Cardiac tissue function of the teleost fish Oreochromis niloticus under different thermal conditions

Effects of exposure to sediment-associated fipronil on cardiac function of Neotropical armored catfish Hypostomus regani

Fipronil is widely used as a broad-spectrum insecticide in agriculture, urban environments, and veterinary medicine. Fipronil can enter aquatic ecosystems and spread to sediment and organic matter, representing a risk to non-target species. This study aimed to evaluate the effects of short-term (96 h) exposure to a low and realistic concentration of sediment-associated fipronil (4.2 µg.kg^-1 of Regent® 800 WG) on myocardial contractility of armored catfish Hypostomus regain, a benthic fish species. Fipronil exposure induced increased inotropism and acceleration of contractile kinetics, although no alterations in the relative ventricular mass were observed. This better cardiac function was associated with an elevated expression and/or function of the Na⁺/Ca²⁺ exchanger and its marked contribution to contraction and relaxation, probably due to a stress-induced adrenergic stimulation. Ventricle strips of exposed fish also exhibited a faster relaxation and a higher cardiac pumping capacity, indicating that armored catfish were able to perform cardiac adjustments to face the exposure. However, a high energetic cost to maintain an increased cardiac performance can make fish more susceptible to other stressors, impairing developmental processes and/or survival. These findings highlight the need for regulations of emerging contaminants, such as fipronil, to ensure adequate protection of the aquatic system.

A multi-biomarker approach to assess the sublethal effects of settleable atmospheric particulate matter from an industrial area on Nile tilapia (Oreochromis niloticus)

Iron and steel industries discharge a large amount of atmospheric particulate matter (PM) containing metals and metallic nanoparticles (NPs) that contaminate not only the air, but also settle into the aquatic environments. However, the effects of settleable atmospheric particulate matter (SePM) on aquatic fauna are still poorly understood. This study aimed to evaluate the sublethal effects of a short-term exposure to a realistic concentration of SePM on Nile tilapia (Oreochromis niloticus) using a multi-biomarker approach: relative ventricular mass (RVM) and heart function, blood oxidative stress, stress indicators, hemoglobin concentration, metallic NPs internalization, and metal bioaccumulation. Exposed fish exhibited reduced hemoglobin content and elevated plasma cortisol and glucose levels, reflecting stressed states. Furthermore, SePM caused blood oxidative stress increasing lipid and protein oxidation, decreasing glutathione levels, and inhibiting superoxide and glutathione reductase activities. SePM exposure also increased RVM and improved cardiac performance, increasing myocardial contractile force and rates of contraction and relaxation. In the heart tissue there was a significant accumulation of Fe > Zn > > Cr > Cu > Rb > Ni > V > Mn > Se > Mo > As. On the other hand, in the erythrocytes there was significant accumulation of Sn > Zn > > Cr > Ti > Mn = Ni > Nb > As > Bi. The highest bioaccumulation factors were found for Cr, Zn and Ni in both tissues. NPs (Ti, Sn, Al, Fe, Cu, Si, Zn) were also detected in ventricular myocardium of fish exposed and nanocrystallographic analysis revealed a predominance of anatase phase of TiO₂-NP, which is regarded to be more cytotoxic. The association between blood oxidative stress and energy expenditure to sustain increased cardiac pumping capacity under stress condition suggests that SePM has negative impacts on fish physiological performance, threatening their survival, growth rate and/or population establishment.

Cardiac contractility of the African sharptooth catfish, Clarias gariepinus: role of extracellular Ca2+, sarcoplasmic reticulum, and β-adrenergic stimulation

This study investigated the dependence of contraction from extracellular Ca²⁺, the presence of a functional sarcoplasmic reticulum (SR), and the effects of β-adrenergic stimulation using isometric cardiac muscle preparations. Moreover, the expression of Ca²⁺-handling proteins such as SR-Ca²⁺-ATPase (SERCA), phospholamban (PLN), and Na⁺/Ca²⁺ exchanger (NCX) were also evaluated in the ventricular tissue of adult African sharptooth catfish, Clarias gariepinus, a facultative air-breathing fish. In summary, we observed that (1) contractility was strongly regulated by extracellular Ca²⁺; (2) inhibition of SR Ca²⁺-release by application of ryanodine reduced steady-state force production; (3) ventricular myocardium exhibited clear post-rest decay, even in the presence of ryanodine, indicating a decrease in SR Ca²⁺ content and NCX as the main pathway for Ca²⁺ extrusion; (4) a positive force-frequency relationship was observed above 60 bpm (1.0 Hz); (5) ventricular tissue was responsive to β-adrenergic stimulation, which caused significant increases in twitch force, kept a linear force-frequency relationship from 12 to 96 bpm (0.2 to Hz), and improved the cardiac pumping capacity (CPC); and (6) African catfish myocardium exhibited similar expression patterns of NCX, SERCA, and PLN, corroborating our findings that both mechanisms for Ca²⁺ transport across the SR and sarcolemma contribute to Ca²⁺ activator. In conclusion, this fish species displays great physiological plasticity of E-C coupling, able to improve the ability to maintain cardiac performance under physiological conditions to ecological and/or adverse environmental conditions, such as hypoxic air-breathing activity.

