D. J. Randall

7 - The Respiratory and Circulatory Systems During Exercise

Exercise is accompanied by an increase in the rate of energy conversion from the resting rate. This increase provides for the energy requirements of the locomotory muscles, as well as for extra work performed by the heart and respiratory muscles in supplying oxygen demanded in exercise. This chapter discusses the ability of fish to increase the rate of gas exchange at the gills and tissues and the changes that occur in the components of the respiratory and circulatory systems facilitating this increase in gas exchange. On many occasions, circulatory and respiratory adjustments have been measured along with indirect estimates of metabolism during exercise. Metabolism, power output, and swimming speed are related, but, for virtually all fish species, the relationship is too imprecisely known to use any one in calculations of the others. Consequently, before discussing respiratory and circulatory compensations in exercise, the literature on the metabolic adjustments to exercise have been reviewed in an attempt to establish criteria that could be used in assessing the exercise performance of a particular fish.

Ammonia toxicity, tolerance, and excretion

Ammonia is an unusual toxicant in that it is produced by, as well as being poisonous to, animals. In aqueous solution ammonia has two species, NH3 and NH4+, total ammonia is the sum of [NH3] + [NH4+] and the pK of this ammonia/ammonium ion reaction is around 9.5. The NH3/NH4+ equilibrium both internally in animals and in ambient water depends on temperature, pressure, ionic strength, and pH; pH is most often of greatest significance to animals. Elevated ammonia levels in the environment are toxic. Temperature has only minor effects on ammonia toxicity expressed as total ammonia in water, and ionic strength of the water can influence ammonia toxicity, but pH has a very marked effect on toxicity. Acid waters ameliorate, whereas alkaline waters exacerbate ammonia toxicity. The threshold concentration of total ammonia ([NH3] + [NH4+]) resulting in unacceptable biological effects in freshwater, promulgated by the EPA (1998), is 3.48 mg N/liter at pH 6.5 and 0.25 mg N/liter at pH 9.0. There is only a relatively small saltwater data set, and a paucity of data on ammonia toxicity in marine environments, particularly chronic toxicity. The national criteria promulgated in the EPA (1989) saltwater document is a criterion continuous concentration (chronic value) of 0.99 mg N/liter total ammonia and a criterion maximum concentration (half the mean acute value) of 6.58 mg N/liter total ammonia, somewhat less than the equivalent freshwater pH 8.0 values of 1.27 and 8.4 mg N/liter total ammonia, respectively. This is consistent with marine species being somewhat more sensitive to ammonia than freshwater species. Toxicity studies are usually carried out on unfed, resting fish in order to facilitate comparison of results. Based on recent studies, however, environmental stresses, including swimming, can have dramatic effects on ammonia toxicity. It is also clear that feeding results in elevated postprandial body ammonia levels. Thus, feeding will probably also exacerbate ammonia toxicity. Fish may be more susceptible to elevated ammonia levels during and following feeding or when swimming. Thus, present ammonia criteria may fail to protect migrating fish and may be inappropriate for fish fed on a regular basis. Most teleost fish are ammonotelic, producing and excreting ammonia by diffusion of NH3 across the gills. They are very susceptible to elevated tissue ammonia levels under adverse conditions. Some fish avoid ammonia toxicity by utilizing several physiologic mechanisms. Suppression of proteolysis and/or amino acid catabolism may be a general mechanism adopted by some fishes during aerial exposure or ammonia loading. Others, like the mudskipper, can undergo partial amino acid catabolism and use amino acids as an energy source, leading to the accumulation of alanine, while active on land. Some fish convert excess ammonia to less toxic compounds including glutamine and other amino acids for storage. A few species have active ornithine—urea cycles and convert ammonia to urea for both storage and excretion. Under conditions of elevated ambient ammonia, the mudskipper P. schlosseri can continue to excrete ammonia by active transport of ammonium ions. There are indications that some fish may be able to manipulate the pH of the body surface to facilitate NH3 volatilization during aerial exposure, or that of the external medium to lower the toxicity of ammonia during ammonia loading. Future investigation of these aspects of “environmental ammonia detoxification” may produce new information on how fish avoid ammonia intoxication.

