Recovery of trout myocardial function following anoxia: preconditioning in a non-mammalian model

Repeated hypoxic episodes allow hematological and physiological habituation in rainbow trout

Introduction: Under climate change, the increase in temperature in aquatic environments may induce oxygen depletion. In extreme cases, low oxygen may become a limiting factor for fish, thus generating stress. In addition, consecutive hypoxic episodes may complicate the recovery of individuals and hinder their ability to modulate physiological and biochemical responses to maintain homeostasis. Thus, the aim of this study was to determine the hematological and physiological responses of rainbow trout under a condition of repeated hypoxic and manipulation stresses at three different time points. Methods: Every hypoxic episode consisted of exposing the fish to low dissolved oxygen concentrations (2 mgO2/L for 1 h). Following the exposure, the fish were allowed to recover for 1 h, after which they were sampled to investigate hematological and physiological parameters. Results and discussion: The results showed a pattern of habituation reflected by values of hematocrit, hemoglobin, and mean corpuscular volume, indicating a certain ability of rainbow trout to resist this type of repeated hypoxic events, provided that the fish can have some recovery time between the exposures.

Shaping the cardiac response to hypoxia: NO and its partners in teleost fish

The reduced availability of dissolved oxygen is a common stressor in aquatic habitats that affects the ability of the heart to ensure tissue oxygen supply. Among key signalling molecules activated during cardiac hypoxic stress, nitric oxide (NO) has emerged as a central player involved in the related adaptive responses. Here, we outline the role of the nitrergic control in modulating tolerance and adaptation of teleost heart to hypoxia, as well as major molecular players that participate in the complex NO network. The purpose is to provide a framework in which to depict how the heart deals with limitations in oxygen supply. In this perspective, defining the relational interplay between the multiple (sets of) proteins that, due to the gene duplication events that occurred during the teleost fish evolutive radiation, do operate in parallel with similar functions in the (different) heart (districts) and other body districts under low levels of oxygen supply, represents a next goal of the comparative research in teleost fish cardiac physiology.

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The flexibility of the teleost heart to O₂ limitations is illustrated by using cyprinids as hypoxia tolerance models.

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Major molecular mediators of the teleost cardiac response are discussed with a focus on the nitrergic system.

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A comparative analysis of gene duplication highlights conserved targets which may orchestrate the cardiac response to hypoxia.

The evolutionary and physiological significance of the Hif pathway in teleost fishes

The hypoxia-inducible factor (HIF) pathway is a key regulator of cellular O2 homeostasis and an important orchestrator of the physiological responses to hypoxia (low O2) in vertebrates. Fish can be exposed to significant and frequent changes in environmental O2, and increases in Hif-α (the hypoxia-sensitive subunit of the transcription factor Hif) have been documented in a number of species as a result of a decrease in O2. Here, we discuss the impact of the Hif pathway on the hypoxic response and the contribution to hypoxia tolerance, particularly in fishes of the cyprinid lineage, which includes the zebrafish (Danio rerio). The cyprinids are of specific interest because, unlike in most other fishes, duplicated paralogs of the Hif-α isoforms arising from a teleost-specific genome duplication event have been retained. Positive selection has acted on the duplicated paralogs of the Hif-α isoforms in some cyprinid sub-families, pointing to adaptive evolutionary change in the paralogs. Thus, cyprinids are valuable models for exploring the evolutionary significance and physiological impact of the Hif pathway on the hypoxic response. Knockout in zebrafish of either paralog of Hif-1α greatly reduces hypoxia tolerance, indicating the importance of both paralogs to the hypoxic response. Here, with an emphasis on the cardiorespiratory system, we focus on the role of Hif-1α in the hypoxic ventilatory response and the regulation of cardiac function. We explore the effects of the duration of the hypoxic exposure (acute, sustained or intermittent) on the impact of Hif-1α on cardiorespiratory function and compare relevant data with those from mammalian systems.

