Chapter 28 Water and salt physiology in different environments

Probable mechanism of epithelial NaCl secretion

-Cl-: >Secondary transport >NKCC •linked to Na+/K+-ATPase that supplies ATP •removes sodium chloride and moves potassium in •Cl- diffuses out(gradient)>apical Cl- channel>creates outside -ve charge •K+ diffuses back into blood> basolateral K+ channel •electroneutral> one Na+, one K+ and 2Cl- move in -Na+: > electrochemical gradient >Cl- transfer is electrogenic •Apical side=-ve •Na+ removed by diffusion •no loss of water

why is there conflict between volume regulation and ionic regulation?

-Due to the opposing conditions of water and ions in blood plasma and the environmental waters.

Contrasting water-salt relations-Freshwater teleost

-Hyperosmotic to ambient water Problems: •increased water uptake by osmosis •increases salt loss Solutions: •actively take up sodium chloride •produce hyperosmotic urine to plasma •excess water is removed

Freshwater animals tend to gain water and lose ions

-Hyperosmotic to surroundings: >influx/Passive exchange of water by osmosis>water potential outside is higher than inside >efflux/passive exchange of ions by diffusion> Na+ and Cl-> ion concentration inside is higher than outside -energy is required to counteract these processes> Na+ and Cl- ions are actively transported back into blood

Marine teleosts-Replacement of water

-Marine teleost fish lose water by osmosis and by urine production -to replace the water they lose and to regulate volume>fish drink seawater: •some drink 1% body weight/day and others drink 50%>average=10-20% •seawater in gut is strongly hyperosmotic to fish's blood plasma ( blood plasma hyposmotic{higher water content} to seawater ingested in gut) 1-water moves from blood plasma into gut by osmosis 2-Na+ and Cl- diffuse into the blood across the gut wall 3-H20 enters the gut fluids by osmosis 4-ingested seawater in gut expands in volume and is diluted> water uptake from gut fluid eventually occurs Further along intestine: 5-Na+ and Cl- are ATed out of the gut contents into the blood(97%)>requires ATP and creates conditions that favor osmotic uptake of water. 6- Water flows by osmosis(50-85%)- amount that can be extracted back to blood plasma 7-a much greater portion of NaCl from the ingested water is absorbed(97%) >NaCl absorption is required to drive absorption of H2O >solves problems of Na+ and Cl- regulation faced by fish

a mitochondria-rich cell surrounded by pavement cells in the gill epithelium of a freshwater teleost fish

-SEM image of the outer surface of the gill epithelium of a brown bullhead that lives in freshwater -portion of the gill epithelium of the freshwater fish> includes MRCs surrounded by pavement cells.

Osmoconformers

-adopt local conditions> need to find right ambient environment -2 types: 1-poikilosmotic: •match the ambient osmotic pressures 2-ocean invertebrates: •stenohaline osmoconformers •die in brakish waters 3-oysters and mussels •euryhaline •cells continue to function over wide range of blood osmotic pressure •cells able to regulate cell-volume >advantages: -protects from parasitic infection which might harm the>if they move to right environment where they can survive and the parasite cant live e.g. freshwater

Animals adapt to their surroundings

-animals live in vast array of aquatic environments -animals evolved ways to survive with very little water -seawater: high ionic concentrations -freshwater: very dilute conditions and low ion concentrations -salt lakes: very high conc.s of ions -glacial ponds: even more dilute than freshwater>less ion concentration -estuaries: can change from high salt low water environment to low salt high water environment>change in ambient environment -rainforests> humid environment -extreme desserts> extremely dry

anhydrobiosis

-animals that can stay alive without water -e.g. Polypedilum vanderplanki -dries out but stays alive>will return to life after addition of water if desiccated

Immunocytochemistry

-applied to transport proteins -principal method used at present to study gill ion-transport functions throughout the animal kingdom Procedure: 1-two-flour labelled antibodies used: one against Na+-K+-ATPase and the other against a cotransport protein NKCC-1(transports Na+,K+,Cl- ions) 2-when filament was exposed to antibodies>bound where Na+-K+-ATPase was found and NKCC-1 3-filament was scanned with lasers that excited the flours >antibody against Na+-K+-ATPase glowed red >antibody against NKCC-1 glowed green

What ways does migratory fish use to survive in 2 diff. habitats?

