Interdependence of Ca2+ and proton movements in trout hepatocytes

This study was undertaken to investigate possible interrelationships between Ca2+ homeostasis and pH regulation in trout hepatocytes. Exposure of cells to Ca2+ mobilizing agents ionomycin (0.5 μmol l–1) and thapsigargin (0.1 μmol l–1)induced an increase in intracellular pH (pHi) that was dependent on Ca2+ influx from the extracellular medium as well as Ca2+ release from intracellular pools. Surprisingly, this increase in pHi and intracellular Ca2+ concentration,[Ca2+]i, was not accompanied by any change in proton secretion. By contrast, removal of extracellular Ca2+(Ca2+e) using EGTA (0.5 mmol l–1)briefly increased proton secretion rate with no apparent effect on pHi, while chelation of Ca2+i using BAPTA-AM (25 μmol l–1) resulted in a drop in pHi and a sustained increase in proton secretion rate. [Ca2+]i therefore affected intracellular proton distribution and/or proton production and also affected the distribution of protons across the cell membrane. Accordingly, changes in pHi were not always compensated for by proton secretion across the cell membrane.

Alteration in pHe below and above normal values induced a slow,continuous increase in [Ca2+]i with a tendency to stabilize upon exposure to high pHe values. Rapid pHi increase induced by NH4Cl was accompanied by an elevation in[Ca2+]i from both extracellular and intracellular compartments. Ca2+e appeared to be involved in pHi regulation following NH4Cl-induced alkalinization whereas neither removal of Ca2+e nor chelation of Ca2+i affected pHi recovery following Na-propionate exposure. Similarly, [Ca2+]i increase induced by hypertonicity appeared to be a consequence of the changes in pHi as Na-free medium as well as cariporide diminished the hypertonicity-induced increase in[Ca2+]i. These results imply that a compensatory relationship between changes in pHi and proton secretion across cell plasma membrane is not always present. Consequently, calculating proton extrusion from buffering capacity and rate of pHi change cannot be taken as an absolute alternative for measuring proton secretion rate, at least in response to Ca2+ mobilizing agents.

How does blood cell concentration modulate cardiovascular parameters in developing zebrafish (Danio rerio)?

Blood flow and shear forces are considered to be important parameters possibly stimulating angiogenesis or cardiovascular remodeling. The main objective of this study was to test the hypothesis that a significant reduction in shear forces as a consequence of a significant isovolemic anemia induced by microsurgical techniques during early larval development of the zebrafish might induce a compensatory stimulation of erythropoiesis and/or induce a modification of cardiac activity or even the formation of the heart and may influence the shaping of the vascular bed. Blood from 2 day old zebrafish larvae was withdrawn and replaced by zebrafish Ringer's solution, so that the blood cell concentration was reduced by at least 75%. At 5 days post fertilization (dpf) a partial recovery in blood cell concentration was observed and reached a value of 814.55+/-85.42 cells/nL, while in control animals blood cell concentration amounted to 1856.00+/-131.59 cells/nL. At 7 dpf the value of blood cell concentration was 1023.89+/-95.75 cells/nL versus 1701.54+/-146.03 cells/nL in control animals. Compared to control animals, heart rate and cardiac output were significantly reduced in anemic animals and alterations in the formation of the vascular bed were also observed. A significant decrease in the end-diastolic volume suggested that ventricular volume was reduced. Thus, within a few days zebrafish larvae were nearly able to compensate for an isovolemic anemia by an enhanced erythropoiesis. However, several changes in cardiovascular system indicated that phenotypic plasticity is established even at an early developmental stage.

Signalling pathways involved in hypertonicity- and acidification-induced activation of Na+/H+ exchange in trout hepatocytes