Hypoxia Performance Curve: Assess a Whole-Organism Metabolic Shift from a Maximum Aerobic Capacity towards a Glycolytic Capacity in Fish

The utility of measuring whole-animal performance to frame the metabolic response to environmental hypoxia is well established. Progressively reducing ambient oxygen (O₂) will initially limit maximum metabolic rate as a result of a hypoxemic state and ultimately lead to a time-limited, tolerance state supported by substrate-level phosphorylation when the O₂ supply can no longer meet basic needs (standard metabolic rate, SMR). The metabolic consequences of declining ambient O₂ were conceptually framed for fishes initially by Fry's hypoxic performance curve, which characterizes the hypoxemic state and its consequences to absolute aerobic scope (AAS), and Hochachka's concept of scope for hypoxic survival, which characterizes time-limited life when SMR cannot be supported by O₂ supply. Yet, despite these two conceptual frameworks, the toolbox to assess whole-animal metabolic performance remains rather limited. Here, we briefly review the ongoing debate concerning the need to standardize the most commonly used assessments of respiratory performance in hypoxic fishes, namely critical O₂ (the ambient O₂ level below which maintenance metabolism cannot be sustained) and the incipient lethal O₂ (the ambient O₂ level at which a fish loses the ability to maintain upright equilibrium), and then we advance the idea that the most useful addition to the toolbox will be the limiting-O₂ concentration (LOC) performance curve. Using Fry & Hart's (1948) hypoxia performance curve concept, an LOC curve was subsequently developed as an eco-physiological framework by Neil et al. and derived for a group of fish during a progressive hypoxia trial by Claireaux and Lagardère (1999). In the present review, we show how only minor modifications to available respirometry tools and techniques are needed to generate an LOC curve for individual fish. This individual approach to the LOC curve determination then increases its statistical robustness and importantly opens up the possibility of examining individual variability. Moreover, if peak aerobic performance at a given ambient O₂ level of each individual is expressed as a percentage of its AAS, the water dissolved O₂ that supports 50% of the individual's AAS (DO_AAS-50) can be interpolated much like the P₅₀ for an O₂ hemoglobin dissociation curve (when hemoglobin is 50% saturated with O₂). Thus, critical O₂, incipient lethal O₂, DO_AAS-50 and P₅₀ and can be directly compared within and across species. While an LOC curve for individual fish represents a start to an ongoing need to seamlessly integrate aerobic to anaerobic capacity assessments in a single, multiplexed respirometry trial, we close with a comparative exploration of some of the known whole-organism anaerobic and aerobic capacity traits to examine for correlations among them and guide the next steps.

Effects of hypoxic acclimation on contractile properties of the spongy and compact ventricular myocardium of steelhead trout (Oncorhynchus mykiss)

The trout ventricle has an outer compact layer supplied with well-oxygenated arterial blood from the coronary circulation, and an inner spongy myocardium supplied with oxygen poor venous blood. It was hypothesized that: (1) the spongy myocardium of steelhead trout (Oncorhynchus mykiss), given its routine exposure to low partial pressures of oxygen (PO₂), would be better able to maintain contractile performance (work) when exposed to acute hypoxia (100 to 10% air saturation) relative to the compact myocardium, and would show little benefit from hypoxic acclimation; and (2) the compact myocardium from hypoxia-acclimated (40% air saturation) fish would be better able to maintain work during acute exposure to hypoxia relative to normoxia-acclimated individuals. Consistent with our expectations, when PO₂ was acutely lowered, net work from the compact myocardium of normoxia-acclimated fish declined more (by ~ 73%) than the spongy myocardium (~ 50%), and more than the compact myocardium of hypoxia-acclimated fish (~ 55%), and hypoxic acclimation did not benefit the spongy myocardium in the face of reduced PO₂. Further, while hypoxic acclimation resulted in a 25% (but not significant) decrease in net work of the spongy myocardium, the performance of the compact myocardium almost doubled. This research suggests that, in contrast to the spongy myocardium, performance of the compact myocardium is improved by hypoxic acclimation; and supports previous research suggesting that the decreased contractile performance of the myocardium upon exposure to lowered PO₂ may be adaptive and mediated by mechanisms within the muscle itself.