Effects of change in temperature on the cardiac contractility of broad-snouted caiman (Caiman latirostris) during digestion

In many reptiles, digestion has been associated with the selection of higher body temperatures, the so-called post-prandial thermophilic response. This study aimed to investigate the excitation-contraction (E-C) coupling in postprandial broad-snouted caimans (Caiman latirostris) in response to acute warming within a preferred body temperature range of crocodiles. Isometric preparations subjected to a temperature transition from 25°C to 30°C were used to investigate myocardial contractility of postprandial caimans, that is, 48 h after the animals ingested a rodent meal corresponding to 15% of body mass. The caiman heart exhibits a negative force-frequency relationship that is independent of the temperature. At 25°C, cardiac muscle was able to maintain a constant force up to 36 bpm, above which it decreased significantly, reaching minimum values at the highest frequency of 84 bpm. Moreover, E-C coupling is predominantly dependent on transsarcolemmal Ca²⁺ transport denoted by the lack of significant ryanodine effects on force generation. On the contrary, ventricular strips at 30°C were able to sustain the cardiac contractility at higher pacing frequencies (from 12 to 144 bpm) due to an important role of Na⁺ /Ca²⁺ exchanger in Ca²⁺ cycling, as indicated by the decay of the post-rest contraction, and a significant contribution of the sarcoplasmic reticulum above 72 bpm. Our results demonstrated that the myocardium of postprandial caimans exhibits a significant degree of thermal plasticity of E-C coupling during acute warming. Therefore, myocardial contractility can be maximized when postprandial broad-snouted caimans select higher body temperatures (preferred temperature zone) following feeding.

Postprandial cardiorespiratory responses and the regulation of digestion-associated tachycardia in Nile tilapia (Oreochromis niloticus)

Cardiorespiratory adjustments that occur after feeding are essential to supply the demands of digestion in vertebrates. The well-documented postprandial tachycardia is triggered by an increase in adrenergic activity and by non-adrenergic non-cholinergic (NANC) factors in mammals and crocodilians, while it is linked to a withdrawal of vagal drive and NANC factors in non-crocodilian ectotherms-except for fish, in which the sole investigation available indicated no participation of NANC factors. On the other hand, postprandial ventilatory adjustments vary widely among air-breathing vertebrates, with different species exhibiting hyperventilation, hypoventilation, or even no changes at all. Regarding fish, which live in an environment with low oxygen capacitance that requires great ventilatory effort for oxygen uptake, data on the ventilatory consequences of feeding are also scarce. Thus, the present study sought to investigate the postprandial cardiorespiratory adjustments and the mediation of digestion-associated tachycardia in the unimodal water-breathing teleost Oreochromis niloticus. Heart rate (f_H), cardiac autonomic tones, ventilation rate (f_V), ventilation amplitude, total ventilation and f_H/f_V variability were assessed both in fasting and digesting animals under untreated condition, as well as after muscarinic cholinergic blockade with atropine and double autonomic blockade with atropine and propranolol. The results revealed that digestion was associated with marked tachycardia in O. niloticus, determined by a reduction in cardiac parasympathetic activity and by circulating NANC factors-the first time such positive chronotropes were detected in digesting fish. Unexpectedly, postprandial ventilatory alterations were not observed, although digestion triggered mechanisms that were presumed to increase oxygen uptake, such as cardiorespiratory synchrony.