Ammonia distribution and excretion in fish

This paper reviews the literature concerning ammonia production, storage and excretion in fish. Ammonia is the end product of protein catabolism and is stored in the body of fish in high concentrations relative to basal excretion rates. Ammonia, if allowed to accumulate, is toxic and is converted to less toxic compounds or excreted. Like other weak acids and bases, ammonia is distributed between tissue compartments in relation to transmembrane pH gradients. NH3 is generally equilibrated between compartments but NH4+ is distributed according to pH. Ammonia is eliminated from the blood upon passage through the gills. The mechanisms of branchial ammonia excretion vary between different species of fish and different environments, and primarily involves NH3 passive diffusion and NH4+/Na+ exchange. Water chemistry near the gill surface may also be important to ammonia excretion, but a more accurate measurement of the NH3 gradient across the gill epithelium is required before a more detailed analysis of NH3 and NH4+ excretion can be made.

5 Oxygen and Carbon Dioxide Transfer Across Fish Gills

Publisher Summary This chapter discusses oxygen and carbon dioxide transfer across fish gills. The gills of fish are the major site, though not the only one, for oxygen and carbon dioxide transfer. The skin and fins may also serve in this capacity and many fish have evolved accessory air-breathing organs. These may be the modifications of the skin, buccal, pharyangeal, or gill surface, or they may be the regions of the gut or the swim bladder. In general, the gills of fish are the major pathway for oxygen and carbon dioxide transfer between the environment and the body tissues. The oxygen stores within the body, with the exception of that in the swim bladder, are small. Carbon dioxide stores in the body are large as compared with the rate of production. At resting rates of CO 2 production, it would take the animal several hours to accumulate the equivalent of the body CO 2 stores. Thus, minor changes in the magnitude of the CO 2 stores—for example, related to the acidification of the body tissues—can have a marked effect on CO 2 excretion across the gills.

The effects of changes in pH and Pco2, in blood and water on breathing in rainbow trout, Salmo gairdneri

The effect of sustained hypercapnia on the acid-base balance and gill ventilation in rainbow trout, Salmo gairdneri, was studied. The response to an increase in PICO2 from 0.3 to 5.2 mm Hg was a five-fold increase in gill ventilation volume and a slight increase in breathing frequency. There was a concomitant rise in PACO2 and an immediate fall in pHa. If PICO2 was maintained at 5.2 mm Hg for several days, ventilation volume gradually returned to the initial, prehypercapnic level within three days. Arterial pH also returned to the initial level within 2-3 days. These results are consistent with the hypothesis that under these conditions fish regulate pH via HCO3/C1 exchange across the gills rather than by changes in ventilation and subsequent adjustment of PACO2. A reduction in environmental pH causes a reduction in pHa but only a slow gradual increase in VG. Injections of HC1 or NaHCO3 into the blood have opposite effects on pHa but both cause a marked increase in VG. It is concluded that a rise in PACO2 results in a rise in VG and that changes in pH in blood or water have little direct effect on VG in rainbow trout. Possible location for receptors involved in this reflex response are discussed.

Acute exposure to graded levels of hypoxia in rainbow trout (Salmo gairdneri): metabolic and respiratory adaptations

We have studied the mechanisms of acute hypoxia tolerance in rainbow trout (Salmo gairdneri). Fish held at 9 degrees C were exposed to various levels of hypoxia for 24 h. At an environmental PO2 of 30 Torr, the fish showed an initial plasma acidosis probably of metabolic origin which was subsequently offset such that blood pH returned to normal within about 4 h. Over this time period, red cell pH was maintained constant. Comparing the effects of different levels of hypoxia following 24 h exposure, oxygen consumption of the animal remained unchanged over a broad range of inspired oxygen tensions but declined by over 30% of normoxic values at inspired water PO2 levels of 80 Torr. This appeared to be a true metabolic depression because signs of increased anaerobic metabolism did not occur until there was a further reduction in water oxygen levels. Rainbow trout appear to be able to maintain a relatively high energy status in their white muscle during 24 h exposure to severe hypoxia (water PO2 = 30 Torr). As the level of hypoxia was intensified, there was a reduction in the oxygen gradient across the gills, probably facilitated in part by the release of catecholamines into the blood. The erythrocytic ATP: Hb4 molar ratios declined with increasing hypoxic stress as did the pH gradient between the erythrocyte and plasma. The overall effect was no change in Hb O2-affinity after 24 h exposure to severe hypoxia.

