Mode of locomotion places selective pressures on Antarctic and temperate labriform swimming fish

The physiological responses to exercise and stress of the Antarctic labriform swimmer Pagothenia borchgrevinki were compared to the temperate labriform swimmers Notolabrus celidotus and Notolabrus fucicola. Basic swimming characteristics were very similar amongst the three species with P. borchgrevinki showing a reduced capacity for exercise. P. borchgrevinki showed large increases in haematocrit (Hct) following exercise that were not seen in the temperate species. Lactate dehydrogenase (LDH) activities were high in the white myotomal muscle from the Antarctic fish, with a distinct indication of metabolic cold adaptation in this enzyme. Nevertheless, although the temperate fish showed elevated muscle lactate concentrations following either exercise or electrical stimulation the Antarctic fish did not. The data suggest that poor anaerobic performance of white muscle is associated with the labriform mode of locomotion.

Physiological basis of temperature-dependent biogeography: trade-offs in muscle design and performance in polar ectotherms

Polar, especially Antarctic, oceans host ectothermic fish and invertebrates characterized by low-to-moderate levels of motor activity; maximum performance is reduced compared with that in warmer habitats. The present review attempts to identify the trade-offs involved in adaptation to cold in the light of progress in the physiology of thermal tolerance. Recent evidence suggests that oxygen limitations and a decrease in aerobic scope are the first indications of tolerance limits at both low and high temperature extremes. The cold-induced reduction in aerobic capacity is compensated for at the cellular level by elevated mitochondrial densities, accompanied by molecular and membrane adjustments for the maintenance of muscle function. Particularly in the muscle of pelagic Antarctic fish, among notothenioids, the mitochondrial volume densities are among the highest known for vertebrates and are associated with cold compensation of aerobic metabolic pathways, a reduction in anaerobic scope, rapid recovery from exhaustive exercise and enhanced lipid stores as well as a preference for lipid catabolism characterized by high energy efficiency at high levels of ambient oxygen supply. Significant anaerobic capacity is still found at the very low end of the activity spectrum, e.g. among benthic eelpout (Zoarcideae).

In contrast to the cold-adapted eurytherms of the Arctic, polar (especially Antarctic) stenotherms minimize standard metabolic rate and, as a precondition, the aerobic capacity per milligram of mitochondrial protein,thereby minimizing oxygen demand. Cost reductions are supported by the downregulation of the cost and flexibility of acid—base regulation. At maintained factorial scopes, the reduction in standard metabolic rate will cause net aerobic scope to be lower than in temperate species. Loss of contractile myofilaments and, thereby, force results from space constraints due to excessive mitochondrial proliferation. On a continuum between low and moderately high levels of muscular activity, polar fish have developed characteristics of aerobic metabolism equivalent to those of high-performance swimmers in warmer waters. However, they only reach low performance levels despite taking aerobic design to an extreme.

Juvenile sturgeon exhibit reduced physiological responses to exercise

Experiments were conducted to determine the physiological responses to exercise of Atlantic sturgeon (Acipenser oxyrhynchus) and shortnose sturgeon (A. brevirostrum). We measured the rates of oxygen consumption and ammonia excretion in both species and a variety of physiological parameters in both muscle (e.g. lactate, glycogen, pyruvate, glucose and phosphocreatine concentrations) and blood (e.g. osmolality and lactate concentration) in juvenile shortnose sturgeon following 5 min of exhaustive exercise.

In both species, oxygen consumption and ammonia excretion rates increased approximately twofold following exhaustive exercise. Post-exercise oxygen consumption rates decreased to control levels within 30 min in both sturgeon species, but post-exercise ammonia excretion rates remained high in Atlantic sturgeon throughout the 4 h experiment. Resting muscle energy metabolite levels in shortnose sturgeon were similar to those of other fish species, but the levels decreased only slightly following the exercise period and recovery occurred within an hour. Under resting conditions, muscle lactate levels were low (<1 μmol g–1) but they increased to approximately 6 μmol g–1 after exercise, returning to control levels within 6 h. Unlike similarly stressed teleost fish, such as the rainbow trout, plasma lactate levels did not increase substantially and returned to resting levels within 2 h. Plasma osmolality was not significantly affected by exercise in shortnose sturgeon.

