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.Exit at the basolateral membrane probably occurs downelectrochemical gradients (Stoner, 1985; Dantzler, 1989).One of the most strikingfeatures of the distal tubules is its low permeability to water.Nishimura et al.(1983)observed that distal tubules from freshwater-acclimated rainbow trout were nearlyimpermeable to water.As net absorption of Na+ and Cl- was demonstrated, this equals adilution process.A long distal tubule has been observed in several teleost speciesinhabiting extremely ion-poor water, such as the Amazon rivers (Hochachka et al., 1977),further verifying the distal tubule as a diluting segment in freshwater species.Althoughmost stenohaline do not possess a distal tubule, some do, which may indicate somedegree of euryhalinity similar to species which are known to be euryhaline (e.g.salmonids sp.).The distal tubule in marine fish must, however, be modified becausereabsorption of ions in this part would only increase ion loading.It is most likely that thepermeability to water is increased in these animals as this would allow water to bereabsorbed concomitantly with Na+ and Cl-.Furthermore, the function of the distaltubule may even have been reversed to secretion, e.g.in species where chloride cellssimilar to gill chloride cells have been observed.The function of the collecting tubule/duct system is probably similar to the function ofthe distal tubule, but information about this is rather limited.It is now clear that the urinary bladder plays an important role in teleosts, not only as astorage organ but also for modification of the urine.In earlier studies, implantation ofcatheters into the bladder meant that kidney urine was not modified by the bladder, andthe capacity of the whole kidney/urine bladder system was thereby unknown.In otherstudies, urine was collected directly from the bladder, which may include, but does notseparate, kidney function and bladder function.Curtis and Wood (1991) used a newexternal catheterization technique to collect naturally discharged urine.Theydemonstrated that freshwater-acclimated trout urinate in intermittent bursts at 20- to 30-min intervals, allowing the urine to be stored for approximately 25 min.They alsodemonstrated that natural urine flow is at least 20 percent lower and the excretion ratesfor Na+ and Cl- at least 40 percent lower than those determined by traditional techniquesin which catheters implanted into the bladder collect urine continuously.It is clear thatNa+ and Cl- is absorbed in the urinary bladder, and that in freshwater species this isaccompanied by an epithelium with low water permeability, allowing the urine to be Target organ toxicity in the kidney 129further diluted.Furthermore, in freshwater, water permeability seems to be influenced bythe hardness of the surrounding water, such that animals in soft water have tighterbladder epithelia than animals in hard water.This may be related to differences inprolactin titers because prolactin is well known to lower epithelial permeability (seebelow) and the prolactin level is highest in soft water (Marshall, 1995).In marine species,the urinary bladder is not impermeable to water.Beyenbach and Kirschner (1975)compared kidney urine with bladder urine from seawater-acclimated rainbow trout.Theyobserved that both kidney urine and bladder urine contained high amounts of Mg2+ (~140 and ~ 170 mM respectively), and that Na+ concentration was high in kidney urine (~100 mM) but very low in bladder urine (~ 10 mM).This clearly shows that the urinarybladder reabsorbs Na+, and that this is followed by osmotic water uptake.The differencein bladder water permeability between freshwater and marine species was also observedwhen flounder were acclimated to both media, and demonstrated a sixfold decrease inwater permeability in flounder acclimated to freshwater (Demarest, 1984).Themechanisms by which Na+ and Cl are reabsorbed seem to differ between media (Figure2.7).Basolateral Na+,K+-ATPase is in both situations the driving force, creating lowintracellular Na concentrations.In seawater, Na+ and Cl transport over the apicalmembrane seems to be coupled, whereas in freshwater (particularly soft water) Na uptakeseems to be in exchange for NH4+ (or H+) and Cl in exchange for HCO3 (Marshall,1995).Control of kidney functionIn light of how urine is produced in glomerular fish, and where functional/ morphologicchanges occur, sites of regulation could include:" renal blood flow;" glomerular filtration pressure;" hydraulic permeability of glomerular filtration barrier;" tubular transport activity;" water permeability of tubules/urinary bladder;" ion permeability of tubules/urinary bladder.Furthermore, as glomerular and possibly tubular intermittency have been observed infish, individual nephrons could probably be regulated.How this occurs is not known, butone possibility is that individual or subpopulations of nephrons could have a differentsensitivity to circulating hormones, e.g.via different hormone receptor concentrationsand/or receptor affinities to hormones.It is, however, very problematic to measureindividual nephron responses in fish partly because of the renal anatomy
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