A SURVEY OF IONOREGULATORY RESPONSES TO EXTENDED EXERCISE AND ACUTE HYPOXIA IN FRESHWATER AMAZONIAN AND SOUTHERN ONTARIAN TELEOSTS: INVESTIGATING THE OSMORESPIRATORY COMPROMISE

Abstract

<p>The osmorespiratory compromise is the trade-off between high gill permeability for oxygen uptake and low gill permeability for conservation of ions in fish. The fundamental purpose of this study was to examine facets of the osmorespiratory compromise in freshwater fish under conditions of extended exercise and acute hypoxia, in light of previous research identifying very different gill morphometric and ionoregulatory modifications in the hypoxia-tolerant Amazonian oscar (<em>Astronotus ocellatus</em>) and the hypoxia-intolerant rainbow trout (<em>Oncorhynchus mykiss</em>). A technique using [³H]polyethylene-4000 ([³H]PEG-4000) for branchial paracellular permeability measurement was developed, and then applied to investigate the osmorespiratory compromise during extended swimming. Methods were developed to overcome the challenges of renal [³H]PEG-4000 loss, respirometer surface adsorption, and freshwater drinking of the chemical. In both trout and oscar, corrections were employed for these sources of error – leading to findings that in both species, branchial [³H]PEG-4000 permeability was not rectified and freshwater drinking was quite high. In both species, during an 8-h swim (1.2BL/s), oxygen consumption rate increased by 75-90%; drinking rate remained high but did not increase. Branchial paracellular permeability increased by 61% during exercise in trout but remained constant in oscar. The methods developed here can be widely applied to future studies of branchial paracellular permeability.</p> <p>Unidirectional fluxes (by ²²Na) of sodium, and net fluxes of potassium, ammonia, and urea were observed during a 2-h nomoxia:2-h hypoxia (30% O₂ saturation):2-h normoxic recovery protocol – to identify adaptive trends across phylogenies and/or environments in North and South American teleosts. Strategies for coping with hypoxia appeared to be environmentally, rather than phylogenetically linked, since both the oscar (perciform) and the tambaqui (<em>Colossoma macropomum</em> – characiform) displayed characteristic permeability reduction (of apparent transcellular origin); both frequently encounter severe hypoxia in their natural habitat. Two North American perciforms, pumpkinseed (<em>Lepomis gibbosus</em>) and bluegill sunfish (<em>Lepomis macrochirus</em>) which live in less hypoxic environments, increased branchial ion leakage as in the hypoxia-intolerant trout. Four Amazonian tetra species (all characiformes: <em>Paracheirodon axelrodi, Hemigrammus rhodostomus, Moenkausia diktyota,</em> <em>Hyphessobrycon bentosi rosaceus</em>) which experience intermediate hypoxia in their native Rio Negro presented variable responses. Finally, during a 4-h swim at1.2BL/s, branchial ion fluxes were not reduced but elevated in oscar, indicating that ionoregulation in this species occurs primarily transcellularly, and that adaptive strategies to one manifestation of the osmorespiratory compromise (hypoxia) may not apply to another (exercise).</p>