Hypoxic acclimation negatively impacts the contractility of steelhead trout (Oncorhynchus mykiss) spongy myocardium

Cardiac stroke volume (SV) is compromised in Atlantic cod and rainbow trout following acclimation to hypoxia (i.e., 40% air saturation; ~8 kPa O2) at 10-12°C, and this is not due to changes in heart morphometrics or maximum achievable in vitro end-diastolic volume. To examine if this diminished SV may be related to compromised myocardial contractility, we used the work-loop method to measure work and power in spongy myocardial strips from normoxic- and hypoxic-acclimated steelhead trout when exposed to decreasing Po2 levels (21 to 1.5 kPa) at several frequencies (30-90 contractions/min) at 14°C (their acclimation temperature). Work required to lengthen the muscle, as during filling of the heart, was strongly frequency dependent (i.e., increased with contraction rate) but was not affected by hypoxic acclimation or test Po2. In contrast, although shortening work was less frequency dependent, this parameter and network (and power) 1) were consistently lower (by ~30-50 and ~15%, respectively) in strips from hypoxic-acclimated fish and 2) fell by ~40-50% in both groups from 20 to 1.5 kPa Po2, despite the already-reduced myocardial performance in the hypoxic-acclimated group. In addition, strips from hypoxic-acclimated trout showed a poorer recovery of net power (by ~15%) when returned to normoxia. These results strongly suggest that hypoxic acclimation reduces myocardial contractility, and in turn, may limit SV (possibly by increasing end-systolic volume), but that this diminished performance does not improve the capacity to maintain myocardial performance under oxygen limiting conditions.

Diel cycling hypoxia enhances hypoxia-tolerance in rainbow trout (Oncorhynchus mykiss): evidence of physiological and metabolic plasticity

Many fish naturally encounter a daily cycle of hypoxia but it is unclear whether this exposure hardens hypoxia-intolerant fish to future hypoxia or leads to accumulated stress and death. Rainbow trout (Oncorhynchus mykiss) is a putatively hypoxia-sensitive species found in rivers and estuaries that may routinely experience hypoxic events. Trout were exposed to 1 of 4 135h treatments in a swim-tunnel respirometer: 1) air-saturated control (20.7 kPa PO2); 2) diel cycling O2 (20.7-4.2 kPa over 24h); 3) acute hypoxia (130h at 20.7 kPa PO2 followed by 5h at 4.2 kPa PO2); 4) the mean oxygen tension (12.4 kPa PO2) experienced by the diel cycled fish. Some responses were similar in diel O2 cycled and mean PO2-treated fish but overall exposure to ecologically-representative diel hypoxia cycles improved hypoxia tolerance. Diel hypoxia-induced protective responses included increased inducible HSP70 concentration and mean corpuscular hemoglobin concentration, as well as reduced plasma cortisol. Acclimation to diel hypoxia allowed metabolic rates to decline during hypoxia, reduced oxygen debt following subsequent exposures, and allowed fish to return to an anabolic phenotype. The data demonstrate that acute diel cycling hypoxia improves hypoxia tolerance in previously intolerant fish through the activation of cellular protective mechanisms and a reduction in metabolic O2 requirements.

Physiological Responses to Swimming-Induced Exercise in the Adult Zebrafish Regenerating Heart

Exercise promotes a set of physiological responses known to provide long-term health benefits and it can play an important role in cardioprotection. In the present study, we examined cardiac responses to exercise training in the adult zebrafish and in the context of cardiac regeneration. We found that swimming-induced exercise increased cardiomyocyte proliferation and that this response was also found under regenerating conditions, when exercise was performed either prior to and after ventricular cryoinjury (CI). Exercise prior to CI resulted in a mild improvement in cardiac function and lesion recovery over the non-exercise condition. Transcriptomic profiling of regenerating ventricles in cryoinjured fish subjected to exercise identified genes possibly involved in the cardioprotective effects of exercise and that could represent potential targets for heart regeneration strategies. Taken together, our results suggest that exercise constitutes a physiological stimulus that may help promote cardiomyogenic mechanisms of the vertebrate heart through the induction of cardiomyocyte proliferation. The zebrafish exercise model may be useful for investigating the potential cardioprotective effects of exercise in teleost fish and to contribute to further identify and develop novel avenues in basic research to promote heart regeneration.

