Respiratory rate,haemolymph oxygen tension and haemocyanin level in the shrimp Palaemon adspersus: Rathke

The marine shrimp Palaemon adspersus Rathke, frequenting Zostera meadows, shows an oxygen consumption rate (MO₂) that is independent of water oxygen tension (P_wO₂) as PO₂ decreases to a critical point (P_cr) near 70 mm Hg. This respiratory independence is associated with maintenance of a relatively constant “arterial” (post-branchial) haemolymph tension (P_aO₂) at 70–80 mm Hg. At lower P_wO₂ values, both MO₂ and P_aO₂ fall, reflecting, in contrast to the above independence, a direct dependence of MO₂ on internal PO₂.Haemolymph copper measurements demonstrate relatively high haemocyanin concentrations and oxygen-carrying capacities, but MO₂ levels reflect an insignificant rôle for the pigment as an oxygen store.The data are discussed as regards adaptations for aerobic metabolism.     

The cardiac responses of the scallop Pecten maximus (L.) to respiratory stress

Heart activity of Pecten maximus (L.) has been recorded during various forms of experimentally induced respiratory stress. There was considerable variation in the responses of individual scallops but bradycardia generally occurred in response to all forms of respiratory stress, with the rate of fall in heart rate dependent upon the severity of hypoxia.When oxygen tension declined slowly in a closed respirometer there was regulation of both heart rate and oxygen consumption. The critical tension, P_c, for oxygen consumption lay between 70 and 80 mm Hg, and corresponded with a slight regulatory upswing of the heart rate, whereas the P_c for heart rate was much lower at 20–30 mm Hg. Sudden transfer to deoxygenated water for 3 h resulted in very rapid bradycardia and there was a rapid recovery and initial overshoot of the normal rate on return to well-oxygenated sea water. Aerial exposure for 3 h produced more gradual bradycardia followed by gradual recovery on return to sea water.The results of this work are compared in some detail with previous work on other species of bivalve from different geographical areas and habitats, and the mechanisms controlling cardiac and respiratory regulation are discussed. It is concluded that there are few clear-cut general differences between littoral and sublittoral species in their behavioural and physiological adaptations to hypoxia; the main distinguishing feature of littoral-adapted species is their ability to control air-gaping. Changes in heart activity generally indicate variations in metabolic rate, the speed at which the metabolic rate may be altered reflecting the degree of adaptation to the littoral environment.     

Metabolic and ventilatory responses to hypoxia in two altitudinal populations of the toad, Bufo bankorensis

Effects of hypoxia on resting oxygen consumption ( ), lung ventilation, and heart rate at different ambient P_O2 were compared between lowland and high altitude populations of the toad, Bufo bankorensis. Resting decreased significantly in mild hypoxia (P_O2=120 mm Hg) at 10°C and in moderate hypoxia (P_O2=80 mm Hg) at 25°C in both altitudinal populations; however, resting did not differ significantly between the two populations. Numbers of lung ventilation periods (VP) and total inspired volume (V_L) did not change with P_O2 at 10°C, but did increase at moderate and severe hypoxia (40 mm Hg), respectively, at 25°C. Resting heart rates did not change during hypoxia and did not differ between altitude populations. The results suggest (1) the effect of P_O2 change on should be considered in future studies involving transfer of anurans to a different altitude; and (2) the metabolic and ventilatory physiology in B. bankorensis does not compensate for the low temperature and P_O2 at high altitude.     

The influence of dissolved oxygen concentration on phosphofructokinase and the glucose metabolism ofEscherichia coli K-12

The glucose metabolism and the response of phosphofructokinase activity to oxygen were investigated using glucose-limited chemostat cultures ofE. coli K-12. With a dilution rate of 0.2 hr^–1 and a glucose input concentration of 0.83 g/litre, 10 steady states were obtained ranging from 320 to 0 mm HgO₂. Dissolved oxygen reached zero level at a pO₂ of 25.8 mm Hg. The specific phosphofructokinase activity was constant above 28 mm Hg O₂ and increased linearly at lower pO₂ levels until it reached highest activity at 0 mm Hg O₂. Cell dry weight also started to decrease linearly from 28 to 5.9 mm Hg O₂, and fell sharply thereafter. Acid production rate did not start before pO₂ reached 25.6 mm Hg, increased progressively with an additional sharp increase below 5.9 mm Hg O₂. The main endproducts formed were acetic acid and ethanol with lactic acid appearing below 5.9 mm Hg O₂. The results suggest an effect of oxygen on phosphofructokinase synthesis rather than an ATP inhibition of the enzyme.This work was supported by a grant from the Australian Research Grant Commission.     