In trout hepatocytes, hypertonicity and cytosolic acidification are known to stimulate Na+/H+ exchanger (NHE) activity, which contributes to recovery of cell volume and intracellular pH (pHi),respectively. The present study investigated the signalling mechanisms underlying NHE activation under these conditions. Exposing trout hepatocytes to cariporide, a specific inhibitor of NHE-1, decreased baseline pHi,completely blocked the hypertonicity-induced increase of pHi and reduced the hypertonicity-induced proton secretion by 80%. Changing extracellular pH (pHe)above and below normal values, and allowing cells to adjust pHi accordingly,significantly delayed alkalinization during hypertonic exposure, whereas following an acid load an enhanced pHi recovery with increasing pHe was seen. Chelating Ca2+, and thereby preventing the hypertonicity-induced increase in intracellular Ca2+ ([Ca2+]i), significantly diminished hypertonic elevation of pHi, indicating that Ca2+signalling might be involved in NHE activation. A reduction in alkalinization and proton secretion was also observed in the presence of the protein kinase A(PKA) inhibitor H-89 or the calmodulin (CaM) inhibitor calmidazolium. A complete inhibition of hypertonic- and acidification-induced changes of pHi concurrent with an increase in hypertonically induced proton efflux was seen with the protein kinase C (PKC) inhibitor chelerythrine. Recovery of pHi following sodium propionate addition was reduced by more than 60% in the presence of cariporide, was sensitive to PKA inhibition, and tended to be reduced by CaM inhibition. In conclusion, we showed that NHE-1 is the main acid secretion mechanism during hypertonicity and recovery following acid loading. In addition, Ca2+-, PKA- and CaM-dependent pathways are involved in NHE-1 activation for recovery of cell volume and pHi. On the other hand, PKC appeared to have an impact on NHE-independent pathways affecting intracellular acid-base homeostasis.

Extracellular signal regulated MAP-kinase signalling in osmotically stressed trout hepatocytes

Activation of the extracellular signal-regulated MAP-kinase (ERK) by anisoosmotic conditions, the underlying signalling pathways, and the role of protein kinases in cell volume regulation were investigated in trout hepatocytes. While hyperosmolarity left phosphorylated ERK (pERK) levels unaffected, hypoosmolarity caused a significant increase of pERK within 2 min which peaked at around 30 min. Chelating extracellular Ca2+ to prevent the influx of Ca2+ associated with swelling reduced iso- and abolished hypoosmotic ERK activation. Similarly, inhibiting the ERK activator MEK, tyrosine kinases, or PKC inhibited the increase of pERK. In contrast, exposing cells to chelerytrine or staurosporine, PKC inhibitors of little specificity, increased pERK independently from osmotic conditions. Blocking PI3 kinase, application of 8-Br-cAMP, exposure to a P-receptor antagonist, and inhibition of p38 MAP-kinase had no effect on ERK activity. A significant reduction of regulatory volume decrease (RVD) after hypoosmotic swelling caused by MEK-inhibition and an even more pronounced reduction due to p38 inhibition indicates a role for MAP-kinases in volume regulation, but a lack of correlation between the impact of protein kinase inhibitors on pERK levels and on RVD suggests that ERK may merely modulate volume recovery. Immunocytochemical detection of pERK indicated cytoplasmic activation, but no nuclear accumulation within 30 min, supporting the notion that ERK exerts non-genomic effects. Overall, our data underscore the complexity of hypoosmotic ERK signalling and suggest a role of ERK and p38 in acute cell volume regulation.

Development of the sympatho-vagal balance in the cardiovascular system in zebrafish (Danio rerio) characterized by power spectrum and classical signal analysis

The development of sympatho-vagal control of cardiac activity was analyzed in zebrafish (Danio rerio) larvae from 2 to 15 days post fertilization (d.p.f.) by pharmacological studies as well as by assessing short term heart rate variability. Changes in heart rate in response to cholinergic and adrenergic receptor stimulation or inhibition were investigated using in situ preparations and digital video-microscopic techniques. The data revealed that the heart responded to adrenergic stimulation starting at 4 d.p.f. and to cholinergic stimulation starting at 5 d.p.f. Atropine application resulted in an increase in heart rate beyond 12 d.p.f., while the inhibitory effect of cholinergic stimulation ceased at this time of development. Adrenergic inhibition (propranolol) reduced heart rate for the first time at 5 d.p.f., but the reduction was only very small (3.8%). Between 5 and 12 d.p.f. propranolol application always resulted in a minor reduction in heart rate, but because the effect was so small it was not always significant. Because the presence of an adrenergic or cholinergic tone may influence the stability of heart rate, we analyzed short-term heart rate variability (HRV). The frequency band width of heart rate variability revealed that HRV increased between 4 d.p.f. and 15 d.p.f. From 13 to 15 d.p.f. atropine reduced the frequency band width of HRV, whereas the combination of atropine and propranolol effectively reduced the frequency band width between 11 and 15 d.p.f. Classical power spectrum analysis using electrocardiograms is not possible in tiny zebrafish larvae and juveniles. It was therefore performed using optical methods, recording cardiac movement and cardiotachograms calculated from these measurements. Whereas heart movements contained frequency components characterizing HRV, the cardiotachogram did not show typical frequency spectra as known from other species.

