Control of cardiorespiratory function in response to hypoxia in an air-breathing fish, the African sharptooth catfish, Clarias gariepinus

Arapaima gigas maintains gas exchange separation in severe aquatic hypoxia but does not suffer branchial oxygen loss

One of the most air-reliant obligate air-breathing fish is the South American Arapaima gigas, with substantially reduced gills impeding gas diffusion, thought to be a result of recurring aquatic hypoxia in its habitat. In normoxic water, A. gigas is reported to satisfy 70-80% of its O2 requirement from the air while excreting 60-90% of its CO2 to the water. If this pattern of gas exchange were to continue in severely hypoxic water, O2 loss at the gills would be expected. We hypothesized therefore that partitioning of CO2 would shift to the air phase in severe aquatic hypoxia eliminating the risk of branchial O2 loss. By adapting a respirometer designed to measure aquatic MO2/MCO2 we were able to run intermittent closed respirometry on both water and air phase for both of these gasses as well as sample water for N-waste measurements (ammonia-N, urea-N) so as to calculate metabolic fuel utilization. In contrast to our prediction, we found that partitioning of CO2 excretion changed little between normoxia and severe hypoxia (83% vs 77% aquatic excretion respectively) and at the same time there was no evidence of branchial O2 loss in hypoxia. This indicates that A. gigas can utilize distinct transfer pathways for O2 and CO2. Routine and standard MO2, N-waste excretion, and metabolic fuel utilization did not change with water oxygenation. Metabolism was fueled mostly by protein oxidation (53%) while carbohydrates and lipids accounted for 27% and 20% respectively.

A Decerebrate Preparation of the Rattlesnake, Crotalus durissus, Provides an Experimental Model for Study of Autonomic Modulation of the Cardiovascular System in Reptiles

AbstractThe South American rattlesnake, Crotalus durissus, has been successfully used as an experimental model to study control of the cardiovascular system in squamate reptiles. Recent technical advances, including equipment miniaturization, have lessened the impact of instrumentation on in vivo recordings, and an increased range of anesthetic drugs has improved recording conditions for in situ preparations. Nevertheless, any animal-based experimental approach has to manage limitations regarding the avoidance of pain and stress the stability of the preparation and duration of experiments and the potentially overriding effects of anesthesia. To address such aspects, we tested a new experimental preparation, the decerebrate rattlesnake, in a study of the autonomic control of cardiovascular responses following the removal of general anesthesia. The preparation exhibited complex cardiovascular adjustments to deal with acute increases in venous return (caused by tail lifting), to compensate for blood flow reduction in the cephalic region (caused by head lifting), for body temperature control (triggered by an external heating source), and in response to stimulation of chemoreceptors (triggered by intravenous injection of NaCN). The decerebrate preparation retained extensive functional integrity of autonomic centers, and it was suitable for monitoring diverse cardiac and vascular variables. Furthermore, reanesthetizing the preparation markedly blunted cardiovascular performance. Isoflurane limited the maintenance of recovered cardiovascular variables in the prepared animal and reduced or abolished the observed cardiovascular reflexes. This preparation enables the recording of multiple concomitant cardiovascular variables for the study of mechanistic questions regarding the central integration of autonomic reflex responses in the absence of anesthesia.

Histological Study of Suprabranchial Chamber Membranes in Anabantoidei and Clariidae Fishes

Simple Summary

Air-breathing fish constitute a broad evolutionary group of fish, which are generally characterized by distinctive phenotypical plasticity. These fishes usually inhabit waters where oxygen deficiency occurs periodically, which is why they have developed a variety of accessory respiratory organs (AROs) that may be used in an obligatory or a facultative manner. Knowledge of the structure of these organs is important for both the breeding and the conservation of these fish species. The aim of this study was to conduct a comparative histological analysis of two types of AROs found in the Anabantoidei suborder and the Clariidae family, both of which are freshwater fish taxa of high ecological and commercial importance.

