Responses of the red blood cells from two high-energy-demand teleosts, yellowfin tuna (Thunnus albacares) and skipjack tuna (Katsuwonus pelamis), to catecholamines

A lack of red blood cell swelling in five elasmobranch fishes following air exposure and exhaustive exercise

In teleost fishes, catecholamine-induced increases in the activity of cation exchangers compensate for decreases in hemoglobin oxygen affinity and maximum blood oxygen carrying capacity caused by decreases in plasma pH (i.e., metabolic acidosis). The resultant red blood cell (RBC) swelling has been documented in sandbar (Carcharhinus plumbeus) and epaulette (Hemiscyllium ocellatum) sharks following capture by rod-and-reel or after a 1.5 h exposure to anoxia (respectively), although the underlying mechanisms remain unknown. To determine if RBC swelling could be documented in other elasmobranch fishes, we collected blood samples from clearnose skate (Rostroraja eglanteria), blacktip reef shark (Carcharhinus melanopterus), and sicklefin lemon shark (Negaprion acutidens) subjected to exhaustive exercise or air exposure (or both) and measured hematocrit, hemoglobin concentration, RBC count, RBC volume, and mean corpuscular hemoglobin content. We did likewise with sandbar and epaulette sharks to further explore the mechanisms driving swelling when present. We could not document RBC swelling in any species; although hematocrit increased in all species (presumably due to RBC ejection from the spleen or fluid shifts out of the vascular compartment) except epaulette shark. Our results indicate RBC swelling and associated ion shifts in elasmobranch fishes is not inducible by exercise or hypoxia, thus implying this response maybe of lesser importance for maintaining oxygen delivery during acute acidosis than in teleost fishes.

Effects of temperature on feed intake and plasma chemistry after exhaustive exercise in triploid brown trout (Salmo trutta L)

The physiological effect of temperature on feed intake and haematological parameters after exhaustive swimming in diploid and triploid brown trout (Salmo trutta) was investigated. Trout were exposed to an incremental temperature challenge (2 °C/day) from ambient (6 °C) to either 10 or 19 °C. Feed intake profiles did not differ between ploidy at 10 °C; however, triploids had a significantly higher total feed intake at 19 °C. After 24 days, each temperature-ploidy group was exposed to exhaustive swimming for 10 min. The haematological response differed between ploidy, with the magnitude of the response affected by temperature and ploidy. Post-exercise, acid-base and ionic differences were observed. Plasma lactate increased significantly from rest for both temperature and ploidy groups, but glucose increased significantly at higher temperature. Post-exercise, triploids at 19 °C had significantly higher osmolality and cholesterol than diploids, but differences were resumed within 4 h. Elevated alkaline phosphatase (ALP) and aspartate aminotransferase (AST) in fish at higher temperature suggested greater tissue damage; however, both ploidy responded similarly. Despite no significant differences in deformity prevalence, the type and location of deformities observed differed between ploidy (decreased intervertebral space with higher prevalence in tail area and fin regions for diploids, while vertebral compression, fusion in cranial and caudal trunks for triploids). These results suggest triploids have greater appetite than diploids at elevated temperature and that triploids suffer similar blood disturbances after exercise as diploids. These findings have implications for the management of freshwater ecosystems and suggest that stocking triploid brown trout may offer an alternative to diploid brown trout.

Temperature dependence of haemoglobin-oxygen affinity in heterothermic vertebrates: mechanisms and biological significance