Effects of hypoxia and hyposalinity on the heart beat of the intertidal limpets Patella granvlaris (prosobranchia) and Siphonaria capensis (pulmonata)

• 1.1. Exposure to hypoxic sea water (p_O2 = 50mm Hg) and hyposalinity (20%.) caused the heart rate of Patella granularis to decline rapidly. This was particularly marked in hypoxia where normal heart rate (50 beats/min) fell initially to between 15–30 beats/min, and later (after 2 hr) cardiac arrest occurred. When oxygen tension and salinity were eventually normalized, heart rate became significantly elevated, above the normal rate.
• 2.2. In Siphonaria capensis, exposure to reduced oxygen tension and salinity usually induced a regular, although often delayed (after 2 hr) bradycardia (< 10 beats/min). No significant cardiac overshoot was observed for this limpet species.
• 3.3. The significance of the different heart beat patterns by the limpet species, which may be linked with respiratory/metabolic responses, is discussed with regard to the respective capacities of the species for colonization of upper-shore pools.

     

Gill ventilation and O2 extraction during graded hypoxia in two ecologically distinct species of flatfish,the flounder (Platichthys flesus) and the plaice (Pleuronectes platessa)

Synopsis Gill ventilation, breathing frequency, breath volume, oxygen extraction from the ventilatory water current and oxygen uptake through the gills were measured in flounder, Platichthys flesus, and plaice, Pleuronectes platessa, at water O₂ tensions ranging from 35 to 155 mm Hg at 10° C. Ventilation volumes were similar in the two species at high water O₂ tension. Exposure to hypoxic water elicited a larger increase in ventilation in the flounder. The per cent extraction of O₂ from water decreased slightly in both species as water O₂ tension was lowered. At comparable levels of ventilation O₂ extraction was higher in flounder. At the higher levels of water O₂ tension, O₂ uptake across the gills of flounder was stable, the critical O₂ tension being between 60 and 100 mm Hg. The plaice behaved as an oxygen conformer over the entire range of O₂ tensions investigated. The superior ability of the flounder in maintaining OZ uptake across the gills during a reduction in water O₂ tension may in part explain why the species, unlike plaice, inhabits very shallow waters with large fluctuations in dissolved oxygen.     

Oxygen uptake of carp,Cyprinus carpio,swimming in normoxic and hypoxic water

Synopsis Oxygen uptake (V_{o 2}) was measured in carp of approximately 40 cm length swimming at controlled variable oxygen tensions (P_{o 2}). At P_{o 2}> 120 mm Hg V_{o 2} increased with an increase in swimming speed from 5.6 to 11.3 cm · sec^–1. Extrapolation of V_{o 2} to zero activity at P_{o 2} = 140 mm Hg revealed a standard O₂ uptake of 36.7 ml O₂ · kg^–1 · h^–1 at 20° C. At the lowest swimming speed (5.6 cm · s^–1) the oxygen uptake increased when the water P_{o 2} was reduced. A near doubling in V_{o 2} was seen at P_{o 2} = 70 mm Hg compared to 140 mm Hg. At higher swimming speeds in hypoxic water V_{o 2} decreased relative to the values at low swimming speeds. As a result the slope of the lines expressing log V_{o 2} as a function of swimming speed decreased from positive to negative values with decreasing P_{o 2} of the water. pH of blood from the caudal vein drawn before and at termination of swimming at P_{o 2} = 70 mm Hg and 100 mm Hg did not show any decrease in relation to rest values at P_{o 2} = 140 mm Hg. Blood lactate concentration did not increase during swimming at these tensions.     