Trade‐Offs in Thermal Adaptation: The Need for a Molecular to Ecological Integration

Through functional analyses, integrative physiology is able to link molecular biology with ecology as well as evolutionary biology and is thereby expected to provide access to the evolution of molecular, cellular, and organismic functions; the genetic basis of adaptability; and the shaping of ecological patterns. This paper compiles several exemplary studies of thermal physiology and ecology, carried out at various levels of biological organization from single genes (proteins) to ecosystems. In each of those examples, trade-offs and constraints in thermal adaptation are addressed; these trade-offs and constraints may limit species' distribution and define their level of fitness. For a more comprehensive understanding, the paper sets out to elaborate the functional and conceptual connections among these independent studies and the various organizational levels addressed. This effort illustrates the need for an overarching concept of thermal adaptation that encompasses molecular, organellar, cellular, and whole-organism information as well as the mechanistic links between fitness, ecological success, and organismal physiology. For this data, the hypothesis of oxygen- and capacity-limited thermal tolerance in animals provides such a conceptual framework and allows interpreting the mechanisms of thermal limitation of animals as relevant at the ecological level. While, ideally, evolutionary studies over multiple generations, illustrated by an example study in bacteria, are necessary to test the validity of such complex concepts and underlying hypotheses, animal physiology frequently is constrained to functional studies within one generation. Comparisons of populations in a latitudinal cline, closely related species from different climates, and ontogenetic stages from riverine clines illustrate how evolutionary information can still be gained. An understanding of temperature-dependent shifts in energy turnover, associated with adjustments in aerobic scope and performance, will result. This understanding builds on a mechanistic analysis of the width and location of thermal windows on the temperature scale and also on study of the functional properties of relevant proteins and associated gene expression mechanisms.

NO as a mediator during the early development of the cardiovascular system in the zebrafish

As a general pattern innervation of the cardiovascular system appears late during development in vertebrate embryos, and cardiovascular control may be achieved by hormonal activity in early stages. However, very little is known about the onset of NO-responsiveness during development, which in adult vertebrates is known to play a key function in many physiological processes such as control of vascular tone, neurotransmission, macrophage activity, and angiogenesis. Analysis of the effect of NO on the cardiovascular system in zebrafish (Danio rerio) embryos and larvae revealed almost no effect on cardiac activity during chronic exposure to NO-producing chemicals, whereas vascular reactivity was observed in veins and arteries of the zebrafish in early developmental stages (5-6 days post fertilization). Chronic exposure also modified the development of the vascular system. The presence of an NO donor (sodium nitroprusside) did not change the patterning of the vascular bed, but it induced an earlier appearance of some blood vessels in the trunk region of the zebrafish larvae. The data reveal that NO plays an important role in the development of the cardiovascular system and in the ontogeny of the cardiovascular control system in fish.

Cardiac performance in the zebrafish breakdance mutant

In the Tubingen screen a breakdance mutant of zebrafish (bre) was described as an arrhythmia, in which the ventricle beats only with every second atrial contraction (2:1 rhythm). Surprisingly, a careful analysis of the effect of the breakdance mutation on cardiac performance of the zebrafish during development between 3 d.p.f. and 14 d.p.f revealed that homozygous bre mutants did not always show the 2:1 rhythm. Cardiac activity was continuously recorded for a period of 20 min in each larva, and during this period we observed that heart rate randomly switched between the 2:1 rhythm and a 1:1 rhythm. Furthermore, at 28 degrees C and at 31 degrees C the expression of the 2:1 rhythm decreased with development. At 31 degrees C this was in part due to a significantly reduced survival rate of mutants beyond 4 d.p.f. Besides development, temperature had a marked effect on the expression of the 2:1 rhythm, and during the first days of development the expression of the 2:1 rhythm was significantly higher at elevated incubation temperatures. By contrast, in the 2:1 beating heart ventricular contraction rate was about 80 beats min(-1) throughout development irrespective of the temperature, and even in the 1:1 rhythm mutants showed a significant bradycardia at all three temperatures (25 degrees C, 28 degrees C or 31 degrees C). Compared to wild-type animals, cardiac output was significantly lower in bre mutants. Pressure traces recorded in the ventricle of mutants revealed a prolonged relaxation phase, indicating that the second pacemaker current could not be conveyed to the ventricle (AV-block). This phenotype is comparable to the human Long QT Syndrome, an arrhythmia caused by a modification of an ion channel involved in cardiac repolarization. The bradycardia and the modified temperature sensitivity of heart rate suggested that the activity of the pacemaker cells was also affected by this mutation.

