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Notes: Abstract  We found blood from bigeye tuna (Thunnus obesus) to have a significantly higher O2 affinity than blood from other tunas. Its P50 (partial pressure of oxygen, PO2 required to reach 50% saturation) was 1.6 to 2.0 kPa (12 to 15 mmHg) when equilibrated with 0.5% CO2. Previous studies employing similar methodologies found blood from yellowfin tuna (T. albacares), skipjack tuna (Katsuwonus pelamis), and kawakawa (Euthynnus affinis) to have a P50 of 2.8 to 3.1 kPa (21 to 23 mmHg). These observations suggest that bigeye tuna are more tolerant of low ambient oxygen than other tuna species, and support similar conclusions derived from laboratory whole-animal studies, depth-of-capture data, and directly-recorded vertical movements of fish in the open ocean. We also found the O2 affinity of bigeye tuna blood to be essentially unaffected by a 10 C° open-system temperature change (as is the blood of all tuna species studied to date). The O2 affinity of bigeye tuna blood was, however, more affected by a 10 C° closed-system temperature change than the blood of any tuna species yet examined. In other words, bigeye tuna blood displayed a significantly enhanced Bohr effect (change in log P50 per unit change in plasma pH at P50) when subjected to the inevitable changes in partial pressure of carbon dioxide (PCO2) and plasma pH that accompany closed-system temperature shifts, than when subjected to changes in plasma pH accomplished by changing PCO2 alone. In vivo, the resultant large decrease in O2 affinity (i.e. the increase in P50) that occurs as the blood of bigeye tuna is warmed during its passage through the vascular counter-current heat exchangers ensures adequate rates of O2 off-loading in the swimming muscles of this high-energy-demand teleost.

Notes: Primary and secondary stress responses were measured in wild Scorpis violaceus subjected to burst swimming from angling. Fish were blood sampled from 20 s to 30 min after hooking. Consequent rises in plasma adrenaline (14–316 nmol l−1), noradrenaline (25–345 nmol l−1), and cortisol (0.4–197 ng ml−1) correlated with time since capture, and plasma lactate (0.1–12.2 mmol l−1) reflected work done during intense exercise. Haemoglobin concentration and haematocrit also increased with exercise, and erythrocyte swelling occurred. Wild S. violaceus demonstrated a spontaneity and intensity of exercise not seen in fish acclimatized to aquarium conditions. By contrast, the stress responses of fish in captivity, despite careful husbandry, differed qualitatively and quantitatively from those in the wild. Cannulated fish had higher resting plasma cortisol concentrations (61.9±9.5 ng ml−1) than did rapidly caught wild fish (〈5 ng ml−1) and these values were not significantly changed with burst swimming. Catecholamine secretion, possibly suppressed by cortisol, was insufficient to cause erythrocyte swelling. Erythrocyte nucleotides do not play a role in exercise, but are elevated in captive fish. It is hypothesized that primary endocrine responses are triggered by higher cortical processing of sensory information which is fundamentally different in the natural environment.

Notes: Abstract  Burst swimming increased haematocrit (partly from erythrocyte swelling) in the cryopelagic nototheniid Pagothenia borchgrevinki, but not in the benthic species Trematomus bernacchii. Erythrocyte nucleotides, which regulate haemoglobin-oxygen affinity, remained constant. Plasma cortisol was high in all captive fish and raised questions about the effects of chronic stress on metabolic measurements from Antarctic fish held in captivity. Glycogen was reduced in white trunk muscle, but not in the red pectoral muscle of exercised P. borchgrevinki. Red pectoral muscle glycogen was nearly 3 times higher in T. bernacchii than in P. borchgrevinki but post-exercise lactate rises were modest. Lactate values were, however, higher in exercised P. borchgrevinki white muscle than in T. bernacchii, and correlated with muscle-buffering capacity. Resting adenylate energy charge (AEC) was unexpectedly low in both species and reduced with exercise only for white muscle in P. borchgrevinki. While it appears that the capacity for burst swimming is limited by endogenous metabolic fuels, confirmation of low resting values of ATP and AEC in Antarctic fishes requires the development of methods that maintain high phosphocreatine levels in the muscle.

Notes: Abstract In fishes, catecholamines increase red blood cell intracellular pH through stimulation of a sodium/proton (Na+/H+) antiporter. This response can counteract potential reductions in blood O2 carrying capacity (due to Bohr and Root effects) when plasma pH and intracellular pH decrease during hypoxia, hypercapnia, or following exhaustive exercise. Tuna physiology and behavior dictate exceptionally high rates of O2 delivery to the tissues often under adverse conditions, but especially during recovery from exhaustive exercise when plasma pH may be reduced by as much as 0.4 pH units. We hypothesize that blood O2 transport during periods of metabolic acidosis could be especially critical in tunas and the response of rbc to catecholamines elevated to an extreme. We therefore investigated the in vitro response of red blood cells from yellowfin tuna (Thunnus albacares) and skipjack tuna (Katsuwonus pelamis) to catecholamines. Tuna red blood cells had a typical response to catecholamines, indicated by a rapid decrease in plasma pH. Amiloride reduced the response, whereas 4,4′diisothiocyanatostilbene-2,2′-disulphonic acid enhanced both the decrease in plasma pH and the increase in intracellular pH. Changes in plasma [Na+], [Cl−], and [K+] were consistent with the hypothesis that tuna red blood cells have a Na+/H+ antiporter similar to that described for other teleost red blood cells. Red blood cells from both tuna species were more responsive to noradrenaline than adrenaline. At identical catecholamine concentrations, the decrease in plasma pH was greater in skipjack tuna blood, the more active of the two tuna species. Based on changes in plasma pH, the response of red blood cells to catecholamines from both tuna species was less than that of rainbow trout (Oncorhynchus mykiss) red blood cells, but greater than that of cod (Gadus morhua) red blood cells. Noradrenaline had no measurable influence on the O2 affinity of skipjack tuna blood and only slightly increased the O2 affinity of yellowfin tuna blood. Our results, therefore, do not support our original hypothesis. The catecholamine response of red blood cells from high-energy-demand teleosts (i.e., tunas) is not enhanced compared to other teleosts. There are data on changes in cardio-respiratory function in tunas caused by acute hypoxia and modest increases in activity, but there are no data on the changes in cardio-respiratory function in tunas accompanying the large increases in metabolic rate seen during recovery from exhaustive exercise. However, we conclude that during those instances where high rates of O2 delivery to the tissues are needed, tunas' ability to increase cardiac output, ventilation volume, blood O2 carrying capacity, and effective respiratory (i.e., gill) surface area are probably more important than are the responses of red blood cells to catecholamines. We also use our data to investigate the extent of the Haldane effect and its relationship to blood O2 and CO2 transport in yellowfin tuna. Yellowfin tuna blood shows a large Haldane effect; intracellular pH increases 0.20 units during oxygenation. The largest change in intracellular pH occurs between 40–100% O2 saturation, indicating that yellowfin tuna, like other teleosts, fully exploit the Haldane effect over the normal physiological range of blood O2 saturation.