Hypoxia impairs visual acuity in snapper (Pagrus auratus)

We investigated the effect of environmental hypoxia on vision in snapper (Pagrus auratus). Juvenile snapper inhabit estuarine environments where oxygen conditions fluctuate on a seasonal basis. Optomotor experiments demonstrated that visual acuity is impaired by environmental hypoxia, but not until levels approach the critical oxygen tension (P _crit) of this species (around 25 % air-saturated seawater). In 100, 80, and 60 % air-saturated seawater, a positive optomotor response was present at a minimum separable angle (M _SA) of 1°. In 40 % air-saturated seawater, vision was partially impaired with positive responses at M _SAs of 2° and above. However, in 25 % air-saturated seawater, visual acuity was seriously impaired, with positive responses only present at M _SAs of 6° and above. Snapper were found to possess a choroid rete, facilitating the maintenance of high ocular oxygen partial pressures (PO₂) during normoxia and moderate hypoxia (PO₂, between 269 and 290 mmHg). However, at 40 and 25 % water oxygen saturation, ocular PO₂ was reduced to below 175 mmHg, which is perhaps linked to impairment of visual acuity in these conditions. The ability to preserve visual function during moderate hypoxia is beneficial for the maintenance of a visual lifestyle in the fluctuating oxygen environments of estuaries.     

Respiratory responses and tolerance to hypoxia in two marine teleosts,Epinephelus akaara (Temminck & Schlegel) and Mylio macrocephalus (Basilewsky)

The respiratory responses and tolerance of hypoxia were studied in two marine teleosts, the red grouper (Epinephelus akaara, a sluggish species) and the black sea bream (Mylio macrocephalus, an active species). Neither species showed abnormal behaviour or mortality when exposed to 2 mg O₂ l^–1 for 7 h. The black sea bream was, however, comparatively more tolerant when exposed to 1 mg O₂ l^–1, but tolerance of both species became similar under extremely hypoxic conditions (i.e. 0.5 mg O₂ l^–1). In contrast to most other teleosts, both species showed a reduction in opercular beating rate during hypoxia, and oxygen conformity was found in the range of 0.5 to 7.0 mg O₂l ^–1. O₂ dissociation curves were constructed, and the P₅₀ value of the black sea breams (27 ± 5.6 mm Hg) was found to be much lower than that of the red groupers (50 ± 2.5 mm Hg). For both species, the general levels of venous PO₂ showed a direct relationship to ambient PO₂, and were markedly reduced after 1 h exposure to various levels of hypoxia. Compared with the red groupers, the black sea breams appeared to be more able to maintain its venous PO₂ levels during prolonged hypoxic exposure.     

Impaired Synthesis of Acetylcholine by Mild Hypoxic Hypoxia or Nitrous Oxide

The effect of mild hypoxic hypoxia on brain metabolism and acetylcholine synthesis was studied in awake, restrained rats. Since many studies of hypoxia are done with animals anesthetized with nitrous oxide (N₂O), the effects of N²O were evaluated. N²O (70%) increased the cerebral cortical blood flow by 33% and the cortical metabolic rate of oxygen by 26%. In addition, the synthesis of acetylcholine in N²O-anesthetized animals, measured with [U-¹⁴C]glucose and [1-²H₂,2-²H₂]choline, decreased by 45 and 53%, respectively. Consequently, mild hypoxia was studied in unanesthetized rats. Control rats breathing 30% O₂ (partial pressure of oxygen, Pao₂= 120 mm Hg) were compared with rats exposed to 15% O₂ (Pao₂= 57 mm Hg) or 10% O₂ (Pao₂= 42 mm Hg). The synthesis of acetylcholine, measured with [U-¹⁴C]glucose, was decreased by 35 and 54% with 15% O₂ and 10% O₂ respectively; acetylcholine synthesis, measured with [1-²H₂,2-²H₂]choline, was decreased by 50 and 68% with 15% O₂ and 10% O₂ respectively. Animals breathing either 15% or 10% O₂ had normal cerebral metabolic rates of oxygen but had increased brain lactates and increased cortical blood flows compared with animals breathing 30% O₂. These results show that even mild hypoxic hypoxia impairs acetylcholine synthesis, which in turn may account for the early symptoms of brain dysfunction associated with hypoxia.     