Dichloroacetate selectively improves cardiac function and metabolism in female and male rainbow trout

Cardiac tissue from female rainbow trout demonstrates a sex-specific preference for exogenous glucose and glycolysis, impaired Ca(2+) handling, and a greater tolerance for hypoxia and reoxygenation than cardiac tissue from male rainbow trout. We tested the hypothesis that dichloroacetate (DCA), an activator of pyruvate dehydrogenase, enhances cardiac energy metabolism and Ca(2+) handling in female preparations and provide cardioprotection for hypoxic male tissue. Ventricle strips from sexually immature fish with very low (male) and nondetectable (female) plasma sex steroids were electrically paced in oxygenated or hypoxic Ringer solution with or without 1 mM DCA. In the presence of 5 mM glucose, aerobic tissue from male trout could be paced at a higher frequency (1.79 vs. 1.36 Hz) with lower resting tension and less contractile dysfunction than female tissue. At 0.5 Hz, DCA selectively reduced resting tension below baseline values and lactate efflux by 75% in aerobic female ventricle strips. DCA improved the functional recovery of developed twitch force, reduced lactate efflux by 50%, and doubled citrate in male preparations after hypoxia-reoxygenation. Independent of female sex steroids, reduced myocardial pyruvate dehydrogenase activity and impaired carbohydrate oxidation might explain the higher lactate efflux, compromised function of the sarcoplasmic reticulum, and reduced mechanical performance of aerobic female tissue. Elevated oxidative metabolism and reduced glycolysis might also underlie the beneficial effects of DCA on the mechanical recovery of male cardiac tissue after hypoxia-reoxygenation. These results support the use of rainbow trout as an experimental model of sex differences of cardiovascular energetics and function, with the potential for modifying metabolic phenotypes and cardioprotection independent of sex steroids.

Sustained cardioprotection: exploring unconventional modalities

Since Murry et al. [Murry, C.E., Jennings, R.B., Reimer, K.A., 1986. Preconditioning with ischemia: a delay of lethal cell injury in ischemic myocardium. Circulation. 74, 1124-36.] initially reported on the powerful protective effects of ischemic preconditioning (PC), a plethora of experimental investigations have identified varied preconditioning protocols or mimetics to achieve cardioprotection. These stimuli predominantly act via archetypal mediators identified in associated signalling studies (including PI3-K, Akt, PKC, mitochondrial K(ATP) channels). Despite an intense research effort over the last 20 years, there remains a paucity of evidence that this protective paradigm is clinically exploitable. This may arise due to a number of drawbacks to conventional protection, including effects of age, disease, and interactions with other pharmacological agents. This encourages investigation of alternate strategies that trigger protection via unconventional signalling (distinct from conventional PC) and/or mediate sustained shifts in ischemic tolerance in hearts of varying age and disease status. This review considers briefly drawbacks to conventional PC, and focuses on alternate strategies for generating prolonged states of cardiac protection.

Reappraisal of H2S/sulfide concentration in vertebrate blood and its potential significance in ischemic preconditioning and vascular signaling

Hydrogen sulfide (H(2)S) is rapidly emerging as a biologically significant signaling molecule. Studies published before 2000 report low or undetectable H(2)S (usually as total sulfide) levels in blood or plasma, whereas recent work has reported sulfide concentrations between 10 and 300 microM, suggesting it acts as a circulating signal. In the first series of experiments, we used a recently developed polarographic sensor to measure the baseline level of endogenous H(2)S gas and turnover of exogenous H(2)S gas in real time in blood from numerous animals, including lamprey, trout, mouse, rat, pig, and cow. We found that, contrary to recent reports, H(2)S gas was essentially undetectable (<100 nM total sulfide) in all animals. Furthermore, exogenous sulfide was rapidly removed from blood, plasma, or 5% bovine serum albumin in vitro and from intact trout in vivo. To determine if blood H(2)S could transiently increase, we measured oxygen-dependent H(2)S production by trout hearts in vitro and in vivo. H(2)S has been shown to mediate ischemic preconditioning (IPC) in mammals. IPC is present in trout and, unlike mammals, the trout myocardium obtains its oxygen from relatively hypoxic systemic venous blood. In vitro, myocardial H(2)S production was inversely related to Po(2), whereas we failed to detect H(2)S in ventral aortic blood from either normoxic or hypoxic fish in vivo. These results provide an autocrine or paracrine mechanism for myocardial coupling of hypoxia to H(2)S in IPC, i.e., oxygen sensing, but they fail to provide any evidence that H(2)S signaling is mediated by the circulation.