Epigenetic variations in early cardiovascular performance and hematopoiesis can be explained by maternal and clutch effects in developing zebrafish (Danio rerio)

This is the first study to show maternal and clutch effects on early developmental parameters like blood cell concentration and cardiac performance (heart rate, stroke volume and cardiac output) in developing zebrafish larvae (2-8 days post fertilization, dpf). Ten individuals per parental pair and developmental stage were analyzed. A pronounced interclutch variation of heart rate has been found in younger stages (2-4 dpf), while interclutch variation of heart rate was small in later stages (8 dpf). This effect was more pronounced in offspring from parental fish nourished with living food. The opposite effect was observed in interclutch variation of blood cell concentration. Here only older stages showed significant interclutch variations. Stroke volume and cardiac output had very small interclutch variations throughout all stages. Heart rate was strongly dependent with developmental stage in all groups. Nutritional maternal effects on heart performance and also in blood cell concentration could be detected in the offspring of parent animals either fed with flake food or with living food. Red blood cell count, calculated as a product from red blood cell concentration, was not significantly different in both feeding groups. The number of spawned eggs was not different. In summary, these data indicate that "clutch effects" caused by maternal and/or genetic influences can affect the developmental pattern of cardiac performance and blood cell concentration.

Importance of cytoskeletal elements in volume regulatory responses of trout hepatocytes

The role of cytoskeletal elements in volume regulation was studied in trout hepatocytes by investigating changes in F-actin distribution during anisotonic exposure and assessing the impact of cytoskeleton disruption on volume regulatory responses. Hypotonic challenge caused a significant decrease in the ratio of cortical to cytoplasmic F-actin, whereas this ratio was unaffected in hypertonic saline. Disruption of microfilaments with cytochalasin B (CB) or cytochalasin D significantly slowed volume recovery following hypo- and hypertonic exposure in both attached and suspended cells. The decrease of net proton release and the intracellular acidification elicited by hypotonicity were unaltered by CB, whereas the increase of proton release in hypertonic saline was dramatically reduced. Because amiloride almost completely blocked the hypertonic increase of proton release and cytoskeleton disruption diminished the associated increase of intracellular pH (pH(i)), we suggest that F-actin disruption affected Na(+)/H(+) exchanger activity. In line with this, pH(i) recovery after an ammonium prepulse was significantly inhibited in CB-treated cells. The increase of cytosolic Na(+) under hypertonic conditions was not diminished but, rather, enhanced by F-actin disruption, presumably due to inhibited Na(+)-K(+)-ATPase activity and stimulated Na(+) channel activity. The elevation of cytosolic Ca(2+) in hypertonic medium was significantly reduced by CB. Altogether, our results indicate that the F-actin network is of crucial importance in the cellular responses to anisotonic conditions, possibly via interaction with the activity of ion transporters and with signalling cascades responsible for their activation. Disruption of microtubules with colchicine had no effect on any of the parameters investigated.

The influence of environmentalPO2 on hemoglobin oxygen saturation in developing zebrafishDanio rerio

Several studies suggest that during early larval development of lower vertebrates convective blood flow is not essential to supply oxygen to the tissues, but information about the oxygenation status of larvae during the time of cutaneous respiration is still missing. If convective oxygen transport contributes to the oxygen supply to tissues, venous blood in the central circulatory system should be partly deoxygenated, and hyperoxia should increase the oxygen saturation of the hemoglobin. To analyze the changes in hemoglobin oxygen saturation induced by hyperoxic incubation, zebrafish larvae were incubated in a tiny chamber between polytetrafluoroethylene membranes(Teflon), so that the oxygen supply could be rapidly modified. Hemoglobin oxygen saturation was measured in vivo by combining video imaging techniques with a spectrophotometrical analysis of hemoglobin light absorption at specific wavelengths for maximal absorption of oxygenated and deoxygenated blood (413 nm and 431 nm, respectively) under normoxic conditions and after a 10 min period of hyperoxia (PO2=100 kPa),assuming that at a PO2 of 100 kPa the hemoglobin is fully saturated. The results demonstrated that red blood cell oxygenation of zebrafish larvae at 4 days post fertilization (d.p.f.), 5 d.p.f. and 12 d.p.f. could be increased by hyperoxia. The data suggest that at the time of yolk sac degradation (i.e. 4 d.p.f. and 5 d.p.f.), when the total surface area of the animal is reduced, bulk diffusion of oxygen may not be sufficient to prevent a partial deoxygenation of the hemoglobin. The decrease in hemoglobin oxygenation observed at 12 d.p.f. confirms earlier studies indicating that at 12–14 d.p.f., convective oxygen transport becomes necessary to ensure oxygen supply to the growing tissues.