As demonstrated by August Krogh et al. a century ago, the oxygen-binding reaction of vertebrate haemoglobin is cooperative (described by sigmoid O(2) equilibrium curves) and modulated by CO(2) and protons (lowered pH) that - in conjunction with later discovered allosteric effectors (chloride, lactate and organic phosphate anions) - enhance O(2) unloading from blood in relatively acidic and oxygen-poor tissues. Based on the exothermic nature of the oxygenation of the haem groups, haemoglobin-O(2) affinity also decreases with rising temperature. This thermal sensitivity favours oxygen unloading in warm working muscles, but may become detrimental in regionally heterothermic animals, for example in cold-tolerant birds and mammals and warm-bodied fish, where it may perturb the balance between O(2) unloading and O(2) requirement in organs with substantially different temperatures than at the respiratory organs and thus commonly is reduced or obliterated. Given that the oxygenation of haemoglobin is linked with the endothermic release of allosteric effectors, increased effector interaction is an effective strategy that is widely exploited to achieve adaptive reductions in the temperature dependence of blood-O(2) affinity. The molecular mechanisms implicated in heterothermic vertebrates from different taxonomic groups reveal remarkable variability, both as regards the effectors implicated (protons in tunas, organic phosphates in sharks and billfish, chloride ions in ruminants and chloride and phosphate anions in the extinct woolly mammoth, etc.) and binding sites for the same effectors, indicating multiple evolutionary origins, but convergent physiological functionality (reductions in temperature dependence of O(2) -binding affinity that safeguard tissue O(2) supply).

Hematological characteristics of the spotted rose snapper Lutjanus guttatus (Steindachner, 1869) healthy and naturally infected by dactylogyrid monogeneans

The aim of the present study was to obtain a basic knowledge of the hematology in order to determine changes in blood parameters of the spotted rose snapper Lutjanus guttatus. The morphological features of blood cells were described according to the observations made by light microscopy of Wright-Giemsa-stained blood films. The reference intervals and the mean value were determined for each hematological parameter evaluated in healthy fish and data were compared to those of naturally infected, with dactylogyrid monogeneans fish. Infected fish showed a prevalence of 100% and a mean intensity of 246.6 parasites per fish. Mean values of HCT, WBC, thrombocytes percentage and eosinophils percentage were significantly higher (P<0.05) in the infected fish. In addition, lymphocytes percentage and total protein were significantly lower (P<0.05) in the infected fish compared to healthy fish. Only total WBC count, lymphocytes percentage and eosinophils percentage in infected fish were outside reference interval. The hematology of the spotted rose snapper of this study might serve as a basis for future studies and diagnosis. Changes observed in blood parameters in infected fish suggest that the immune system of L. guttatus was affected by the presence of the parasites.

ATP-induced temperature independence of hemoglobin-O2 affinity in heterothermic billfish

The inverse relationship between temperature and hemoglobin-O(2) affinity resulting from the exothermic nature of heme oxygenation favors O(2) unloading from blood to warm, metabolically active tissues. However, this temperature sensitivity is maladaptive, and commonly countered in regional heterotherms, where it may hamper unloading (e.g. in cold extremities of arctic mammals) or increase the diffusive arterio-venous short-circuiting of O(2) (e.g. in counter-current heat exchangers of warm swimming muscles of tuna). We hypothesized analogous blood specializations in heterothermic billfish, whose warm eyes and brains increase the temporal resolution of vision, and measured hemoglobin-O(2) binding properties in three species over a wide pH range, at two temperatures, and in the absence and presence of the major red cell effector, ATP, permitting detailed assessment of overall oxygenation enthalpies (DeltaH') and contributions from oxygenation-linked proton and ATP dissociation. Billfish express multiple isohemoglobins with similar O(2) affinities and pronounced sensitivities to pH and ATP. Compared with the moderate effects associated with proton dissociation upon oxygenation, dissociation of ATP and coupled extra Bohr protons virtually obliterates the temperature sensitivities. At pH 7.4, where this effect is maximal, ATP changes DeltaH' values of blue marlin, striped marlin and shortbill spearfish hemoglobins from -39, -49 and -44 kJ mol(-1) O(2), respectively, to +26, +4 and -7 kJ mol(-1). Thus in addition to allosterically modulating hemoglobin-O(2) affinity, ATP diminishes its temperature sensitivity, reducing deleterious arterio-venous short-circuiting of oxygen in the cranial billfish heat exchangers. The mechanism underlying this reduction in oxygenation enthalpy differs fundamentally from that in tuna, supporting independent evolution of this trait in these scombroid lineages.