The role of adrenergic stimulation in maintaining maximum cardiac performance in rainbow trout (Oncorhynchus mykiss) during hypoxia,hyperkalemia and acidosis at 10°C

As rainbow trout approach exhaustion during prolonged exercise, they maintain maximum cardiac output despite the fact their venous blood, which bathes the heart, becomes hypoxic, acidotic and hyperkalemic. Because these factors are individually recognized to have detrimental inotropic and chronotropic effects on cardiac performance, we hypothesized that adrenergic stimulation is critical in maintaining maximum cardiac performance under these collectively adverse conditions in vivo. To test this hypothesis,maximum cardiac performance in the presence and absence of maximal adrenergic stimulation was assessed with in situ rainbow trout hearts using relevant hyperkalemic (5.0 mmol l–1 K+), acidotic(pH 7.5) and hypoxic challenges. With tonic adrenergic stimulation (5.0 nmol l–1 adrenaline), hearts produced only 44.8±14.6% of their normal maximum cardiac output when exposed under normoxic conditions (20 kPa) to the hyperkalemic, acidotic perfusate, indicating that in vivothere was no refuge from cardiac impairment even if venous blood was fully oxygenated. By contrast, maximum adrenergic stimulation (500 nmol l–1 adrenaline), fully protected maximum cardiac performance under hyperkalemic and acidotic conditions over a wide range of oxygen availability, from normoxia to 2.0 kPa, a venous oxygen tension close to routine values in vivo. Extending the level of hypoxia to 1.3 kPa resulted in a 43.6±2.8% decrease in maximum cardiac output, with hearts failing when tested at 1.0 kPa. Our results suggest that adrenergic stimulation of the trout heart is critical in maintaining maximum performance during prolonged swimming tests, and probably during all forms of exhaustive activity and recovery, when venous blood is hyperkalemic, acidotic and hypoxic.

Cardiac plasticity in fishes: environmental influences and intraspecific differences

Fish cardiac physiology and anatomy show a multiplicity of intraspecific modifications when exposed to prolonged changes in environmentally relevant parameters such as temperature, hypoxia and food availability, and when meeting the increased demands associated with training/increased activity and sexual maturation. Further, there is evidence that rearing fish under intensive aquaculture conditions significantly alters some, but not all, aspects of cardiac anatomy and physiology. This review focuses on the responses of cardiac physiology and anatomy to these challenges, highlighting where applicable, the importance of hyperplastic (i.e. the production of new cells) vs hypertrophic (the enlargement of existing cells) growth to the adaptive response of the heart. In addition, we summarize recent studies that have explored the relationship between the myocardial protection afforded by preconditioning and myocardial hypoxia tolerance. This latter research clearly demonstrates the capacity of the fish heart to adjust to short-term perturbations, and shows that it can be difficult to predict how short-term and long-term alterations in cardiac physiology will interact.

Hypoxia tolerance and preconditioning are not additive in the trout(Oncorhynchus mykiss) heart