Taken together, these results suggest that shortnose and Atlantic sturgeon do not exhibit the physiological responses to exhaustive exercise typical of other fish species. They may possess behavioural or endocrinological mechanisms that differ from those of other fishes and that lead to a reduced ability to respond physiologically to exhaustive exercise.

Limits to exhaustive exercise in fish

Exercise to exhaustion leads to severe metabolic, acid-base and ionic changes in fish. It has been shown that several abiotic and biotic factors can limit burst exercise performance and the recovery process in fish. This article reviews the importance of body size, temperature, fasting/starvation and training on the ability of fish to perform and recover from exhaustive exercise. It is concluded that the constraints placed on a fish prior to and following exercise reflects the large intra-specific variability in the physiological response to exercise in fish.

High-energy turnover at low temperatures: recovery from exhaustive exercise in Antarctic and temperate eelpouts

Earlier work on Notothenioids led to the hypothesis that a reduced glycolytic capacity is a general adaptation to low temperatures in Antarctic fish. In our study this hypothesis was reinvestigated by comparing changes in the metabolic status of the white musculature in two related zoarcid species, the stenothermal Antarctic eelpout Pachycara brachycephalum and the eurythermal Zoarces viviparus during exercise and subsequent recovery at 0 degreesC. In both species, strenuous exercise caused a similar increase in white muscle lactate, a drop in intracellular pH (pHi) by about 0.5 pH units, and a 90% depletion of phosphocreatine. This is the first study on Antarctic fish that shows an increase in white muscle lactate concentrations. Thus the hypothesis that a reduced importance of the glycolytic pathway is characteristic for cold-adapted polar fish cannot hold. The recovery process, especially the clearance of white muscle lactate, is significantly faster in the Antarctic than in temperate eelpout. Based on metabolite data, we calculated that during the first hour of recovery aerobic metabolism is increased 6.6-fold compared with resting rates in P. brachycephalum vs. an only 2.9-fold increase in Z. viviparus. This strong stimulation of aerobic metabolism despite low temperatures may be caused by a pronounced increase of free ADP levels, in the context of higher levels of pHi and ATP, which is observed in the Antarctic species. Although basal metabolic rates are identical in both species, the comparison of metabolic rates during situations of high-energy turnover reveals that the stenothermal P. brachycephalum shows a higher degree of metabolic cold compensation than the eurythermal Z. viviparus. Muscular fatigue after escape swimming may be caused by a drop of the free energy change of ATP hydrolysis, which is shown to fall below critical levels for cellular ATPases in exhausted animals of both species.

Maintenance of high extracellular pH does not influence cell pH or metabolism in submerged anoxic bullfrogs

We compared extracellular and intracellular acid-base states in paralyzed bullfrogs subjected to 4 h of anoxic submergence at 15 degrees C with or without maintenance of extracellular pH at preanoxic levels by bicarbonate infusion. We also assessed anaerobic metabolism under these conditions by measuring tissue lactate and glycogen concentrations in liver, heart, and skeletal muscle. Although bicarbonate infusion resulted in a significantly higher arterial blood pH (pHe) than saline infusion, intracellular pH (pHi) of heart and skeletal muscle, as determined by the DMO equilibration technique, were not significantly different after 4 h of anoxia. We were also unable to demonstrate any differences in anaerobic metabolic rate, since both tissue lactate accumulation and glycogen depletion were identical in bicarbonate- and saline-infused frogs in the tissues studied. We conclude that (1) alterations in the extracellular acid-base state by bicarbonate infusion are not necessarily reflected in the intracellular compartment, perhaps due to powerful intracellular buffering processes, and (2) maintenance of an alkaline extracellular pH during anoxia in bullfrogs does not influence the anaerobic metabolic rate. We could not, however, rule out a possible role for intracellular pH in regulating anaerobic metabolism during anoxia in frogs.