Abstract

Accessory respiratory organs (AROs) are a group of anatomical structures found in fish, which support the gills and skin in the process of oxygen uptake. AROs are found in many fish taxa and differ significantly, but in the suborder Anabantoidei, which has a labyrinth organ (LO), and the family Clariidae, which has a dendritic organ (DO), these structures are found in the suprabranchial cavity (SBC). In this study, the SBC walls, AROs, and gills were studied in anabantoid (Betta splendens, Ctenopoma acutirostre, Helostoma temminckii) and clariid (Clarias angolensis, Clarias batrachus) fishes. The histological structure of the investigated organs was partially similar, especially in relation to their connective tissue core; however, there were noticeable differences in the epithelial layer. There were no significant species-specific differences in the structure of the AROs within the two taxa, but the SBC walls had diversified structures, depending on the observed location. The observed differences between species suggest that the remarkable physiological and morphological plasticity of the five investigated species can be associated with structural variety within their AROs. Furthermore, based on the observed histology of the SBC walls, it is reasonable to conclude that this structure participates in the process of gas exchange, not only in clariid fish but also in anabantoids.

Striped catfish (Pangasianodon hypophthalmus) use air-breathing and aquatic surface respiration when exposed to severe aquatic hypercarbia

We investigated the extent to which the facultative air-breathing fish, the striped catfish (Pangasianodon hypophthalmus), uses air-breathing to cope with aquatic hypercarbia, and how air-breathing is influenced by the experimental exposure protocol and level of hypercarbia. We exposed individuals to severe aquatic hypercarbia (up to P_w CO₂  = 81 mmHg) using step-wise and progressive exposure protocols while measuring gill ventilation rate, heart rate, mean arterial blood pressure, and air-breathing frequency, as well as arterial blood pH and PCO₂ . We confirm that P. hypophthalmus is tolerant of hypercarbia. Under both protocols gill ventilation rate, heart rate, and mean arterial blood pressure were maintained near control levels even at very high CO₂ levels. We observed a marked amount of individual variation in the PwCO₂ at which air-breathing was elicited, with some individuals not responding at all. The experimental protocol also influenced the onset of air-breathing. Air-breathing began at lower P_w CO₂ in the step-wise protocol (23 ± 4.1 mmHg) compared with the progressive protocol (46 ± 7.8 mmHg). Air-breathing was often followed by aquatic surface respiration, at higher PCO₂ (71 ± 5.2 mmHg) levels. On average, the blood PCO₂ was approximately 43% lower (46 ± 2.5 mmHg) than water P_w CO₂ (~81 mmHg) at our highest tested CO₂ level. While this suggests that aerial CO₂ elimination is an effective, and perhaps critical, respiratory strategy used by P. hypophthalmus to cope with severe hypercarbia, this observation may also be explained by a long lag time required for equilibration.

The baroreflex in aquatic and amphibious teleosts: Does terrestriality represent a significant driving force for the evolution of a more effective baroreflex in vertebrates?

All vertebrates have baroreflexes that provide fast regulation of arterial blood pressure (P_A) to maintain adequate tissue perfusion and avoid vascular lesions from excessive pressures. The baroreflex is a negative feedback loop, where altered P_A results in reciprocal changes in heart rate (f_H) and systemic vascular conductance to restore pressure. In terrestrial environments, gravity usually leads to blood pooling in the lower body reducing venous return, cardiac filling, cardiac output and P_A. Conversely, in aquatic environments, the hydrostatic pressure of surrounding water mitigates blood pooling and prevents vascular distensions. In this context, we aimed to test the hypothesis that vertebrate species that were exposed to gravity-induced hemodynamic disturbances throughout their evolutionary histories have a more effective barostatic reflex than those that were not. We examined the cardiac baroreflex of fish that perform (Clarias gariepinus and Hoplerythrinus unitaeniatus) and do not perform (Hoplias malabaricus and Oreochromis niloticus) voluntary terrestrial sojourns, using pharmacological manipulations of P_A to characterize reflex changes in f_H using a four-variable sigmoidal logistic function (i.e. the "Oxford technique"). Our results revealed that amphibious fish exhibit higher baroreflex gain and responsiveness to hypotension than strictly aquatic fish, suggesting that terrestriality and the gravitational circulatory stresses constitute a relevant driving force for the evolution of a more effective baroreflex in vertebrates. We also demonstrate that strictly aquatic teleosts have considerable baroreflex gain, supporting the view that the baroreflex is an ancient cardiovascular trait that appeared before vertebrates colonized the gravity-dominated realm of land.

Learning to Air-Breathe: The First Steps

Air-breathing in vertebrates has evolved many times among the bony fish while in water. Its appearance has had a fundamental impact on the regulation of ventilation and acid-base status. We review the physico-chemical constraints imposed by water and air, place the extant air-breathing fish into this framework, and show how that the advantages of combining control of ventilation and acid-base status are only available to the most obligate of air-breathing fish, thus highlighting promising avenues for research.