Research has shown that the trout heart is normally hypoxia-sensitive, and that it can be preconditioned. However, we have identified a group of rainbow trout Oncorhynchus mykiss that shows a surprising degree of myocardial hypoxia tolerance. In this study, we used in situ hearts from these fish as a comparative model to examine whether the cardioprotective effects afforded by hypoxic adaptation and preconditioning are additive. In situ trout hearts were exposed to severe hypoxia (perfusate PO2 5–10 mmHg) in the absence and presence of a transient hypoxic pre-exposure (preconditioning). The four groups studied were: (1) control (no hypoxia); (2) 5 min of severe hypoxia;(3) 30 min of severe hypoxia; and (4) 5 min of severe hypoxia (hypoxic preconditioning) followed 20 min later by 30 min of severe hypoxia. 30 min of severe hypoxia significantly decreased maximum cardiac output and stroke volume by 15–30%. However, hypoxic preconditioning failed to confer any protection against post-hypoxic myocardial dysfunction. This work shows that the protection afforded by inherent myocardial hypoxia tolerance and preconditioning are not additive in this population of trout, and strongly suggests that the relationship between hypoxic adaptation and preconditioning in fishes resembles that of the neonatal/immature, not adult, mammalian heart. Further, our results (1) indicate that stretch (volume loading) and chronic exposure to low levels of adrenaline (15 nmol l-1) do not confer any protection against hypoxia-related myocardial dysfunction in this population, and (2) validate the use of the in situ trout heart as a comparative model for studying aspects of myocardial hypoxia tolerance and preconditioning in vertebrates.

All rainbow trout (Oncorhynchus mykiss) are not created equal:intra-specific variation in cardiac hypoxia tolerance

All of our previous work, and that of other investigators, shows that the trout heart only partially recovers following brief exposure to severe hypoxia or anoxia (i.e. it is hypoxia-sensitive). However, in preliminary studies, we found evidence to suggest that rainbow trout reared at a farm in Oregon (USA)have a significant degree of inherent myocardial hypoxia tolerance. To evaluate whether hearts from these trout are indeed hypoxia-tolerant, and thus to determine whether intra-specific variation in rainbow trout myocardial hypoxia tolerance exists, we measured in situ cardiac function and monitored myoglobin and lactate dehydrogenase (LDH) release (both indices of myocardial damage) in hearts that were exposed to varying durations(10–30 min) of severe hypoxia (PO =5–10 mmHg). There was a strong positive relationship between the duration of severe hypoxia and the degree of post-hypoxic myocardial dysfunction. However, the resulting dysfunction was modest, with hearts exposed to 30 min of severe hypoxia recovering 77% of their initial maximum cardiac output. Furthermore,myoglobin was not detected in the perfusate, and ventricular LDH activity did not vary in response to the duration of severe hypoxia. These data (1)indicate that trout from this farm have extremely hypoxia-tolerant hearts; (2)suggest that considerable intra-specific variation exists in trout myocardial hypoxia tolerance; and (3) provide preliminary evidence that trout hearts are not irreversibly damaged, but are merely `stunned', following brief periods(10–30 min) of severe hypoxia.

Effects of temperature and anoxia upon the performance ofin situperfused trout hearts

Rainbow trout (Oncorhynchus mykiss) are likely to experience acute changes in both temperature and oxygen availability and, like many other organisms, exhibit behavioural selection of low temperatures during hypoxia that acts to reduce metabolism and alleviate the demands on the heart. To investigate whether low temperature protects cardiac performance during anoxia, we studied the effects of an acute temperature change, from 10°C to either 5°C, 15°C or 18°C, upon the performance of in situ perfused trout hearts before, during and after exposure to 20 min of anoxia. Routine cardiac workload mimicked in vivo conditions at the given temperatures, and the effects of anoxia were evaluated as maximal cardiac performance before and after 20 min of anoxic perfusion. Functional data were related to maximal activities of glycolytic enzymes and energetic status of the heart at the termination of the experiment.

At high oxygenation, maximum cardiac output and power output increased with temperature (Q10 values of 1.8 and 2.1, respectively) as a result of increased heart rate. Hypoxia tolerance was inversely related to temperature. At 5°C, the hearts maintained routine cardiac output throughout the 20 min period of anoxia, and maximal cardiac performance was fully restored following reoxygenation. By contrast, cardiac function failed sooner during anoxia as temperature was increased and maximal performance after reoxygenation was reduced by 25%, 35% and 55% at 10°C, 15°C and 18°C, respectively. Increased functional impairment following anoxic exposure at elevated temperature occurred even though both cardiac glycolytic enzyme activity and the rate of lactate production were increased proportionally with cardiac work. Nonetheless, there was no indication of myocardial necrosis, as biochemical and energetic parameters were generally unaffected by anoxia.