Isoforms vatB1 and vatB2 of the vacuolar type ATPase subunit B are differentially expressed in embryos of the zebrafish (Danio rerio)

The v-type ATPase is a membrane anchored, multi-subunit proton pump, which in freshwater fish appears to play a major role in ionoregulative processes in the apical membrane of specialized gill cells. Very little is known about free-living fish embryos and larvae that are exposed to hypo-osmotic conditions with spawning but do not have their gills fully developed. By using reverse transcriptase-polymerase chain reaction and immunological methods, we could demonstrate the presence of two isoforms of the subunit B of this v-type ATPase in the early development of the zebrafish. Immunohistochemical analysis revealed the presence of one isoform (vatB1) in the apical membrane of embryonic skin cells, while vatB2 has been found ubiquitously. This differential localization of the two isoforms supports the hypothesis that vatB1 is preferentially involved in ionoregulative functions, while vatB2 may be preferentially responsible for acidification of intracellular vesicles.

pH regulation and swimbladder function in fish

Gas gland cells of the European eel (Anguilla anguilla) are specialized for the production and secretion of acidic metabolites. Although typically exposed to high oxygen partial pressures, they convert glucose mainly into lactate, but also produce CO2 in the pentose phosphate shunt. Only a very small fraction of glucose is oxidized via aerobic metabolism. Although the buffer capacity of gas gland cells appears to be high, even at low extracellular pH values intracellular pH is always kept about 0.2-0.3 pH-units more acidic. Thus, under all physiological conditions proton concentration within gas gland cells is higher than in the extracellular fluid, facilitating proton extrusion. Diffusion of CO2, Na+/H+-exchange, sodium-dependent anion exchange and a V-ATPase represent the pathways available for proton secretion. While under resting conditions the sodium-dependent pathways and diffusion of CO2 appear to be the dominating mechanisms for acid secretion, at low intracellular pH the contribution of Na+/H+-exchange and of V-ATPase appear to increase, while sodium-dependent anion exchange becomes less important. The mechanisms regulating the activity of these acid-secreting pathways and of the metabolism responsible for the production of protons are largely unknown.

Ontogeny of the gut motility control system in zebrafish Danio rerio embryos and larvae

Using digital motion analysis, the ontogeny of the cholinergic, tachykinin and pituitary adenylate cyclase-activating polypeptide (PACAP) control systems was studied in zebrafish Danio rerio larvae, in vivo. For the first time we show that the regular propagating anterograde waves that occur in the zebrafish larval gut before and around the onset [at 5–6 days post fertilization (d.p.f.)] of feeding are modulated by acetylcholine or atropine, PACAP and NKA (neurokinin A). At 3 d.p.f., when no spontaneous motility has developed, application of acetylcholine did not affect the gut. However, at 4 d.p.f., acetylcholine increased and atropine reduced the frequency of propagating anterograde waves. At 5 d.p.f., NKA increased and PACAP reduced the wave frequency. This suggests that both excitatory and inhibitory pathways develop at an early stage in the gut, independent of exogenous feeding. Immunohistochemistry established the presence of gut neurons expressing PACAP and NKA in the proximal part of the developing gut from the first stage investigated (2 d.p.f.) and before regular motility was observed. 1 d.p.f. (PACAP) or 2 d.p.f. (NKA) stages later the whole gut was innervated. This supports physiological results that gut motility is under neuronal control during the period when regular motility patterns develop.

Acid-base regulation in isolated gill cells of the goldfish (Carassius auratus)

Mechanisms of acid release and intracellular pH (pH(i)) homeostasis were analysed in goldfish (Carassius auratus) gill cells in primary culture. The rate of acid secretion was measured using a cytosensor microphysiometer, and pH(i) was determined using the fluorescent probe 2',7'-bis-(3-carboxypropyl)-5-(and-6)-carboxyfluorescein (BCPCF). Amiloride, a Na(+) channel and Na(+)/H(+) exchanger (NHE) inhibitor, had no effect on pH(i), but acid secretion of the gill cells was significantly impaired. In the presence of amiloride, the intracellular acidification (achieved using the NH(4)Cl pulse technique) was more severe than in the absence of amiloride, and recovery from the acidosis was slowed down. Accordingly, acid secretion of gill cells was severely reduced in the absence of extracellular Na(+). Under steady-state conditions, 4,4'-diisothiocyanatodihydro-stilbene-2,2'-disulfonic acid (DIDS), a HCO(3)(-)-transport inhibitor, caused a slow acidification of pH(i), and acid secretion was significantly reduced. No recovery from intracellular acidification was observed in the presence of DIDS. Bafilomycin A(1), an inhibitor of V-ATPase, had no effect on steady-state pH(i) and recovery from an intracellular acidification, whereas the rate of acid secretion under steady-state conditions was slightly reduced. Immunohistochemistry clearly revealed the presence of the V-ATPase B-subunit in goldfish gill lamellae. Taken together, these results suggest that a Na(+)-dependent HCO(3)(-) transport is the dominant mechanism besides an NHE and V-ATPase to control pH(i) in goldfish gill cells.