Control of air-breathing in fishes: Central and peripheral receptors

This review considers the environmental and systemic factors that can stimulate air-breathing responses in fishes with bimodal respiration, and how these may be controlled by peripheral and central chemoreceptors. The systemic factors that stimulate air-breathing in fishes are usually related to conditions that increase the O₂ demand of these animals (e.g. physical exercise, digestion and increased temperature), while the environmental factors are usually related to conditions that impair their capacity to meet this demand (e.g. aquatic/aerial hypoxia, aquatic/aerial hypercarbia, reduced aquatic hidrogenionic potential and environmental pollution). It is now well-established that peripheral chemoreceptors, innervated by cranial nerves, drive increased air-breathing in response to environmental hypoxia and/or hypercarbia. These receptors are, in general, sensitive to O₂ and/or CO₂/H⁺ levels in the blood and/or the environment. Increased air-breathing in response to elevated O₂ demand may also be driven by the peripheral chemoreceptors that monitor O₂ levels in the blood. Very little is known about central chemoreception in air-breathing fishes, the data suggest that central chemosensitivity to CO₂/H⁺ is more prominent in sarcopterygians than in actinopterygians. A great deal remains to be understood about control of air-breathing in fishes, in particular to what extent control systems may show commonalities (or not) among species or groups that have evolved air-breathing independently, and how information from the multiple peripheral (and possibly central) chemoreceptors is integrated to control the balance of aerial and aquatic respiration in these animals.

Lactate provides a strong pH-independent ventilatory signal in the facultative air-breathing teleost Pangasianodon hypophthalmus

Fish regulate ventilation primarily by sensing O₂-levels in the water and arterial blood. It is well established that this sensory process involves several steps, but the underlying mechanisms remain frustratingly elusive. Here we examine the effect of increasing lactate ions at constant pH on ventilation in a teleost; specifically the facultative air-breathing catfish Pangasianodon hypophthalmus. At lactate levels within the physiological range obtained by Na-Lactate injections (3.5 ± 0.8 to 10.9 ± 0.7 mmol L⁻¹), gill ventilation increased in a dose-dependent manner to levels comparable to those elicited by NaCN injections (2.0 µmol kg⁻¹), which induces a hypoxic response and higher than those observed in any level of ambient hypoxia (lowest PO₂ = 20 mmHg). High lactate concentrations also stimulated air-breathing. Denervation of the first gill arch reduced the ventilatory response to lactate suggesting that part of the sensory mechanism for lactate is located at the first gill arch. However, since a residual response remained after this denervation, the other gill arches or extrabranchial locations must also be important for lactate sensing. We propose that lactate plays a role as a signalling molecule in the hypoxic ventilatory response in fish.

Characterization and functional analysis of hypoxia-inducible factor HIF1α and its inhibitor HIF1αn in tilapia

Hypoxia is a major cause of fish morbidity and mortality in the aquatic environment. Hypoxia-inducible factors are very important modulators in the transcriptional response to hypoxic stress. In this study, we characterized and conducted functional analysis of hypoxia-inducible factor HIF1α and its inhibitor HIF1αn in Nile tilapia (Oreochromis niloticus). By cloning and Sanger sequencing, we obtained the full length cDNA sequences for HIF1α (2686bp) and HIF1αn (1308bp), respectively. The CDS of HIF1α includes 15 exons encoding 768 amino acid residues and the CDS of HIF1αn contains 8 exons encoding 354 amino acid residues. The complete CDS sequences of HIF1α and HIF1αn cloned from tilapia shared very high homology with known genes from other fishes. HIF1α show differentiated expression in different tissues (brain, heart, gill, spleen, liver) and at different hypoxia exposure times (6h, 12h, 24h). HIF1αn expression level under hypoxia is generally increased (6h, 12h, 24h) and shows extremely highly upregulation in brain tissue under hypoxia. A functional determination site analysis in the protein sequences between fish and land animals identified 21 amino acid sites in HIF1α and 2 sites in HIF1αn as significantly associated sites (α = 0.05). Phylogenetic tree-based positive selection analysis suggested 22 sites in HIF1α as positively selected sites with a p-value of at least 95% for fish lineages compared to the land animals. Our study could be important for clarifying the mechanism of fish adaptation to aquatic hypoxia environment.