Plasma lactate and glucose flushes following burst swimming in silver trevally (Pseudocaranx dentex: Carangidae) support the “releaser” hypothesis

Silver trevally (Pseudocaranx dentex) are highly athletic marine teleosts inhabiting the tropical waters of the Great Barrier Reef, Australia. Burst swimming increased plasma lactate from 1.6 +/- 0.4 S.D. to 21.6 +/- 3.3 mM (N = 6), among the highest values reported for functional hypoxia in fish. These data support the hypothesis that elite swimmers release lactate produced in the myotome into the circulation following anaerobic burst activity. The fish further developed a hyperglycaemic response to burst exercise with plasma glucose increasing from 6.6 +/- 2.0 to 13.2 +/- 2.3 mM (N = 6). Post-exercise erythrocyte swelling also occurred, but nucleoside triphosphate levels remained unaltered and do not provide a mechanism to modulate haemoglobin function during exercise. Metabolism of the blood cells appeared to be fuelled by both lactate and glucose.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

Red blood cell pH, the Bohr effect, and other oxygenation-linked phenomena in blood O2 and CO2 transport

The discovery of the S-shaped O2 equilibrium curve and the Bohr effect in 1904 stimulated a fertile and continued research into respiratory functions of blood and allosteric mechanisms in haemoglobin (Hb). The Bohr effect (influence of pH/CO2 on Hb O2 affinity) and the reciprocal Haldane effect (influence of HbO2 saturation on H+/CO2 binding) originate in the Hb oxy-deoxy conformational change and allosteric interactions between O2 and H+/CO2 binding sites. In steady state, H+ is passively distributed across the vertebrate red blood cell (RBC) membrane, and intracellular pH (pHi) changes are related to changes in extracellular pH, Hb-O2 saturation and RBC organic phosphate content. As the Hb molecule shifts between the oxy and deoxy conformation in arterial-venous gas transport, it delivers O2 and takes up CO2 and H+ in tissue capillaries (elegantly aided by the Bohr effect). Concomitantly, the RBC may sense local O2 demand via the degree of Hb deoxygenation and release vasodilatory agents to match local blood flow with requirements. Three recent hypotheses suggest (1) release of NO from S-nitroso-Hb upon deoxygenation, (2) reduction of nitrite to vasoactive NO by deoxy haems, and (3) release of ATP. Inside RBCs, carbonic anhydrase (CA) provides fast hydration of metabolic CO2 and ensures that the Bohr shift occurs during capillary transit. The formed H+ is bound to Hb (Haldane effect) while HCO3- is shifted to plasma via the anion exchanger (AE1). The magnitude of the oxylabile H+ binding shows characteristic differences among vertebrates. Alternative strategies for CO2 transport include direct HCO3- binding to deoxyHb in crocodilians, and high intracellular free [HCO3-] (due to high pHi) in lampreys. At the RBC membrane, CA, AE1 and other proteins may associate into what appears to be an integrated gas exchange metabolon. Oxygenation-linked binding of Hb to the membrane may regulate glycolysis in mammals and perhaps also oxygen-sensitive ion transport involved in RBC volume and pHi regulation. Blood O2 transport shows several adaptive changes during exposure to environmental hypoxia. The Bohr effect is involved via the respiratory alkalosis induced by hyperventilation, and also via the pHi change that results from modulation of RBC organic phosphate content. In teleost fish, beta-adrenergic activation of Na+/H+ exchange rapidly elevates pHi and O2 affinity, particularly under low O2 conditions.