Influence of swim training on cardiac activity, tissue capillarization, and mitochondrial density in muscle tissue of zebrafish larvae

Larval zebrafish (Danio rerio) of two different age classes ("swim-up" larvae, 9 days old; "free-swimming" larvae, 21 days old) were exposed to either an endurance/continuous training or interval training. Control animals were kept in stagnant water. A comparison of cardiac activity of trained (either endurance or interval) and untrained animals at the end of the training regime revealed no differences in heart rate, end-diastolic and end-systolic ventricular volume, and cardiac output. Training also had no influence on the concentration of erythrocytes in the blood. Thus, at the level of total oxygen transport in the blood, training did not provoke any improvement during the first 32 days of development. Significant changes, however, were observed at the tissue level. In free-swimming larvae [i.e., between 21 and 32 days postfertilization (dpf)] endurance training increased the capillarization of both axial muscle caudal to the anus and the tail fin. In addition, mitochondrial density of red and intermediate muscle fibers increased significantly. In contrast to capillarization, even swim-up larvae, trained between 9 and 15 dpf, were affected. The observed increase in mitochondrial content indicates a high demand for oxygen and energy-rich metabolites for oxidative phosphorylation. In older larvae, this is met by the increase in capillarization that improves the blood supply and with it the required oxygen and metabolite supply of muscle tissue. Both of these adaptational changes result in a reduction of diffusion distances (between capillary and muscle fiber as well as mitochondria) and may contribute to a higher resistance toward oxygen deficiency. Furthermore, this study indicates that plasticity of muscle tissue is already established in early stages of development at both the tissue and cellular levels.

Expression of two isoforms of the vacuolar-type ATPase subunit B in the zebrafish Danio rerio

In the present study we tested the hypothesis that two isoforms of the regulatory subunit B of vacuolar-type ATPase (V-ATPase) are expressed in the zebrafish Danio rerio. The complete coding sequences for both isoforms, vatB1 and vatB2, were cloned and sequenced. BLASTX analysis revealed the greatest similarity to amino acid sequences of B subunits from the European eel Anguilla anguilla and rainbow trout Oncorhynchus mykiss. The isoforms were expressed in a bacterial system and the recombinant proteins verified using isoform-specific antibodies directed against vatB isoforms of the eel. The distribution of both isoforms in zebrafish tissues was investigated using reverse transcriptase-polymerase chain reaction and western blot analysis. The results revealed that at the RNA level both isoforms were expressed in all tested organs, i.e. the gills, swimbladder, heart, kidney, liver, spleen, intestine and skeletal muscle. At the protein level, however, there were tissue-specific variations in the levels of the two vatB isoforms expressed. The highest amounts of V-ATPase were detected in total protein preparations from gill, heart and liver tissue. In liver tissue, however, the western blot analysis indicated that vatB1 was not as prominent as vatB2, and immunohistochemistry revealed that antibodies directed against vatB1 yielded a very weak staining in a number of cells, while an antibody directed against vatB1 and vatB2 yielded a strong staining in virtually every cell. Similarly, neurosecretory cells of the small intestine were stained with an antibody directed against vatB1 and vatB2, but not with an antibody specific for vatB1. Therefore we conclude that the differential expression of two isoforms of the V-ATPase subunits, which may serve different functions as in several mammalian species, may also be a common phenomenon in teleost fish.