Blood respiratory properties of rainbow trout,Salmo gairdneri: Responses to hypoxia acclimation and anoxic incubation of blood in vitro

Summary Blood respiratory properties of rainbow trout were determined following acclimation to normoxia and two levels of hypoxia.The most prominent response appeared to be an increase in blood O₂ affinity graded to the level of hypoxia. TheP ₅₀ values (at pH 7.8 and 20°C) were 24.1 21.7 and 16.8 mm Hg when specimens were acclimated to water O₂ tensions of 150, 80 and 50 mm Hg, respectively. The blood O₂ affinity was closely correlated with the erythrocytic ATP concentration. The stepwise correlation of ATP andP ₅₀, when trout were exposed to graded oxygen lack in the water, indicates that the blood O₂ affinity is precisely regulated.Anoxic incubation of trout blood in vitro induced a rapid reduction in erythrocytic ATP concentration (t _1/2=75 min), which was closely correlated to theP ₅₀ value. The drop inP ₅₀ value during anoxic exposure can be explained partly by the direct allosteric effect of a decreased erythrocytic ATP concentration and partly by the modified Donnan distribution of protons across the red cell membrane. Reoxygenation of the incubated blood, however, only partly re-established the erythrocytic ATP concentration, with a concurrent rise inP ₅₀ value.The results invite discussion about the mechanism, by which fish regulate their blood O₂ affinity. It is concluded, that it is regulated at the organismal rather than at the red cell level.Abbreviation (E) erythrocytes, erythrocytic     

Modulation of epithelial cell proliferation in culture by dissolved oxygen

Modulation of epithelial cell proliferation by the dissolved oxygen concentration (P_O2) of the growth medium was assessed with primary human foreskin epithelium and a continuous monkey kidney epithelial cell line (LLC-MK₂). Direct measurement of the growth medium P_O2 provides the first quantitative evaluation of epithelial cell proliferation as a function of P_O2 provides the first quantitative evaluation of epithelial cell proliferation as a function of P_O2. Sustained proliferation of LLC-MK₂ cells occurs in serum-free medium equilibrated with a gas phase containing 18% or 30% O₂ v/v. Mid-logarithmic phase cultures rapidly consume dissolved oxygen; this results in a 60–70 mm Hg decline in P_O2 and leads to a stable growth medium P_O2 between 70 and 100 mm Hg, well above anoxic values. In contrast, if culture medium is equilibrated with a gas phase containing 0% or 1% O₂ v/v to yield a growth medium P_O2 ～ 20–40 mm Hg, proliferation of LLC-MK₂ and primary foreskin epithelial cells is retarded, and LLC-MK₂ cells use little dissolved oxygen. Gentle, continuous rocking to prevent diffusion gradient formation enhances proliferation slightly at the higher P_O2, but neither periodic fluid renewals nor continued rocking stimulates cells retarded by a lowered oxygen concentration to resume proliferation. The data collectively demonstrate that epithelial cell proliferation requires a P_O2 > 40 mm Hg, and threshold requirements are probably closer to 70 mm Hg. Glycolysis continues at a P_O2 insufficient for proliferation, but more lactic acid accumulates in actively proliferating cultures than in cultures equilibrated with 0% oxygen. We conclude that epithelial cells in vitro both consume more oxygen and require a higher P_O2 for continued proliferation, and that the oxygen requirement for epithelial cell proliferation exceeds that of a comparable population of fibroblasts for which low oxygen may enhance survival and proliferation.     

Cardiorespiratory response to hypoxia in persons with different levels of creativity

The correlation between the parameters of creativity and tolerance to experimentally induced normobaric hypoxia has been studied in young healthy subjects. The subjects inhaled a low-oxygen gas mixture (10% O₂) until the arterial hemoglobin oxygen saturation (SaO₂) decreased to 80%. In the recovery period, the subjects breathed normoxic air until the SaO₂ returned to its initial value. The parameters of creativity, including the originality and fluency of producing images in Torrance’s subtests of Circles and Incomplete figures, as well as the originality and fluency of sentences composed by using nouns from distant semantic categories, were evaluated before conducting hypoxic tests. Positive relationships were found between the values of figurative originality and tolerance to hypoxia, i.e., by the time of decrease in SaO₂ and the coefficient that reflects the rate of recovery of the respiratory function. Analysis of the reactivity of the cardiovascular system showed negative correlations between the parameters of creativity, namely, the figurative originality and verbal fluency, and the heart rate under the conditions of hypoxia, as well as after the recovery of the functions of the cardiorespiratory system. An increase in the capacity for original figurative thinking also corresponded to smaller differences in the values of the heart rate during the recovery and at rest. These correlations between the characteristics of creative thinking and reactions of the cardiovascular and pulmonary systems to experimentally induced hypoxia indicate that there is a common mechanism of nervous regulation underlying adaptive behavior and maintaining homeostasis.     