The relationship between gas and ion transfer across the gills of fishes

1. 1. The relationship between perfusion pressure and flow of saline through the gills of an isolated head of rainbow trout (Salmo gairdneri) was determined. 2. 2. Noradrenaline, adrenaline and isoprenaline decrease the vascular resistance to flow through the gills in the isolated head preparation, indicating the presence of both α- and s-adrenergic receptors in the gills. 3. 3. The rate of 22Na loss across the gills of intact trout was determined in fish with ligated urogenital papillae. 4. 4. The rate of 22Na loss was higher in “active” than in “quiet” fish. Adding noradrenaline or isoprenaline to the water containing the fish also increased the rate of 22Na loss across the gills. 5. 5. The significance of these observations is discussed.

9 Proton Pumps in Fish Gills

Publisher Summary This chapter examines the importance of proton pumps in fish gills. The gills of the fish are the primary site of gas exchange, acid–base regulation, and osmoregulation. Proton-translocating-ATPases are integral membrane proteins that vectorially translocate H + from one surface to the other. The H + -ATPases in the plasma membrane of eukaryotic cells are classified as phosphorylated ion motive enzymes because they form a covalent phosphorylated intermediate as a part of the reaction cycle. The chapter presents a study in which the localization of proton pumps in gill epithelia of the rainbow trout was elucidated by immunofluorescence microscopy using rabbit polyclonal antibodies against the 70-kDa the subunit of clathrin-coated vesicle H + -ATPase from the bovine brain. It has also been found that hypercapnia treatment in freshwater fish induces a rapid increase in the N -ethylmaleimide-sensitive ATPase activity, which stabilizes at a level twice that of normocapnia. The fact that Ca 2+ level has no effect on proton-ATPase activity in gill epithelium when water Na + concentration is high indicates that water sodium level has a predominant regulatory effect on proton pump recruitment.

Concentrations of persistent lipophilic compounds in fish are determined by exchange across the gills, not through the food chain

Uptake of persistent lipophilic toxicants in fish occurs via the food and by transfer across the body surface, notably the gills. Flux rates of most lipid soluble toxicants across the gills is rapid and the animal must eat at very high rates for feeding to have a significant effect on toxicant concentration in the body. The relative rates of uptake via feeding and transfer across the gills are analyzed from a theoretical and experimental standpoint. At the low feeding rates typical of fish, the uptake of toxicants in the food can be ignored when estimating toxicant body concentration.

Oxygen uptake and transport during hypoxic exposure in the sturgeon Acipenser transmontanus

Gill ventilation, stroke volume and frequency, %O2 utilization and oxygen uptake, and dorsal aortic blood oxygen tension, content, pH and oxygen affinity have been determined during normoxia and during a range of hypoxic exposures in the sturgeon, Acipenser Transmontanus. In air-equilibrated water gill ventilation was 350 ml/kg/min, % utilization was 35--40%, and oxygen uptake at 15 degrees C was 55--60 ml O2/kg/h. Dorsal aortic blood PO2 was 90 mm Hg and blood O2 content at a normal pHa of 7.84 was 7.0 vol%. Vg fell considerably through a reduction in branchial stroke volume when PIO2 was reduced from 150 to 100 mm Hg. Although % utilization remained unchanged, VO2 was halved, clearly identifying Acipenser as an O2 conformer with a critical O2 tension just below air saturation. At a PIO2 of 60 mm Hg VO2 was only 15% of that at normoxic levels falling to only 5% at a PIO2 of 30 mm Hg. There was no hypoxic bradycardia. There was no repayment of an oxygen debt even after severe hypoxic exposure in Acipenser, and pHa remained unchanged under all experimental conditions, a response incompatible with lactate or succinate production. It is concluded that the sturgeon reduces total energy expenditure during hypoxic exposure, rather than switching from aerobic to anaerobic metabolism.