Extracellular and intracellular acid-base effects of submergence anoxia and nitrogen breathing in turtles

We compared extracellular and intracellular acid-base state in turtles (Chrysemys picta bellii) subjected to anoxic submergence to turtles made anoxic by N2-breathing. Measurements made on control animals and on animals after 1, 2, 4, or 6 h of anoxia included blood pH, PO2, PCO2, and lactate as well as liver, heart, skeletal muscle, and brain pHi (using DMO equilibration), lactate, and glycogen concentrations. We hypothesized that the anaerobic metabolic rate of submerged turtles would be depressed by the more severe extra- and intracellular acidosis, and that this would be indicated by reduced lactate accumulation and glycogen depletion. Submerged turtles became extremely acidemic due to a combined metabolic and respiratory acidosis and had significantly lower arterial pH than N2-breathing animals (6.98 and 7.34, respectively, after 6 h). In spite of this disparity in pHa, 6 h pHi values for liver, heart, and brain were similar. Likewise, our data on glycogen depletion and lactate accumulation at h 6 in these tissues suggest no dramatic differences in anaerobic metabolic rate. While skeletal muscle pHi was somewhat lower at h 6 in the submerged group (6.73 vs 6.91 for N2-breathers), we observed no differences in either glycogen depletion or lactate accumulation in this tissue between our two treatments. Thus, at h 6, in spite of a 0.37 pH unit difference in pHa and a nearly 70 mm Hg difference in arterial and presumably cytosolic PCO2, pHi and tissue lactate and glycogen concentrations were similar. These results can be explained if the in vivo intracellular buffer values (beta) of turtle tissues are very high. We conclude that extracellular acid-base state is not necessarily reflected intracellularly in vivo in turtles and care must be taken in extrapolating from one compartment to another when attempting to make inferences about metabolic depression or acid-base regulation in this species.

Control of ventilation in the hypercapnic skate Raja ocellata: II. Cerebrospinal fluid and intracellular pH in the brain and other tissues

This study examined the possible role(s) of central acid-base stimuli in the increase in ventilation induced by hypercapnia in the skate, a response that is not due to an O2 signal (Graham et al., Respir. Physiol., 1990, 80: 251-270). Skate were sampled for cerebrospinal fluid (CSF) acid-base status, intracellular pH of the brain (14C-DMO method), and pHi in other tissues throughout 24 h of exposure to PICO2 = 7.5 Torr. CSF PCO2 rapidly equilibrated with the elevated PaCO2. Despite the much lower non-HCO3- buffer capacity in the CSF, CSF pH was not depressed to the same extent as blood pHa. CSF pH was also regulated rapidly, returning to control levels by 8-10 h, whereas pHa remained significantly depressed at 24 h. Similarly, the pHis of the weakly buffered brain and heart ventricle were initially compensated more rapidly than those of more strongly buffered white muscle and red blood cells. However, brain pHi adjustment slowed markedly after 4 h and stabilized at only 70% compensation by 20-24 h, suggesting that brain intracellular acidosis may play a role in the long-term increase in ventilation. CSF and brain were the only compartments which did not exhibit an apparent compounding metabolic acidosis during the initial stages of hypercapnic exposure. While these results illustrate the primacy of central acid-base regulation, they do not support a role for CSF pH in the long-term elevation of ventilation in response to hypercapnia. Depressions in pHa and brain pHi appear the two most likely candidates for proximate stimuli.

Influence of pH and temperature on force development and shortening velocity in skinned muscle fibres from fish

Three species of fish were studied: Atlantic cod (Gadus morhua), sculpin (Myoxocephalus scorpius) (from the North Sea, temperature 2 to 12°C) andNotothenia neglecta (from Antarctica, temperature -2 to +2°C). Single fast muscle fibres were isolated from anterior myotomes and skinned with detergent in order to directly determine the effects of pH and temperature on force production and shortening velocity.In all species maximum force production (Po) was independent of pH over the range 7.3-8.0. Decreasing the pH from 7.3 to 6.6 reduced maximum force by 28% in fibres fromG. morhua andN. neglecta but had no effect on fibres fromM. scorpius. The depression in maximum force with acidosis was accompanied by a proportional decrease in stiffness and an increase in the rate of force recovery after stretch.Unloaded contraction velocity of cod fibres (Vmax) showed a pH optimum at around pH 7.6 decreasing by 31% at pH 6.6. Vmax of fibres from the other species was independent of pH over the range 6.6-8.0.The effects of pH on Po and Vmax were similar at 0 and 10°C. Thus for maximally activated fibres both force and contraction velocity are independent of temperature induced changes in pH. In some species acidosis depresses contractility and is likely to be a contributory factor to muscle fatigue.