Branchial membrane-associated carbonic anhydrase activity maintains CO2excretion in severely anemic dogfish

Plasma CO2reactions in Pacific spiny dogfish ( Squalus acanthias) have access to plasma and gill membrane-associated carbonic anhydrase (CA). Acute severe experimental anemia and selective CA inhibitors were used to investigate the role of extracellular CA in CO2excretion. Anemia was induced by blood withdrawal coupled to volume replacement with saline. Lowering hematocrit from 14.2 ± 0.4% (mean ± SE; N = 31) to 5.2 ± 0.1% ( N = 31) had no significant impact on arterial or venous CO2tensions (PaCO2and PvCO2, respectively) over the subsequent 2 h. Pco2was maintained despite the reduction in red cell number and a significant 32% increase in cardiac output (V̇b), both of which have been found to cause PaCO2increases in teleost fish. By contrast, treatment of anemic dogfish with the CA inhibitors benzolamide (1.3 mg/kg) or F3500 (50 mg/kg), to selectively inhibit extracellular CA, elicited rapid and significant increases in PaCO2of 0.68 ± 0.17 Torr ( N = 6) and 0.53 ± 0.11 Torr ( N = 7), respectively, by 30 min after treatment. These findings provide a functional context in which extracellular CA in dogfish contributes substantially to CO2excretion. Additionally, the apparent lack of effect of V̇bchanges on Pco2suggests that, in contrast to teleost fish, CO2excretion in dogfish does not behave as a diffusion-limited system.

Apparent diffusion limitations on branchial CO2 transfer are revealed by severe experimental anaemia in brown bullhead (Ameiurus nebulosus)

In this study, the rapid (within 2 h) effects of acute anaemia on blood gas and acid-base status, as well as cardiorespiratory variables, were examined in brown bullhead (Ameiurus nebulosus). Anaemia was induced by blood withdrawal coupled to volume replacement with saline. Lowering haematocrit from the control value of 23.5+/-1.0% (mean+/-S.E.M.; N=37) to 5.9+/-0.3% (N=37) resulted in a significant increase (by 2.63+/-0.51 torr; N=7) in arterial CO(2) tension (PaCO(2)) over the subsequent 2-h period in the absence of a change in arterial O(2) tension (PaO(2)). Treatment with bovine carbonic anhydrase (CA) reduced the extent of the PaCO(2) increase to the point where it was not statistically significant. In both control and CA-treated fish, arterial pH decreased during acute anaemia; the acidosis was of mixed respiratory and metabolic origin in control fish and primarily metabolic in CA-treated fish. Inducing anaemia caused increases in both cardiac output (V*b) and heart rate that were similar in control and CA-treated fish. Experimental elevation of V*b equivalent to that observed during anaemia, but in the absence of lowered haematocrit, increased PaCO(2) significantly by 1.49+/-0.74 to 1.64+/-0.78 torr (N=5) without affecting PaO(2). These findings suggest that CO(2) excretion in bullhead, as in rainbow trout, is effectively diffusion-limited, and that approximately half of the increase in PaCO(2) measured during the initial 2 h of anaemia results from the impact of increased blood flow (hence decreased gill transit time) in a diffusion-limited system.

Oxygen transport capacity in the air-breathing fish, Megalops cyprinoides: compensations for strenuous exercise

Tarpon have high resting or routine hematocrits (Hct) (37.6+/-3.4%) and hemoglobin concentrations (120.6+/-7.3 gl(-1)) that increased significantly following bouts of angling-induced exercise (51.9+/-3.7% and 142.8+/-13.5 gl(-1), respectively). Strenuous exercise was accompanied by an approximately tenfold increase in blood lactate and a muscle metabolite profile indicative of a high energy demand teleost. Routine blood values were quickly restored only when this facultative air-breathing fish was given access to atmospheric air. In vitro studies of oxygen transport capacity, a function of carrying capacity and viscosity, revealed that the optimal Hct range corresponded to that observed in fish under routine behaviour. During strenuous exercise however, further increase in viscosity was largely offset by a pronounced reduction in the shear-dependence of blood which conformed closely to an ideal Newtonian fluid. The mechanism for this behaviour of the erythrocytes appears to involve the activation of surface adrenergic receptors because pre-treatment with propranolol abolished the response. High levels of activity in tarpon living in hypoxic habitats are therefore supported by an elevated Hct with adrenergically mediated viscosity reduction, and air-breathing behaviour that enables rapid metabolic recovery.

