Ionoregulatory Aspects of the Osmorespiratory Compromise during Acute Environmental Hypoxia in 12 Tropical and Temperate Teleosts.

In the traditional osmorespiratory compromise, as seen in the hypoxia-intolerant freshwater rainbow trout (Oncorhynchus mykiss), the branchial modifications that occur to improve O2 uptake during hypoxia result in unfavorable increases in the fluxes of ions and water. However, at least one hypoxia-tolerant freshwater species, the Amazonian oscar (Astronotus ocellatus), shows exactly the opposite: decreased branchial flux rates of ions, water, and nitrogenous wastes during acute hypoxia. In order to find out whether the two strategies were widespread, we used a standard 2-h normoxia, 2-h hypoxia (20%-30% saturation), 2-h normoxic recovery protocol to survey 10 other phylogenetically diverse tropical and temperate species. Unidirectional influx and efflux rates of Na(+) and net flux rates of K(+), ammonia, and urea-N were measured. The flux reduction strategy was seen only in one additional species, the Amazonian tambaqui (Colossoma macropomum), which is similarly hypoxia tolerant and lives in the same ion-poor waters as the oscar. However, five other species exhibited evidence of the increased flux rates typical of the traditional osmorespiratory compromise in the trout: the rosaceu tetra (Hyphessobrycon bentosi rosaceus), the moenkhausia tetra (Moenkhausia diktyota), the bluegill sunfish (Lepomis macrochirus), the zebra fish (Danio rerio), and the goldfish (Carassius auratus). Four other species exhibited no marked flux changes during hypoxia: the cardinal tetra (Paracheirodon axelrodi), the hemigrammus tetra (Hemigrammus rhodostomus), the pumpkinseed sunfish (Lepomis gibbosus), and the Atlantic killifish (Fundulus heteroclitus). Overall, a diversity of strategies exist; we speculate that these may be linked to differences in habitat and/or lifestyle.

Gill paracellular permeability and the osmorespiratory compromise during exercise in the hypoxia-tolerant Amazonian oscar (Astronotus ocellatus).

In the traditional osmorespiratory compromise, fish increase their effective gill permeability to O2 during exercise or hypoxia, and in consequence suffer unfavorable ionic and osmotic fluxes. However oscars, which live in the frequently hypoxic ion-poor waters of the Amazon, actually decrease ionic fluxes across the gills during acute hypoxia without changing gill paracellular permeability, and exhibit rapid paving over of the mitochondrial-rich cells (MRCs). But what happens during prolonged exercise? Gill paracellular permeability, ionic fluxes, and gill morphology were examined in juvenile oscars at rest and during aerobic swimming. Initial validation tests with urinary catheterized fish quantified drinking, glomerular filtration, and urinary flow rates, and confirmed that measurements of gill paracellular permeability as [(3)H]PEG-4000 clearances were the same in efflux and influx directions, but far lower than previously measured in comparably sized trout. Although the oscars achieved a very similar proportional increase (90%) in oxygen consumption (MO2) to trout during steady-state swimming at 1.2 body lengths sec(-1), there was no increase in gill paracellular permeability, in contrast to trout. However, oscars did exhibit increased unidirectional Na(+) efflux and net K(+) rates during exercise, but no change in drinking rate. There were no changes in MRC numbers or exposure, or other alterations in gill morphology during exercise. A substantial interlamellar cell mass (ILCM) that covered the lamellae to a depth of 30% was unchanged by 4 h of swimming activity. We conclude that a low branchial paracellular permeability which can be dissociated from changes in O2 flux, as well as the presence of the ILCM, may be adaptive in limiting ionoregulatory costs for a species endemic to ion-poor, frequently hypoxic waters.