Non-invasive imaging of blood cell concentration and blood distribution in zebrafish Danio rerio incubated in hypoxic conditions in vivo

This is the first study to use a combination of digital imaging techniques and vital video microscopy to study hypoxia-induced changes in blood cell concentration, angiogenesis and blood redistribution in entire animals. Zebrafish Danio rerio, which are known to be independent of convective oxygen transport until about 2 weeks post-fertilization, were raised under chronic hypoxia (P(O(2))=8.7 kPa) starting at 1 day after fertilization (d.p.f.) until 15 d.p.f. In control animals, the concentration of red cells (i.e. the number of red cells per nl blood) remained constant until 7 d.p.f., and than decreased by approximately 70% until 15 d.p.f. In hypoxic animals, however, the concentration of red cells remained significantly elevated compared to control animals at 12 and 15 d.p.f. Assuming that the hemoglobin content of the red cells is similar, hypoxic animals have a higher oxygen carrying capacity in their blood. Red cell distribution within the various parts of the circulatory system, taken as an indicator for blood distribution, revealed a significant modification in the number of blood cells perfusing the organs in hypoxic animals. At 12 d.p.f., gut perfusion was reduced by almost 50% in hypoxic animals, while perfusion of the segmental muscle tissue was increased to 350% of control values. No significant changes in brain perfusion were observed under these conditions. At 15 d.p.f., the reduction in gut perfusion was abolished, although muscle perfusion was still significantly elevated. At this time, growth of hypoxic animals was less compared to control animals, revealing that hypoxia had become deleterious for further development. The vascular bed of various organs was not obviously different in hypoxic animals compared to normoxic animals.

Localization of the vacuolar-type ATPase in swimbladder gas gland cells of the European eel (Anguilla anguilla)

The vacuolar ATPase is a multifunctional enzyme that consists of several subunits. Subunit B is part of the catalytic domain of the enzyme and is present in two isoforms in fish as well as in mammals. Possibly, these two isoforms - vatB1 (kidney isoform) and vatB2 (brain isoform) - serve different functions. A localization of the two isoforms was attempted in swimbladder gas gland cells of the European eel Anguilla anguilla by immunohistochemistry. Two antibodies were produced by immunization of rabbits with synthetic peptides. Specificity of the antibodies, on the one hand, an isoform-specific antibody for vatB1 and, on the other hand, an antibody that recognizes both isoforms (vatB1 and vatB2), was confirmed by western blot analysis using recombinant proteins produced in a bacterial expression system. The immunohistochemical localization with the antibody directed against both isoforms of the B subunit revealed a positive staining in apical membranes of swimbladder gas gland cells as well as in the basolateral membranes. Significant staining was observed in vesicles located near the apical membrane. Staining with the vatB1-specific antibody resulted in a similar picture in the apical region of the cells. In contrast to the staining with the first antibody, only a poor signal was observed in the basal region. The nature of the vesicles in the apical region of the gas gland cells was determined by using an antibody directed against surfactant protein D.

Developmental plasticity in the cardiovascular system of fish, with special reference to the zebrafish

During development the circulatory system of vertebrates typically starts operating earlier than any other organ. In these early stages, however, blood flow is not yet linked to metabolic requirements of tissues, as is well established for adults. While the autonomic nervous system becomes functional only quite late during development, in the early stages control of blood flow appears to be possible by blood-borne and/or local hormones. This study presents methods based on video-imaging techniques and fluorescence microscopy to visualize cardiac activity, as well as the vascular bed of developing lower vertebrates, and tests the idea that environmental factors, such as hypoxia, may modify cardiac activity, or even the early formation of blood vessels in embryos and larvae. In zebrafish larvae, adaptations of cardiovascular activity to chronic hypoxia become visible shortly after hatching, and the formation of some blood vessels is enhanced under chronic hypoxia. Exposure of early larval stages of zebrafish to a constant water current induces physiological adaptations, resulting in enhanced swimming efficiency and increased tolerance towards hypoxia. Furthermore, application of hormones such as NO can modify cardiac activity as well as peripheral resistance, and they can stimulate blood vessel formation. In consequence, even during early development of fish or amphibian larvae, the performance of cardiac muscle and of skeletal muscle can be modified by environmental influences and peripheral resistance can be adjusted. Even blood vessel formation can be stimulated by hypoxia, for example, or by the presence of specific hormones. Thus, at approximately the time of hatching the physiological performance of vertebrate larvae is already determined by the combined action of environmental influences and of genetic information.