High plasma buffering and the absence of a red blood cell beta-NHE response in brown bullhead (Ameiurus nebulosus)

The high non-bicarbonate buffer capacity of brown bullhead (Ameiurus nebulosus) plasma was postulated to function as an alternative mechanism for the protection of red blood cell (RBC) intracellular pH (pHi) in the absence or attenuation of a RBC adrenergic response. The requirement for protecting RBC pHi arises from the presence of a Root effect haemoglobin in bullhead. In support of this hypothesis, bullhead RBCs incubated in vitro with isoproterenol (10(-8)-10(-5) mol l(-1)) or forskolin (10(-4) mol l(-1)) exhibited significant cyclic AMP accumulation, but failed to exhibit cell swelling or significant Na(+) or Cl(-) accumulation; plasma pH (pHe) was also unaffected. Similarly, no significant effect on RBC water content, Na(+) or Cl(-) concentration, or pHe was detected in bullhead blood incubated with 8-bromo cyclic AMP (10(-4)-10(-2) mol l(-1)) in vitro. These results suggest that while bullhead RBCs possess a beta-adrenoreceptor linked to cyclic AMP formation, stimulation of this adrenergic receptor does not result in measurable activation of a Na(+)/H(+) exchanger.

Hydrogen ion binding properties of tuna haemoglobins

Tunas are very active fish with a high aerobic capacity, but they also regularly perform burst swimming with massive production of lactic acid. The present study examines whether H(+) buffering by tuna haemoglobin (Hb) is elevated to cope with metabolic acidoses (by analogy with the high buffer capacity of tuna white musculature) or whether the Hb-H(+) binding properties resemble other teleosts that have low buffer values and high Haldane effects. H(+) titration of oxygenated and deoxygenated composite Hb from yellowfin tuna, skipjack tuna and bigeye tuna in 0.1 M KCl revealed low Hb-specific buffer values in all three tunas. Values at physiological pH were comparable to those reported in less active species such as carp and eel. The fixed acid Haldane effect was large (maximal uptakes of close to 4 mol H(+) per mol Hb tetramer upon deoxygenation). Thus, the Hb-H(+) binding properties of very active tuna species correspond to other teleosts. Low Hb buffer values may be a pre-requisite for the regulation of red blood cell pH via Na(+)/H(+) exchange. Approximately nine "neutral" groups were titratable in tuna Hbs, suggesting that two alpha-amino groups and seven histidine residues are titrated within each tetramer.

The interaction between O2 and CO2 exchange in rainbow trout during graded sustained exercise

A quantitative analysis of O2 and CO2 transport was conducted in resting and exercising rainbow trout, and these data were used to quantify the magnitude of coupling between O2 and CO2 exchange, in vivo. The release of Bohr protons during haemoglobin-oxygenation was non-linear over the Hb-O2 equilibrium curve used in trout subjected to different levels of sustained exercise. At low swimming speeds, when venous blood O2 content (CvO2) was high, there was a small acidosis as blood passed through the gills, indicating more protons were released during oxygenation of Hb than were consumed during HCO3- dehydration. At higher swimming speeds, when CvO2 was low, there was a significant alkalosis in arterial relative to venous blood, indicating that fewer protons were released upon oxygenation than HCO3- ions were dehydrated to CO2. Haldane coefficients (moles of protons released per mole of O2 which binds to Hb), calculated from steady state arterial and mixed-venous parameters, revealed that under resting conditions all blood CO2 removed from the blood during gill transit was stoichiometrically related to O2 uptake through the release of Bohr protons during Hb oxygenation. The magnitude of coupling between CO2 excretion and O2 uptake decreased from 100% to less than 40% at the maximal swimming velocity when the largest region of the Hb-O2 equilibrium curve was used for gas exchange. The non-linear release of Bohr protons over the range of Hb-O2 saturation in the blood reduces HCO3- dehydration at the gills during greater work loads elevating arterial P(CO2) levels, leading to an increase in HCO3- buffer capacity of the blood and tissues.