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.It is dependent on the transmembrane Na+gradient generated by the Na+, K+-ATPase (Figure 2.6A).Cl- can then diffuse out overthe apical membrane, and down its electrochemical gradient with cAMP increasing theCl- permeability of the apical membrane (Beyenbach and Frömter, 1985).Na+ excretionoccurs down an electrochemical gradient, via a paracellular route, with Na+ and Cl+existing in almost equimolar concentrations in the secreted fluid.Fluid transport is notstimulated by cAMP in all species (e.g.winter flounder), indicating that othermechanisms for Na+ and Cl- secretion must exist (Beyenbach, 1995).Aglomerular and glomerular fluid secretionThe secretory mechanisms for secretion of ions in glomerular and aglomerular fish aremost likely identical and are the driving forces for aglomerular urine production.Furthermore, tubular fluid secretion has been identified in several glomerular species(Beyenbach, 1995) and contributes significantly to urine production as well.Aglomerularspecies have very low metabolic rates, and exhibit a sedentary lifestyle (e.g.toadfish)and/or live in very cold environments (e.g.icefish) (Dantzler, 1989).The minimal need toeliminate metabolic waste and the need to conserve water could explain evolution ofaglomerular kidneys.Furthermore, in icefish, the need to conserve antifreeze molecules(e.g.glycoproteins), which are relatively small molecules (8 33 kDa) and normallywould be filtered, may contribute as well to the evolution of aglomerular kidneys in theseanimals.The secretory mechanisms in the proximal segment PII, which enable theseanimals to produce urine, and the fluid secretory mechanisms found in glomerular speciesseems to be the same, although the capacity probably varies.Indeed, the proximal tubules(PII) in aglomerular and glomerular marine species are structurally so similar that theycannot be distinguished.Aglomerular species are not restricted to seawater despite thelack of a distal tubule.As urine formation is secondary to ion secretion, the ability tosurvive in freshwater must depend on very efficient reabsorption mechanisms in lattersegments of the nephron (collecting tubule) or urinary bladder, in combination withepithelia basically impermeable to water.Transfer of the marine toadfish (Opsanus tau)(stenohaline) to 10 percent seawater showed that their limited ability to survive in dilutedseawater is related to an inability to produce the necessary diluted urine.They were ableto eliminate water but lost large amounts of ions in this process (Table 2.2) (Baustian etal, 1997).Target organ toxicity in marine and freshwater teleosts 124Mechanism for tubular fluid secretionFluid secretion in aglomerular as well as in glomerular species was formerly thought tobe secondary to transport of Mg2+ and SO42- alone or possibly involving other divalentions as well.Secretion of Na+ and Cl- did not appear to be involved as the urineconcentrations of these ions are frequently low, and net reabsorption is always observed(urine-plasma ratio 100 mosmol (Beyenbach, 1986), which canbe explained by Na+ and Cl secretion of the proximal tubule.Although the urine Na+ andCl concentration is mostly low in marine fish, tubular secretion is not excluded as Naand Cl can be reabsorbed in the collecting duct and urinary bladder.FLUID SECRETION IN FRESHWATER-ADAPTED TELEOSTSIt may seem surprising that tubular fluid secretion is observed in freshwater species.However, the fluxes may, even though they are small, contribute to formation of urine,for example UFRs greater than GFRs have been measured in the freshwater-adaptedAmerican eel (Anguilla rostrata), although Na+ and Cl- secretion was not thought to bethe driving factor (Schmidt-Nielsen and Renfro, 1975).Furthermore, secretion mayreplace Na+, Cl- and water which may be reabsorbed concomitantly with reabsorption ofamino acids and glucose.Tubular fluid secretion may also provide a basal volume flow innon-filtering nephrons.As these tubules are not perfused, fluid secretion may provide amethod by which other tubule transport mechanisms can be maintained.Finally, thecapacity for fluid secretion may in itself be an indicator of euryhalinity (Cliff andBeyenbach, 1992).If freshwater fish do exhibit tubular fluid secretion, it can be expectedTarget organ toxicity in marine and freshwater teleosts 128that Na+ and Cl- secretion is the absolute driving force because these animals mustconserve Mg2+.Functions of the distal tubule/collecting tubule/urinary bladderThe urine osmolality of freshwater teleosts is usually less than 20 mosmol, which is about5 percent of the plasma concentration, indicating very efficient overall reabsorption ofions.Most of this urinary dilution occurs along the distal tubule, collecting tubule system,and urinary bladder (Nishimura et al., 1983; Dantzler, 1989).The high reabsorptionefficiency is a combination of high ion transport activity, preferentially Na+ and Cl-transport, and an epithelium which is practically impermeable to water (Nishimura et al.,1983; Stoner, 1985; Karnaky, 1998) [ Pobierz caÅ‚ość w formacie PDF ]