What is the most efficient respiratory organ for the loricariid air-breathing fish Pterygoplichthys anisitsi, gills or stomach? A quantitative morphological study

The purpose of the present study was to evaluate the morphometric respiratory potential of gills compared to the stomach in obtaining oxygen for aerobic metabolism in Pterygoplichthys anisitsi, a facultative air-breathing fish. The measurements were done using stereological methods. The gills showed greater total volume, volume-to-body mass ratio, potential surface area, and surface-to-volume ratio than the stomach. The water-blood diffusion barrier of the gills is thicker than the air-blood diffusion barrier of the stomach. Taken together, the surface area, the surface-to-volume ratio and the diffusion distance for O₂ transfer from the respiratory medium to blood yield a greater diffusing capacity for gills than for the stomach, suggesting greater importance of aquatic respiration in this species. On the other hand, water breathing is energetically more expensive than breathing air. Under severe hypoxic conditions, O₂ uptake by the stomach is more efficient than by the gills, although the stomach has a much lower diffusing capacity. Thus, P. anisitsi uses gills under normoxic conditions but the stomach may also support aerobic metabolism depending on environmental conditions.

Gill denervation eliminates the barostatic reflex in a neotropical teleost, the tambaqui (Colossoma macropomum)

The baroreflex is one of the most important regulators of cardiovascular homeostasis in vertebrates. It begins with the monitoring of arterial pressure by baroreceptors, which constantly provide the central nervous system with afferent information about the status of this variable. Any change in arterial pressure relative to its normal state triggers autonomic responses, which are characterized by an inversely proportional change in heart rate and systemic vascular resistance and which tend to restore pressure normality. Although the baroreceptors have been located in mammals and other terrestrial vertebrates, their location in fish is still not completely clear and remains quite controversial. Thus, the objective of this study was to locate the baroreceptors in a teleost, the Colossoma macropomum. To do so, the occurrence and efficiency of the baroreflex were both analyzed when this mechanism was induced by pressure imbalancements in intact fish (IN), first-gill-denervated fish (G1), and total-gill-denervated fish (G4). The pressure imbalances were initiated through the administration of the α1-adrenergic agonist phenylephrine (100 µg kg(-1)) and the α1-adrenergic antagonist prazosin (1 mg kg(-1)). The baroreflex responses were then analyzed using an electrocardiogram that allowed for the measurement of the heart rate, the relationship between pre- and post-pharmacological manipulation heart rates, the time required for maximum chronotropic baroreflex response, and total heart rate variability. The results revealed that the barostatic reflex was attenuated in the G1 group and nonexistent in G4 group, findings which indicate that baroreceptors are exclusively located in the gill arches of C. macropomum.

Developmental cardiorespiratory physiology of the air-breathing tropical gar, Atractosteus tropicus

The physiological transition to aerial breathing in larval air-breathing fishes is poorly understood. We investigated gill ventilation frequency (f_G), heart rate (f_H), and air breathing frequency (f_AB) as a function of development, activity, hypoxia, and temperature in embryos/larvae from day (D) 2.5 to D30 posthatch of the tropical gar, Atractosteus tropicus, an obligate air breather. Gill ventilation at 28°C began at approximately D2, peaking at ∼75 beats/min on D5, before declining to ∼55 beats/min at D30. Heart beat began ∼36-48 h postfertilization and ∼1 day before hatching. f_H peaked between D3 and D10 at ∼140 beats/min, remaining at this level through D30. Air breathing started very early at D2.5 to D3.5 at 1-2 breaths/h, increasing to ∼30 breaths/h at D15 and D30. Forced activity at all stages resulted in a rapid but brief increase in both f_G and f_H, (but not f_AB), indicating that even in these early larval stages, reflex control existed over both ventilation and circulation prior to its increasing importance in older fishes. Acute progressive hypoxia increased f_G in D2.5-D10 larvae, but decreased f_G in older larvae (≥D15), possibly to prevent branchial O₂ loss into surrounding water. Temperature sensitivity of f_G and f_H measured at 20°C, 25°C, 28°C and 38°C was largely independent of development, with a Q₁₀ between 20°C and 38°C of ∼2.4 and ∼1.5 for f_G and f_H, respectively. The rapid onset of air breathing, coupled with both respiratory and cardiovascular reflexes as early as D2.5, indicates that larval A. tropicus develops "in the fast lane."