Hypercapnic acidosis reduces contractile function in the ventricle of the armored catfish, Pterygoplichthys pardalis

The armored catfish, Pterygoplichthys pardalis (formerly Liposarcus pardalis), is a freshwater, facultative air-breathing teleost that experiences seasonal hypercapnia in the water systems of South America. We studied the tolerance of the P. pardalis heart to hypercapnic acidosis using an isolated ventricular muscle strip preparation. Force generation and kinetic variables were examined across a range of contraction frequencies under normocapnic and hypercapnic conditions in the absence and presence of sarcoplasmic reticulum (SR) inhibitors. Pterygoplichthys pardalis ventricle exhibited robust contractile force, on par with athletic fish species such as trout and tuna and a relatively flat force-frequency relationship between 0.2 and 1.5 Hz under normocapnic conditions (1% CO2, pH 7.78 +/- 0.02). Hypercapnic acidosis (7.5% CO2, pH 7.78 +/- 0.03) did not alter the shape of the force-frequency response but reduced force by approximately 50% across all frequencies tested, with only partial recovery upon return to normocapnic conditions. A subsequent and more severe acidotic challenge (15% CO2, pH 6.77 +/- 0.05) caused an additional 20% decrease in force. Force recovered to the level at which it had stablized after the first hypercapnic insult. SR inhibition had no steady state effect on force production at 0.2 Hz but resulted in a negative force-frequency relationship, suggesting that SR Ca2+ is recruited to a greater extent at high contraction frequencies. Surprisingly, SR-inhibited muscle was more resistant to hypercapnic acidosis (force decreased by approximately 40% across all frequencies) and displayed improved recovery upon return to normocapnic conditions. The significance of this latter finding is not clear. In aggregate, our results demonstrate robust contractile force, which extends across a range of frequencies and appears to be supported by SR Ca2+ cycling. Hypercapnic acidosis reduced contractile force but may provide preconditioning-like protection from subsequent insults.

Expression of calsequestrin in atrial and ventricular muscle of thermally acclimated rainbow trout

Calsequestrin (CASQ) is the main Ca(2+) binding protein within the sarcoplasmic reticulum (SR) of the vertebrate heart. The contribution of SR Ca(2+) stores to contractile activation is larger in atrial than ventricular muscle, and in ectothermic fish hearts acclimation to low temperatures increases the use of SR Ca(2+) in excitation-contraction coupling. The hypotheses that chamber-specific and temperature-induced differences in SR function are due to the increased SR CASQ content were tested in rainbow trout (Oncorhynchus mykiss) acclimated at either 4 degrees C (cold acclimation, CA) or 18 degrees C (warm acclimation, WA). To this end, the trout cardiac CASQ (omCASQ2) was cloned and sequenced. The omCASQ2 consists of 1275 nucleotides encoding a predicted protein of 425 amino acids (54 kDa in molecular mass, MM) with a high (75-87%) sequence similarity to other vertebrate cardiac CASQs. The transcript levels of the omCASQ2 were 1.5-2 times higher in CA than WA fish and about 2.5 times higher in the atrium than ventricle (P<0.001). The omCASQ2 protein was measured from western blots using a polyclonal antibody against the amino acid sequence 174-315 of the omCASQ2. Unlike the omCASQ2 transcripts, no differences were found in the abundance of the omCASQ2 protein between CA and WA fish, nor between the atrium and ventricle (P>0.05). However, a prominent qualitative difference appeared between the acclimation groups: two CASQ isoforms with apparent MMs of 54 and 59 kDa, respectively, were present in atrial and ventricular muscle of the WA trout whereas only the 54 kDa protein was clearly expressed in the CA heart. The 59 kDA isoform was a minor CASQ component representing 22% and 13% of the total CASQ proteins in the atrium and ventricle of the WA fish, respectively. In CA hearts, the 59 kDa protein was present in trace amounts (1.5-2.4%). Collectively, these findings indicate that temperature-related and chamber-specific differences in trout cardiac SR function are not related to the abundance of luminal Ca(2+) buffering by cardiac CASQ.

Differences in Ca2+-management between the ventricle of two species of neotropical teleosts: the jeju, Hoplerythrinus unitaeniatus (Spix & Agassiz, 1829), and the acara, Geophagus brasiliensis (Quoy & Gaimard, 1824)

This study analyzed the physiological role of the cardiac sarcoplasmic reticulum (SR) of two neotropical teleosts, the jeju, Hoplerythrinus unitaeniatus (Erythrinidae), and the acara, Geophagus brasiliensis (Cichlidae). While the in vivo heart frequency (fH - bpm) of acara (79.6 ± 6.6) was higher than that of the jeju (50.3 ± 2.7), the opposite was observed for the ventricular inotropism (Fc - mN/mm²) at 12 bpm (acara = 28.66 ± 1.86 vs. jeju = 36.09 ± 1.67). A 5 min diastolic pause resulted in a strong potentiation of Fc (≅ 90%) of strips from jeju, which was completely abolished by ryanodine. Ryanodine also resulted in a ≅ 20% decrease in the Fc developed by strips from jeju at both subphysiological (12 bpm) and physiological (in vivo) frequencies. However, this effect of ryanodine reducing the Fc from jeju was completely compensated by adrenaline increments (10-9 and 10-6 M). In contrast, strips from acara were irresponsive to ryanodine, irrespective of the stimulation frequency, and increases in adrenaline concentration (to 10-9 and 10-6 M) further increased Fc. These results reinforce the hypothesis of the functionality of the SR as a common trait in neotropical ostariophysian (as jeju), while in acanthopterygians (as acara) it seems to be functional mainly in 'athletic' species.