The influence of temperature on the haemocyanin function and oxygen uptake of the freshwater crab Potamonautes warreni calman

• 1.1. At 35°C a maximal VO₂ value of 110 ml O₂/kg/hr was obtained with a significant decrease in the value at 40°C.
• 2.2. The Bohr-effect for P. warreni is — 0.28 and does not change significantly at 15, 25 and 35°C.
• 3.3. The ability of the crab to extract oxygen from the water medium during a single exhalation is on average 41.2% whilst the limitation diffusion (L. diff, Piiper, [1982], A Companion to Animal Physiology, pp. 49–64. Cambridge University Press.) is 0.84.
• 4.4. Compared to land and marine crabs, in P. warreni, the P_aO₂ (29.5 mm Hg) and the P_vO₂ (15.3 mm Hg) is low.

     

Autonomic nervous control of blood pressure and heart rate during hypoxia in the cod,Gadus morhua

Summary The autonomic nervous and possible adrenergic humoral control of blood pressure and heart rate during hypoxia was investigated in Atlantic cod. The oxygen tension in the water was reduced to 4.0–5.3 kPa (i.e.. P_wO₂=30–40 mmHg), and the fish responded with an immediate increase in ventral and dorsal aortic blood pressure (P _va P _da), as well as a slowly developing bradycardia. The plasma concentrations of circulating catecholamines increased during hypoxia with a peak in the plasma level of noradrenaline occurring before the peak for adrenaline. Bretylium was used as a chemical tool to differentiate between neuronal and humoral adrenergic control of blood pressure and heart rate (f _H) during hypoxia. The increase in P _va and P _da in response to hypoxia was strongly reduced in bretylium-treated cod, which suggests that adrenergic nerves are responsible for hypoxic hypertension. In addition, a small contribution by circulating catecholamines to the adrenergic tonus affecting P _va during hypoxia was suggested by the decrease in P _va induced by injection of the -adrenoceptor antagonist phentolamine. The cholinergic and the adrenergic tonus affecting heart rate were estimated by injections of atropine and the -adrenoceptor antagonist sotalol. The experiments demonstrate an increased cholicholinergic as well as adrenergic tonus on the heart during hypoxia.     

Quantitative aspects of oxygen and carbon dioxide exchange through the lungs in Ocypode ceratophthalmus (Crustacea: Decapoda) during rest and exercise in water and air

Ghost crabs Ocypode ceratophthalmus were exercised in air and water to measure CO₂ and O₂ exchange rates using the method of instantaneous measurements of oxygen consumption rate (MO₂) where applicable. Average heart rate increased from 100 to nearly 400 pulses per minute after five minutes of exercise on a treadmill at a run rate of 0.133 m s^?1. It took less than a minute for oxygen taken up through the lung epithelium from the air inside the branchial cavity to reach the maximal oxygen consumption rate of 26.1 mmol O₂ kg^?1 h^?1. Resting MO₂ was 4.06 mmol O₂ kg^?1 h^?1 in air, but decreased to 3.37 mmol O₂ kg^?1 h^?1 in seawater. Radioactive CO₂ from injected l-lactate is released linearly by the lung. The percent accumulated 14-CO₂ in exhaled air, plotted against time, intersects zero time on the x -axis, indicating rapid gas exchange at the lung surface. The P ₅₀ values for native haemocyanin of 4.89 mm Hg before exercise, and 8.99 mm Hg after exercise, are typical of a high-affinity haemocyanin usually associated with terrestrial crabs. The current notion that Ocypode ceratophthalmus drown when submerged in seawater was not substantiated by our experiments. MO₂ in seawater increased from 3.37 mmol O₂ kg^?1 h^?1 for resting crabs to 5.72 mmol O₂ kg^?1 h^?1 during exercise. When submerged by wave-seawater in the natural environment and during exercise in respirometer-seawater O. ceratophthalmus do not swim but, having a specific density of 1.044, float nearly weightless with a minimum of body movements.     