A role for nitric oxide in hypoxia-induced activation of cardiac KATP channels in goldfish (Carassius auratus)

Hypoxia-induced shortening of cardiac action potential duration (APD) has been attributed in mammalian hearts to the activation of ATP-sensitive potassium (KATP) channels. Since KATP channels are also present at high densities in the hearts of vertebrate ectotherms, speculation arises as to their function during periods of reduced environmental oxygen. The purpose of the present study was to determine whether nitric oxide (NO)plays a role in cardiac sarcolemmal KATP channel activation during hypoxia in a species with a high degree of tolerance to low oxygen environments: the goldfish (Carassius auratus). Conventional intracellular and patch-clamp recording techniques were used to record responses from excised ventricles or isolated ventricular myocytes and inside-out patches, respectively, from fish acclimated at 21°C. During moderate, substrate-free hypoxia (6.1±0.2 kPa), ventricular APD was significantly shortened at 50% and 90% of full repolarization, a response that was reversible upon reoxygenation and blocked by the KATP channel antagonist BDM. Under normoxic conditions, APD was also reduced in the presence of the NO-donor SNAP (100 μmol l-1). In cell-attached membrane patches, sarcolemmal KATP channel activity was enhanced after 10 min hypoxia, an effect that was reduced or eliminated by simultaneous exposure to BDM, to the guanylate cyclase inhibitor ODQ or to the NO synthase inhibitor l-NAME. In cell-free patches, KATP channel activity was abolished by 2 mmol l-1 ATP but increased by SNAP; the cGMP analog 8-Br-cGMP (200 μmol l-1) also enhanced activity, an effect that was eliminated by BDM. Our data indicate that NO synthesized in cardiac myocytes could enhance sarcolemmal KATP channel activation during moderate hypoxia in goldfish. This response may serve a cardioprotective role by helping to conserve ATP or by reducing intracellular Ca2+ accumulation.

Novel mechanism for high-altitude adaptation in hemoglobin of the Andean frog Telmatobius peruvianus

In contrast to birds and mammals, no information appears to be available on the molecular adaptations for O(2) transport in high-altitude ectothermic vertebrates. We investigated Hb of the aquatic Andean frog Telmatobius peruvianus from 3,800-m altitude as regards isoform differentiation, sensitivity to allosteric cofactors, and primary structures of the alpha- and beta-chains, and we carried out comparative O(2)-binding measurements on Hb of lowland Xenopus laevis. The three T. peruvianus isoHbs show similar functional properties. The high O(2) affinity of the major component results from an almost complete obliteration of chloride sensitivity, which correlates with two alpha-chain modifications: blockage of the NH(2)-terminal residues and replacement by nonpolar Ala of polar residues Ser and Thr found at position alpha131(H14) in human and X. leavis Hbs, respectively. The data indicate adaptive significance of alpha-chain chloride-binding sites in amphibians, in contrast to human Hb where chloride appears mainly to bind in the cavity between the beta-chains. The findings are discussed in relation to other strategies for high-altitude adaptations in amphibians.

Mitochondrial ATP-sensitive K+ channels influence force development and anoxic contractility in a flatfish, yellowtail flounderLimanda ferruginea, but not Atlantic codGadus morhuaheart

The influence of ATP-sensitive K+ channels (KATPchannels) on cardiac performance during anoxia and reoxygenation was investigated in two species of fish showing different cardiac responses to anoxia. Force production in isometrically contracting ventricular muscle preparations from yellowtail flounder is potentiated at the onset of anoxia,while force immediately declines in Atlantic cod preparations. Glibenclamide,a general KATP blocker, impaired oxygenated force development in yellowtail flounder heart but was without effect on cod preparations. The mitochondrial KATP (mKATP)-specific blocker 5-hydroxydecanoic acid (5HD) improved oxygenated force production in yellowtail flounder heart without influencing contractility during anoxia or reoxygenation. The specific mKATP agonist diazoxide preserved resting tension and eliminated anoxic force potentiation in yellowtail flounder heart preparations. Neither 5HD nor diazoxide affected contractility in cod ventricle preparations. Results indicate that KATP channels can modulate contractility in yellowtail flounder heart and are potentially important in cardiac hypoxia survival in this species.