Comparison of sarcoplasmic reticulum calcium content in atrial and ventricular myocytes of three fish species

Ryanodine (Ry) sensitivity of cardiac contraction differs between teleost species, between atrium and ventricle, and according to the thermal history of the fish. The hypothesis that variability in Ry sensitivity of contraction is due to species-specific, chamber-specific, and temperature-related differences in the sarcoplasmic reticulum (SR) Ca(2+) content, was tested by comparing steady-state (SS) and maximal (Max) Ca(2+) loads of the SR in three teleost fish, rainbow trout (Oncorhynchus mykiss), burbot (Lota lota), and crucian carp (Carassius carassius), which differ in the extent of SR contribution to excitation-contraction coupling. Fish were acclimated at 4 degrees C (cold-acclimation, CA) or 18 degrees C (warm-acclimation, WA), and SR Ca(2+) content was released by a rapid application of 10 mM caffeine to single cardiac myocytes; its amount was determined from the Na(+)-Ca(2+) exchange current at 18 degrees C. SS Ca(2+) load was larger in atrial (304-915 micromol/l) than ventricular (224-540 micromol/l) myocytes in all fish species (P < 0.05), and the same was true for Max SR Ca(2+) content: 550-1,522 micromol/l and 438-840 micromol/l for atrial and ventricular myocytes, respectively (P < 0.05). Consistent with the hypothesis, acclimation to cold increased Ca(2+) load of the cardiac SR in the burbot heart, but contrary to the hypothesis, temperature acclimation did not affect SR Ca(2+) content in rainbow trout and crucian carp hearts. Furthermore, there was an inverse relation between SR Ca(2+) content and Ry sensitivity of contraction force: the species with the smallest SR Ca(2+) content (burbot) is most sensitive to Ry. Collectively, these findings show that SR Ca(2+) content of fish cardiac myocytes is several times larger than that in mammalian cardiac SR.

Cardio-respiratory function and oxidative stress biomarkers in Nile tilapia exposed to the organophosphate insecticide trichlorfon (NEGUVON)

The cardio-respiratory function, oxidative stress and fish antioxidants were analyzed in juvenile Nile tilapia exposed for 96 h to a sublethal trichlorfon (TRC-Neguvon, Bayer) concentration of 0.5 mg L(-1). The exposure to TRC induced oxidative stress in the heart, as manifested by the glutathione S-transferase depletion and hydroperoxide elevation, and was the most sensitive organ when compared to the liver and gills, in which the antioxidant mechanisms against TRC exposure were sufficient to remove reactive oxygen species (ROS), preventing the increase of lipid peroxidation. TRC exposure also reduced O(2) uptake (V O(2)) and increased the critical oxygen tension (PcO(2)), reducing the species capacity to survive prolonged hypoxic conditions. The heart rate and force contraction were significantly impaired, making the heart the most sensitive organ when exposed to the TRC.

Importance of the sarcoplasmic reticulum and adrenergic stimulation on the cardiac contractility of the neotropical teleost Synbranchus marmoratus under different thermal conditions

Experiments were carried out to investigate the heart rate of Synbranchus marmoratus after changing the temperature of the water contained in the experimental chamber of the acclimated fish (from 25 to 35 degrees C and from 25 to 15 degrees C). Then, an isometric cardiac muscle preparation was used to test the relative importance of Ca(2+) released from the sarcoplasmic reticulum and Ca(2+) influx across the sarcolemma for the cardiac performance under different thermal conditions: 25 degrees C (acclimation temperature), 15 and 35 degrees C. Adrenaline and ryanodine were used to modulate the Ca(2+) flux through the sarcolemma and the sarcoplasmic reticulum, respectively. Ryanodine reduced the peak tension by approximately 47% at 25 degrees C, and by 53% at 35 degrees C; however, it had no effect at 15 degrees C. A high adrenaline concentration was able to ameliorate the negative effects of ryanodine. Despite increasing the peak tension, adrenaline increased the times necessary for contraction and relaxation. We conclude that the sarcoplasmic reticulum is active in contributing Ca(2+) to the development of tension at physiological contraction frequencies. The adrenaline-stimulated Ca(2+) influx is able to increase the peak tension, even after addition of ryanodine, at physiologically relevant temperatures and pacing frequencies.