Aerial and Aquatic Respiration in the River Crab Potamonautes Warreni Calman with Notes on Gill Structure

Summary

The oxygen consumption rate (?O₂) for Potamonauteus warreni Calman (= Potamon warreni (Calman) kept in 25 °C water was 34,4 μmol 1^?1 O₂ kg^?1 and after 72 hours in 98% R.H. air the rate was 31,9 μmol 1^?1 O₂ kg^?1 min^?1. The ?O₂ values for each of the two groups are not significantly different (P > 0,05). The partial oxygen tension of pre-branchial (v = venous) haemolymph (P_vCO₂) is 15,3 mm Hg in water and 13,0 mm Hg in air); partial carbon dioxide tension of pre-branchial (v) haemolymph (P_vCO₂) is 13,2 mm Hg in water and 13,0 mm Hg in air); the total carbon dioxide concentration in pre-branchial (v) haemolymph (C_vCO₂) tot. is 12,3 mmol 1^?1 in air and 13,9 mmol 1^?1 in water) are not significantly different for the two groups (P > 0,05). The haemolymph pH and the lactate concentration for crabs in water was found to be 7,51 and 0,38 mmol 1^?1 respectively. No significant differences were found in pre-branchial haemolymph oxygen tension, carbon dioxide tension, total carbon dioxide content, haemolymph pH, lactate level, chloride concentration, P₅₀ and haemocyanin-oxygen cooperativity in control crabs kept in water, and experimental crabs held in air for 72 hours. The chloride concentration, (327,0 mmol 1^?1) for crabs kept in water does not differ from that of crabs held in air for 72 hours but is at least 15% higher than the sodium concentration (255 mmol 1^?1) for crabs kept in water. The gill surface area is 520 mm² g^?1 wet body mass; on average 9,2 gill platelets (lamellae) can be found on a gill length of one millimetre. Each lamella is spaced 60–70 μm apart, each with a thickness of 30–40 μm. It is concluded that P. warreni may be described as a truly amphibious fresh-water crab.     

Temperaturadaptation der Sauerstoffaffinität des Blutes von Rana esculenta L.

Zusammenfassung Die Sauerstoffaffinität des Blutes (ausgedrückt im Sauerstoffhalbsättigungsdruck P ₅₀) von an 5° C adaptierten Fröschen (Rana esculenta L.) ist bei gleicher Umgebungstemperatur geringer als bei an 20° C adaptierten Fröschen. P ₅₀ bei pH 7,4 und 20° C Umgebungstemperatur betrug 42,4 mm Hg bei den kälte- und 39,1 mm Hg bei den wäremadaptierten Fröschen (Abb. 1). Die Sauerstoffkapazität der kälteangepaßten Frösche von 12,33 ml O₂/100 ml Blut ist höher als die der wärmeangepaßten von 10,43 ml O₂/100 ml Blut (Tabelle 1). Die Bedeutung und die Ursache der Sauerstoffkapazitäts- und Sauerstoffaffinitätsadaptation im Hinblick auf die Stoffwechseladaptation werden diskutiert.

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Temperature adaptation of the oxygen affinity of the blood in Rana esculenta L.

Summary The oxygen affinity of blood (expressed by the oxygen half saturation pressure P ₅₀) in frogs (Rana esculenta L.), adapted to 5° C is less as in frogs, adapted to 20° C at equal experimental temperature. P ₅₀ in cold-adapted frogs related to pH 7.4 and 20° C is 42.4 mm Hg and in heat-adapted frogs 39.1 mm Hg (Fig. 1). The oxygen capacity of the cold-adapted frogs is with 12.33 ml O₂/100 ml blood higher than of the heat-adapted frogs with 10.43 ml O₂/100 ml blood (Table 1). The importance and the origin of the adaptation of the oxygen affinity and capacity in view to the adaptation of the metabolic rate are discussed.

     

Respiratory and metabolic responses of the Australian Yabby Cherax destructor to progressive and sustained environmental hypoxia

