the conus with the ventral aorta. Chondrichthyans have retained this basic anatomical design of the heart. In actinopterygians, the heart has been subject to notable changes during evolution. Among them, the following two should be highlighted: (i) a decrease in size of the conus in combination with a remarkable development of the bulbus, especially in teleosts; and (ii) loss of the myocardial compact layer of the ventricle in many teleost species.     

Preferential distribution of left and right auricular blood into the arterial arches of the Tuatara, Sphenodon punctatus

Evidence is presented which shows that in traversing the heart of the Tuatara, systemic venous blood is selectively distributed to the pulmonary arches. This selective distribution of deoxygenated blood into the pulmonary circuit occurs despite the poorly developed primary ventricular septum and the lack of a secondary septum, and yet it appears to be as effective as that seen in reptiles having these anatomical attributes.     

Which came first, the lung or the breath?

Lungs are the characteristic air-filled organs (AO) of the Polypteriformes, lungfish and tetrapods, whereas the swimbladder is ancestral in all other bony fish. Lungs are paired ventral derivatives of the pharynx posterior to the gills. Their respiratory blood supply is the sixth branchial artery and the venous outflow enters the heart separately from systemic and portal blood at the sinus venosus (Polypteriformes) or the atrium (lungfish), or is delivered to a separate left atrium (tetrapods). The swimbladder, on the other hand, is unpaired, and arises dorsally from the posterior pharynx. It is employed in breathing in Ginglymodi (gars), Halecomorphi (bowfin) and in basal teleosts. In most cases, its respiratory blood supply is homologous to that of the lung, but the vein drains to the cardinal veins. Separate intercardiac channels for oxygenated and deoxygenated blood are lacking. The question of the homology of lungs and swimbladders and of breathing mechanisms remains open. On the whole, air ventilatory mechanisms in the actinopterygian lineage are similar among different groups, including Polypteriformes, but are distinct from those of lungfish and tetrapods. However, there is extreme variation within this apparent dichotomy. Furthermore, the possible separate origin of air breathing in actinopterygian and 'sarcopterygian' lines is in conflict with the postulated much more ancient origin of vertebrate air-breathing organs. New studies on the isolated brainstem preparation of the gar (Lepisosteus osseus) show a pattern of efferent activity associated with a glottal opening that is remarkably similar to that seen in the in-vitro brainstem preparation of frogs and tadpoles. Given the complete lack of evidence for AO in chondrichthyans, and the isolated position of placoderms for which buoyancy organs of uncertain homology have been demonstrated, it is likely that homologous pharyngeal AO arose in the ancestors of early bony fish, and was pre-dated by behavioral mechanisms for surface (water) breathing. The primitive AO may have been the posterior gill pouches or even the modified gills themselves, served by the sixth branchial artery. Further development of the dorsal part may have led to the respiratory swimbladder, whereas the paired ventral parts evolved into lungs.     

Activity and inhibition of carbonic anhydrase in Amia calva, a bimodal-breathing holostean fish

Carbonic anhydrase (CA) activity was measured in the respiratory swimbladder, gill filaments and red blood cells of the primitive air-breathing holostean fish, the bowfin, Amia calva . The activity of swimbladder CA, relative to gill and red cell CA activities, was within the range reported previously for unimodally-breathing fishes and was comparable to the CA activities reported for the air-breathing organs of teleosts. It is unlikely that carbon dioxide excretion across the bowfin swimbladder is limited to the uncatalyzed rate of HCO₃-dehydration. Bowfin blood plasma lacked any endogenous inhibitor(s) of CA, in contrast with information on teleostean fishes. This absence may have interesting phylogenetic implications and may offer some potential for investigating the nature and physiological role of plasma CA inhibitors.     

Cardiac natriuretic peptides: a physiological lineage of cardioprotective hormones?

Vertebrate hearts from fish to mammals secrete peptide hormones with profound natriuretic, diuretic, and vasodilatory activity; however, the specific role of these cardiac natriuretic peptides (NPs) in homeostasis is unclear. NPs have been suggested to be involved in salt excretion in saltwater teleosts, whereas they are proposed to be more important in volume regulation in mammals. In this review, we consider an alternative (or perhaps complementary) function of NPs to protect the heart. This hypothesis is based on a number of observations. First, evidence for NPs, or NP-like activity has been found in all vertebrate hearts thus far examined, from osmoconforming saltwater hagfish to euryhaline freshwater and saltwater teleosts to terrestrial mammals. Thus the presence of cardiac NPs appears to be independent of environmental conditions that may variously affect salt and water balance. Second, cardiac stretch is a universal, and one of the most powerful, NP secretagogues. Furthermore, stretch-induced NP release in euryhaline teleosts appears relatively independent of ambient salinity. Third, excessive cardiac stretch that increases end-diastolic volume (EDV) can compromise the mechanical ability of the heart by decreasing actin-myosin interaction (length-tension) or through Laplace effects whereby as EDV increases, the wall tension necessary to maintain a constant pressure must also increase. Excessive cardiac stretch can be produced by factors that decrease cardiac emptying (i.e., increased arterial pressure), or by factors that increase cardiac filling (i.e., increased blood volume, increased venous tone, or decreased venous compliance). Fourth, the major physiological actions of cardiac NPs enhance cardiac emptying and decrease cardiac filling. In fish, NPs promote cardiac emptying by decreasing gill vascular resistance, thereby lowering ventral aortic pressure. In mammals a similar effect is achieved through pulmonary vasodilation. NPs also decrease cardiac filling by decreasing blood volume and increasing venous compliance, the latter producing a rapid fall in central venous pressure. Fifth, the presence of NP clearance receptors in the gill and lung (between the heart and systemic circulation) suggest that these tissues may be exposed to considerably higher NP titers than are systemic tissues. Thus, a decrease in outflow resistance immediately downstream from the heart may be the first response to increased cardiac distension. Because the physiology of cardiac NPs is basically the same in fish and mammals, we propose that the cardioprotective effects of NPs have been well preserved throughout the course of vertebrate evolution. It is also likely that the cardioprotective role of NPs was one of the most primordial homeostatic activities of these peptides in the earliest vertebrates.     

Effect of Urea and Thiourea on Migration Activity of Climbing Perch Anabas testudineus

Journal of Ichthyology - The influence of urea and thiourea on the dynamics of the migration activity of climbing perch Anabas testudineus was studied. The exposure of climbing perch to 0.05%...