SUMMARYThe emphasis in this review will be on Na+ absorption across the skin and gills of vertebrates and the gills of crustaceans. However, some recent studies of Cl– uptake, especially in crustaceans, will also be described.

No abstract

Sodium uptake by rainbow trout gills has been investigated with a small-volume system enabling rapid, successive flux measurements in different solutions. Sodium influx obeys a Michaelis-Menten type relation, with a Km of 0.46 mM, and uptake proceeds unimpaired in the absence of penetrating counterions. This suggests a coupled cation exchange. Ammonia output is about the same as the Na+ influx when external [Na+] is 1 mM, but at higher or lower Na+ influxes, the correlation does not hold. A progressive downward shift in the pH of the irrigating medium as Na+ influx increases indicates that the exchanging cation is hydrogen. In support of this, acetazolamide, which inhibits Na+ uptake, also prevents the downward pH shift. The potential across the gill is about 10 my, body fluids positive, in NaC1 solutions up to 10 mM, and is little affected by changes in Na+ concentration below that. Finally, evidence for locating the rate-limiting step at the outer membrane of the epithelium is presented.

The major mechanisms of urea turnover and of active ion transport in osmoregulatory organs are reviewed. Using published values for the molecular flow through these systems it is possible to estimate the energetic cost of osmotic regulation in hyporegulating marine teleosts and in ureotelic elasmobranchs. Oxygen consumption dedicated to osmotic regulation in the flounder (Platichthys flesus) is about 7 ml kg-' hr-I and in the rainbow trout (Salmo gairdneri) 6 ml kg-' hr-' . For a dogfish (Scyliorhynus canicular) it was 5 ml kg-' hr -' and for the skate (Raja erinacea) 5 or 11 ml kg-' hr-' based on two values for urea loss by the fish. Both groups of fish used 10-15% of their standard metabolism for osmotic regulation. There are a number of uncertainties that may require modification of these values. It is not known whether TMAO lost by ureotelic regulators is replaced by ingestion or by synthesis. If by synthesis, the cost of regulation would be higher than calculated. Even more serious is the uncertainty regarding the use of organic osmolytes to maintain equilibrium between cells and blood. This strategy almost certainly requires some energy input, but there is insufficient information to permit its quantitative estimation, so the factor was omitted from the calculation. Among hyporegulators the sole unknown is whether the esophagus may absorb some of the ingested salt by diffusion. If so, it would decrease the energy expended in the intestinal tract and lower the total cost of regulation. The conclusion, based on known factors for the species examined, is that both modes are about equally efficient. If it becomes possible to factor in the unknowns it may be that hyporegulation is slightly more economical than ureosmotic. o 1993 Wiley-Liss, Inc. ENERGETICS OF OSMOREGULATIONVirtually all modern vertebrates, regardless of habitat, have a blood (plasma) ionic pattern suggesting that they descend from an ancient stock long resident in freshwater (FW).' The main ionic constituents are Na' and C1-both in the concentation range 100-300 mM. This creates a problem for marine fishes; they are markedly hypoionic to seawater (SW), thus facing the problem of osmotic water loss and/or salt loading. As is well known, two totally disparate mechanisms evolved to cope with the problem. In the elasmobranchs, holocephalans and the sole known coelacanth, total blood osmotic concentration is raised above ambient by the addition of organic solutes, largely urea and trimethylamine oxide (TMAO). The marine actinopterygeans, most of them teleosts, maintain the typical FW solute pattern; the main osmolytes are Na+ and C1-at concentrations only a little above those seen in FW fish. Superficially it would seem that this might be more expensive, since these fish have to cope both with water loss and salt loading, while those employing the ureotelic mode have eliminated the osmotic problem and have only to find a way to excrete any salt diffusing into the body fluids. In fact, this 0 1993 WILEY-LISS, INC. was the conclusion reached so...

No abstract