Effects of temperature and calcium availability on cardiac contractility in Synbranchus marmoratus, a neotropical teleost

An isometric muscle preparation was used to investigate the importance of the ventricular sarcoplasmic reticulum (SR) and extracellular Ca2+ (1.25 up to 11.25 mM) to force generation at 25 degrees C (acclimation temperature), 15 and 35 degrees C. The post-rest tension and force-frequency relationship were conducted with and without 10 microM ryanodine in the bathing medium. Increments in extracellular Ca2+ resulted in increases in twitch force development only at 35 degrees C. A significant post-rest potentiation was recorded for the control preparations at 25 degrees C (100% to 119.8+/-4.1%). However, this post-rest potentiation was inhibited by ryanodine only at 25 degrees C (100% to 97.6+/-1.5%). At 35 degrees C, force remained unchanged in the control preparations, but a significant post-rest decay was recorded in the presence of ryanodine (100% to 76.6+/-4.6%) while at 15 degrees C, ryanodine was not able to preventing the post-rest potentiation observed in the control preparations. The increases in the imposed contraction frequency caused a decline of the force at 25 and 35 degrees C and ryanodine decreased significantly peak tension at both temperatures. The findings suggest a high or medium calcium turnover, possibly related to the presence of a functional SR, whose functionality is diminished when temperature is decreased.

Cardiac function of two ecologically distinct Neotropical freshwater fish: Curimbata, Prochilodus lineatus (Teleostei, Prochilodontidae), and trahira, Hoplias malabaricus (Teleostei, Erythrinidae)

An isometric muscle preparation was used to investigate the importance of the ventricular sarcoplasmic reticulum (SR) and extracellular Ca(2+) (2.5 up to 10.5 mM) to force generation at 25 degrees C (acclimation temperature) in two ecologically distinct Neotropical teleost fish: Curimbata (active species), and trahira (sedentary species). The post-rest force was studied with and without 10 muM ryanodine in the medium. The positive inotropism observed for both species in response to increases on extracellular Ca(2+) reflected a greater Ca(2+) influx through sarcolemma, as well as an increase in Ca(2+) liberation from the SR by the Ca(2+)-induced Ca(2+) release mechanism. The significant post-rest potentiation recorded for the curimbata and trahira control preparations (3.22+/-0.24 to 6.55+/-0.77 mN mm(-2) and 0.74+/-0.07 to 2.26+/-0.26 mN mm(-2), respectively), was completely inhibited by the addition of ryanodine to the bathing medium, suggesting a potential functionality of SR for both species. Considering the differences in these species habitats, modes of life and levels of activity and the fact of a probable SR Ca(2+) cycling in a physiological temperature, we suggest that the functionality of the SR in these species is probably related to their phylogeny.

Stressful and behavioral conditions that affect reversible cardiac arrest in the Nile tilapia, Oreochromis niloticus (Teleostei)

The objective of the present investigation was to study the reversible cardiac arrest (RCA) to visual stimuli in the unrestrained Nile tilapia (Oreochromis niloticus) as well as the modulation of this response and its behavioral component (arousal/orientation or startle response) by external and internal factors that interfere with alertness and emotionality. The study was preceded by the determination of the autonomic receptors that contribute to the establishment of the heart rate (HR) and the RCA. Systemic injection of atropine and propranolol showed that a double cardiac autonomic control is present in the tilapia. Basal HR was 79.8+/-1.8 beats min(-1) and HR assessed after double autonomic blockade was 74.1+/-3.3 beats min(-1). The mean interbeat interval was 0.79+/-0.40 s during baseline recording and the magnitude of RCA induced by a moving shadow (2.67+/-0.22 s) was higher than that induced by light (1.53+/-1.11 s). RCA is peripherally mediated by muscarinic receptors for it is abolished by atropine but not by propranolol. Stressful conditions like handling the animal outside the water or a nociceptive stimulus (subcutaneous 2% or 3% formalin injection) reduced the cardiac interbeat interval. A subanesthetic dose of barbiturate (5 mg kg(-1)) inhibited RCA induced by a moving shadow stimulus and the startle response, suggesting that an ideal degree of vigilance is necessary for its occurrence. Benzodiazepine injections (1.0 and 2.0 mg kg(-1)) abolished the reduction in magnitude of RCA induced by handling stress and facilitated the startle response, seen in the dry-cold season, in a dose-dependent manner. These data suggest that drugs that act on alertness and on emotionality modulate the magnitude of cardiac interbeat intervals and the corresponding behavioral response.