Influence of hypoxia and of hypoxemia on the development of cardiac activity in zebrafish larvae

Cardiac activity and anaerobic metabolism were analyzed in zebrafish larvae raised under normoxia (PO(2) = 20 kPa) and under chronic hypoxia (PO(2) = 10 kPa) at three different temperatures (25, 28, and 31 degrees C). Heart rate increased with development and with temperature. Under normoxia, cardiac output increased significantly at high temperature (31 degrees C), but not at 28 or at 25 degrees C. Under chronic hypoxia, however, heart rate as well as cardiac output increased at all temperatures in larvae at about hatching time or shortly thereafter. Cardiac activity of larvae raised for 2 wk after fertilization with a reduced hemoglobin oxygen-carrying capacity in their blood (hypoxemia; due to the presence of CO or of phenylhydrazine in the incubation water) was not different from control animals. Whole body lactate content of these animals did not increase. Thus there was no indication of a stimulated anaerobic energy metabolism. The increase in cardiac activity observed during hypoxia suggests that at about hatching time receptors are present that sense hypoxic conditions, and this information can be used to induce a stimulation of convective oxygen transport to compensate for a reduction in bulk oxygen diffusion in the face of a reduced oxygen gradient between environmental water and tissues. Under normoxia, however, the PO(2) gradient between environmental water and tissues and diffusional oxygen transport assure sufficient oxygen supply even if hemoglobin oxygen transport in the blood is severely impaired. Thus, under normoxic conditions and with a normal metabolic rate of the tissues, convective oxygen transport is not required until approximately 2 wk after fertilization.

Mechanisms of acid secretion in pseudobranch cells of rainbow trout(Oncorhynchus mykiss)

Cell suspensions of rainbow trout Oncorhynchus mykisspseudobranch, prepared by Ca2+ depletion and mechanical maceration,contained a distinct population of cells that always kept their relatively cuboidal shape and did not round up in suspension or proliferate after adhering to the surface of cell culture dishes. Phasecontrast microscopy revealed an extensive system of basal membrane invaginations, and Na+-K+-ATPase- and anionexchanger-like immunoreactivity could be localized in cell membranes. The cells were characterized by a high mitochondrial density. Using specific antibodies, V-ATPase subunit B was localized in the plasma membrane. Using a cytosensor microphysiometer, the rate of acid secretion of these cells was measured and compared with the activity of a gill cell preparation. Incubation of pseudobranch cells with bafilomycin A1 (10-6 moll-1), a specific inhibitor of V-ATPase, reduced the rate of acid secretion by about 10% under control conditions, while no effect of bafilomycin on the rate of acid secretion of gill cells was observed. Application of amiloride (5×10-5moll-1) reduced the rate of acid secretion in cells of both organs,pseudobranch and gills. Incubation of pseudobranch cells with DIDS(10-3 moll-1) resulted in a minor increase in the rate of proton secretion, but in cells prepared from the gills of rainbow trout acid secretion was reduced by about 30-40%. It is concluded that pseudobranch cells are equipped with various pathways to secrete protons, and that the anion exchange activity especially of pseudobranch cells appears to be different from that in gills.

Determinants of intracellular pH in gas gland cells of the swimbladder of the European eel Anguilla anguilla

Gas gland cells of the European eel (Anguilla anguilla) were cultured on collagen-coated coverslips, and intracellular pH was measured using the pH-sensitive fluorescent probe 2′,7′-bis-(2-carboxypropyl)-5-(6)-carboxyfluorescein (BCPCF). The contributions of various proton-translocating mechanisms to homeostasis of intracellular pH (pHi) were assessed by adding specific inhibitors of the various proton-translocating mechanisms at a constant extracellular pH (pHe)of 7.4 and after artificial acidification of the cells using the ammonium pulse technique. The greatest decrease in pHi was observed after addition of 5-(N-ethyl-N-isobutyl)-amiloride (MIA), an inhibitor of Na+/H+ exchange. Na+/H+ exchange was active under steady-state conditions at an extracellular pH of 7.4, and activity increased after intracellular acidification. Incubation of gas gland cells with 4,4′-diisothiocyanostilbene-2,2′-disulphonic acid(DIDS), an inhibitor of anion exchange, also caused a decrease in pHi, but this decrease was not as pronounced as in the presence of MIA. Furthermore, at low pHi, the effect of DIDS was further reduced, suggesting that bicarbonate-exchanging mechanisms are involved in maintaining a steady-state pHi but that their importance is reduced at low pH. Bafilomycin A1,a specific inhibitor of the V-ATPase, had no effect on steady-state pHi. However, recovery of intracellular pH after an artificial acid load was significantly impaired in the presence of bafilomycin. Our results suggest that Na+/H+ exchange and anion exchange are important for the regulation of pHi at alkaline values of pHe. When pHi is low, a situation probably often encountered by gas gland cells during gas secretion,Na+/H+ exchange continues to play an important role in acid secretion and a V-ATPase appears to contribute to proton secretion.