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Chronically catheterized rainbow trout (Oncorhynchus mykiss) were exposed to 4 mg/l (nominal concentration) of Zn2+ and their arterial oxygen tension measured. When the arterial oxygen tension had decreased to less than 50% of the original, one group of fish was sacrificed, and the structure of the gill secondary lamellae analyzed morphometrically. Two other groups of animals were allowed to recover in clean water and their arterial oxygen tension measured. After a recovery period of 24 h or 48 h the fish were sacrificed and their gills analyzed morphometrically. Associated with the decrease in arterial oxygen tension in the exposure, there was a marked increase in the arithmetic mean distance from water to blood across the gill epithelium, from 4.2 μm in control animals to 9.8 μm in exposed animals, and a consecutive decrease in the calculated morphometric diffusion capacity of the secondary lamellae. In clean water, the arterial oxygen tension fully recovered within 48 h, whereas the arithmetic mean distance from water to blood, although it decreased significantly as compared to the value at the end of the exposure, remained at an elevated level even after 48 h recovery (5.6 μm) as compared to control animals. Thus, in resting fish, diffusion limitation starts to influence the arterial oxygen tension when the arithmetic mean distance from water to blood is approximately 6 μm.

The structure of the gill epithelium of adult whitefish (Coregonus lavaretus) was studied in the laboratory at pH 4.75 and pH 5.75 with and without added aluminium (Altot 150 μg 1−1). The gills of whitefish and perch (Perca fluviatilis) from acidic and circumneutral lakes were also studied. The increase in the mean arithmetic thickness of the lamellar epithelium of whitefish gills was not significant at pH 4.75 + Al, when compared with the control fish. The morphometric diffusing capacity decreased significantly, however. It was also significantly smaller in the gills of the whitefish in the acidic lake than in the circumneutral lake. Perch gills were not hypertrophic in any lake. The water pH or Al concentration had no consistent effect on the number of mucous cells in the gill epithelium of whitefish in the laboratory exposure, but mucous cells in the acidic lake whitefish were hypertrophied. No mucous cells were present in the lamellar region of the perch caught in lakes with a low Al concentration. Al was found inside the gill epithelium of whitefish both at pH 4.75 and pH 5.25 when Al (150 μg 1−1) was added to the water in the laboratory exposure. The gills of the whitefish accumulated Al in an acidic lake, also. Al was found inside the perch gill epithelium when the ambient water pH was circa 5, even when the total Al concentration of the water was low.

The absorption rates of 4-bromophenol, 2,4-dibromophenol, 3,4,5-trichloroguaiacol, and tetrachloroveratrol across fish gill epithelium have been measured in a perfused gill preparation from rainbow trout (Oncorhynchus mykiss). In additional experiments the absorption rate of tetrachloroveratrol was measured in free swimming fish. The absorption rates were perfusion limited in gills perfused with a saline containing 2% polyvinylpyrrolidone MW 40,000 (PVP-40) as a plasma protein substitute. Replacement of PVP-40 with 2% bovine serum albumin increased the absorption rates 5-10 times, and in this case rates were not perfusion limited. Furthermore, they were not ventilation limited at ventilatory water flow rates between 0.45 and 1.5 liter/(min 100 g). Our observations suggest that the absorption rates in the presence of albumin are limited by diffusion in the epithelial cell layers. The rates measured in the perfused gills are in close agreement with those measured in vivo. We conclude that the preparation is well suited to investigate the mechanisms behind the absorption of hydrophobic compounds across fish gills and how the absorption rate is related to the physicochemical properties of the compounds, as well as to environmental factors.

Summary The respiratory effects of low pH were studied in the field and laboratory. The thickness of gill epithelium was similar in perch from an acid lake to that in perch from a neutral lake. This observation suggests that gill hypertrophy was not the primary cause of death of fish in the acid lake. A 24-h acid exposure to pH 5 did not affect the blood respiratory properties and ion concentrations of rainbow trout at all. However, when the acid-exposed fish were further subjected to hypoxia, the red cell water content and chloride concentration increased, red cell ATP concentration decreased, and plasma so­ dium and chloride concentration decreased. These changes were not seen in animals maintained in neutral water and subjected to hypoxia. The loss of ions may be caused by hypoxia-induced increase in the diffusion area of the gills, and hyperventilation, which initially increases the diffusion gradient across gill epithelium.

fish to zinc and measur~ residual oxygen concentra­ tion after sealed jar test, secondly by putting healthy fish in sealed jars with known concentration of zinc and measuring residual oxygen concentration, and canparing both the above results with residual oxygen levels of healthy fish in clean water • .Additionally we have investigated the role of gill damage in deter­ mining the residual oxygen concentration by ligating gill arches, thus lllnit~ effective gill surface area, and carrying out sealed jar tests in clean water. In order to see if tissue oxygen consump­ tion may influence the residual oxygen concentration, the tests have been carried out at different tanperatures.

Rainbow trouts (Salmo gairdneri Richardson) were exposed to sublethal concentrations of dehydroabietic acid (DHAA) (1.55 mg.1−1) or zinc (1.25 mg. I−1) in a semistatic system for 96 h. Different secondary lamellar structures were analysed morphometrically from oriented 1 μm sections. DHAA induced hypertrophy of the epithelium. The number of chloride cells was higher than in control gills. Zinc induced detachment of the epithelium from the underlying pillar cell system. The fusion of secondary lamellae reduced the surface area of the epithelium. Both substances induced a marked vasoconstriction in the secondary lamellar capillaries. The haematocrit value of the secondary lamellar blood was higher in both the exposed groups than in the control group.

1. Gill resistance (Rg) and oxygen transfer (To2) were measured in perfused rainbow trout gills. After perfusion the gills were analysed morphometrically. 2. Rg increased with 40% and To2 decreased with 91% during 60 min perfusion without vasoactive substances in the perfusion Ringer. 3. In presence of 10 micro M adrenaline both Rg and To2 remained stable at their starting levels throughout the experimental period (60 min). 4. With 0.2 micro M acetylcholine and 10 micro M adrenaline in Ringer Rg increased nearby to the same extent as in perfusions without vasoactive agents, while To2 was not significantly affected. 1 micro M acetylcholine increased Rg with 76% and decreased To2 with 51%. 5. The changes in Rg and To2 could be explained by structural changes in the secondary lamellae.