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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 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.

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.

Rainbow trout ( Salmo gairdneri ) fingerlings were exposed to sodium hypochlorite (NaClO) in semistatic LC 50 tests for 48 h in dechlorinated Helsinki tap water and water from Lake Vatia (21 km downstream from a kraft pulp mill). Fish were also exposed for 17 days to Lake Vatia water after which an acute exposure to sodium hypochlorite was made, for which physiological samples were analyzed. The 48 h LC 50 of bleached kraft pulp mill effluent was 35%. The 17 days' caging period in the recipient water 21 km downstream from the mill caused disturbances in osmotic regulation, depletion of liver glycogen and inhibition of liver UDP-glucuronosyltransferase. The gills of the exposed fish were microscopically examined. The epithelium of the secondary lamellae in the NaClO exposed gills was thicker than in the controls (48 h LC 50 0.35 mg Cl 2 /l) and the lamellae were curled with constricted blood spaces. The gill epithelium of fish exposed to NaClO in water from Lake Vatia was swollen and vacuolized (48 h LC 50 0.09 mg Cl 2 /l). The gills of fish exposed to 20% pulp mill effluent as well as the gills of fish acclimatized for 17 days in Lake Vatia water were quite normal in structure. However, after hypochlorite exposure the gills of the pre-acclimatized fish were almost totally destroyed (48 h LC 50 0.07 mg Cl 2 /l).