The Australian Yabby, Cherax destructor, inhabits occasionally hypoxic water. The respiratory gas, acid-base, metabolite and energetic status of this crayfish was assessed during progressive hypoxia and during 3 h at a water PO₂ of 1.33 kPa. The O₂ affinity of haemocyanin from C. destructor was increased by lactate (Δlog P ₅₀/Δlog[lactate] = −0.111) and by Ca (Δlog P ₅₀/Δlog[Ca] = −0.62) but not by urate. While the non-bicarbonate buffering capacity was low (Δ[HCO₃ ⁻]/ ΔpH=−4.89) the haemocyanin had a low sensitivity to pH changes (ϕ = −0.33). The crayfish showed a compensatory hyperventilation, which induced a respiratory alkalosis, until the water O₂ partial pressure declined below 2.67 kPa, after which the O₂ uptake rate was approximately 10% of normoxic rates. The high haemocyanin-O₂ affinity maintained haemolymph O₂ content during progressive hypoxia despite the normally low arterial O₂ partial pressure of C. destructor. During severe hypoxia, pH decreased but increased lactate aided in maintaining haemocyanin-O₂ saturation. The importance of regulated haemocyanin-O₂ affinity in hypoxic C. destructor was reduced by lowered metabolism, including reduced cardiac output, and the consequent reduction in O₂ requirement. Anaerobiosis became important only at very low PO₂ but thereafter proceeded rapidly, supported by a marked hyperglycaemia. There was no depletion of adenylates, even after 3 h of severe hypoxia. The tail muscle of C. destructor held small amounts of glycogen which would sustain anaerobiosis for a only a few hours. Hypometabolism seems an important hypoxic response but severe hypoxia may encourage the crayfish to breathe air. Accepted: 26 February 1998     

Oxygen transport and cardiovascular responses in skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) exposed to acute hypoxia

Summary Responses to acute hypoxia were measured in skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) (1–3 kg body weight). Fish were prevented from making swimming movements by a spinal injection of lidocaine and were placed in front of a seawater delivery pipe to provide ram ventilation of the gills. Fish could set their own ventilation volumes by adjusting mouth gape. Heart rate, dorsal and ventral aortic blood pressures, and cardiac output were continuously monitored during normoxia (inhalant water (PO ₂>150 mmHg) and three levels of hypoxia (inhalant water PO ₂130, 90, and 50 mmHg). Water and blood samples were taken for oxygen measurements in fluids afferent and efferent to the gills. From these data, various measures of the effectiveness of oxygen transfer, and branchial and systemic vascular resistance were calculated. Despite high ventilation volumes (4–71·min^-1·kg^-1), tunas extract approximately 50% of the oxygen from the inhalant water, in part because high cardiac outputs (115–132 ml·min^-1·kg^-1) result in ventilation/perfusion conductance ratios (0.75–1.1) close to the theoretically ideal value of 1.0. Therefore, tunas have oxygen transfer factors (ml O₂·min^-1·mmHg^-1·kg^-1) that are 10–50 times greater than those of other fishes. The efficiency of oxygen transfer from water in tunas (65%) matches that measured in teleosts with ventilation volumes and order of magnitude lower. The high oxygen transfer factors of tunas are made possible, in part, by a large gill surface area; however, this appears to carry a considerable osmoregulatory cost as the metabolic rate of gills may account for up 70% of the total metabolism in spinally blocked (i.e., non-swimming) fish. During hypoxia, skipjack and yellowfin tunas show a decrease in heart rate and increase in ventilation volume, as do other teleosts. However, in tunas hypoxic bradycardia is not accompanied by equivalent increases, in stroke volume, and cardiac output falls as HR decreases. In both tuna species, oxygen consumption eventually must be maintained by drawing on substantial venous oxygen reserves. This occurs at a higher inhalant water PO₂ (between 130 and 90 mmHg) in skipjack tuna than in yellowfin tuna (between 90 and 50 mmHg). The need to draw on venous oxygen reserves would make it difficult to meet the oxygen demand of increasing swimming speed, which is a common response to hypoxia in both species. Because yellowfin tuna can maintain oxygen consumption at a seawater oxygen tension of 90 mmHg without drawing on venous oxygen reserves, they could probably survive for extended periods at this level of hypoxia.Abbreviations BP_da, BP_va dorsal, ventral aortic blood pressure - C _aO₂, C _vO₂ oxygen content of arterial, venous blood - DO₂ diffusion capacity - E_b, E_w effectiveness of O₂ uptake by blood, and from water, respectively - Hct hematocrit - HR heart rate - PCO₂ carbon dioxide tension - P _aCO₂, P _vCO₂ carbon dioxide tension of arterial and venous blood, respectively - PO₂ oxygen tension - P _aO₂, P _vO₂, P _iO₂, P _cO₂ oxygen tension of arterial blood, venous blood, and inspired and expired water, respectively - pHa, pHv pH of arterial and venous blood, respectively - P_w—b effective water to blood oxygen partial pressure difference - Pg partial pressure (tension) gradient - cardiac output - R vascular resistance - SV stroke volume - SEM standard error of mean - TO₂ transfer factor - U utilization - _g ventilation volume - O₂ oxygen consumption