-being osmoregulators -change expression of MRC cells(chloride cells) In seawater: -increase in expression in both types of pumps -reduce expression of regulatory proteins -increase drinking -reduce urine output -U/P=1-isosmotic urine -increase intestinal NaCl uptake>change amount of ion transport channels>more sodium potassium channels remove excess sodium In freshwater: -when these channels are no longer needed e.g. in freshwater>expression is reduced>behaves like a freshwater animal

Animals have blood that is substantially more concentrated than freshwater

-blood osmotic pressures of various types of freshwater animals span an order of magnitude -even freshwater mussels which are among most dilute animals on earth have blood that is more concentrated than freshwater>body fluids as dilute as freshwater seem to be incompatible with life -solutes in blood plasma of freshwater animals are mainly inorganic ions such as Na+ and Cl- -each of the individual inorganic ions in the blood plasma of freshwater animals is substantially more concentrated in the blood than in freshwater

How do isosmotic marine invertebrates maintain the differences in ionic composition between blood plasma and seawater?

-by ionic regulatory processes: •animals are relatively permeable to ions and water>ions diffuse with ease between blood and seawater following electrochemical gradient -1st process:AT to take up of ions from seawater at body interface/from ingested seawater in the gut -2nd process: kidney regulation of blood composition >urine: • isosmotic to seawater-no difference in urine and water •produced by excretory organs •alter urine's ionic composition>contribute to ionic regulation •e.g.urine is richer in Mg2+ and SO42- than blood plasma (U/P=1.1-4.2)>helps keep plasma concentrations of these ions lower than seawater concentrations

What does the freshwater animal's energy costs of osmotic and ionic regulation depend on?

-depends directly on rates of passive water gain and passive ion loss -the more rapidly water is taken up by osmosis and the more rapidly ion lost by diffusion the more rapidly an animal will need to expend energy to counteract these processes to maintain a normal blood composition

Animals in freshwater

-descended from ocean-living ancestors>oceans might have had diff. composition then -adapted to life in freshwater> diff. to sea water: >difference in osmotic pressure- hyperosmotic >difference in ion concentration- much higher ion conc. in blood plasma of animals than ambient water -hyperosmotic regulators: >all freshwater animals regulate their blood osmotic pressures at levels of hyperosmotic to freshwater>classified as hyperosmotic regulators >regulate blood plasma to ambient water conc.

Aquatic animals in drying habitats

-e.g. lungfish -lives in lakes that are drying out -vulnerable as it might lose water -adapt to dry conditions

Animals that face changes in salinity

-e.g. migratory salmon and eels -animals that need to adapt to both conditions -move between two types of environments -Brackish environments(e.g. estuaries): >tidal changes: sea moves in changing water potential >drift from high to low salinity and vice versa

Counteracting solutes in stingray

-enzyme piruvate was extracted from round stingrays to be studied -affinity of enzyme for one of its substrates(ADP) was measured -Found: >when enzyme was independently exposed to an increasing concentration of urea(red line) or TMAO(green line) its affinity strongly was strongly affected>driven out of normal range in opposite directions >urea accounts for 40% of osmotic pressure>gains water >increasing urea may damage the enzyme >hypoionic : •ions diffuse ion •ion concentrations are low •do not incur NaCl load-don't need to drink water don't deal with this problem >when it was exposed to a mix of urea and TMAO(black)>exhibited normal affinity regardless of how high concs. of 2 diff. organic solutes was raised >Affinity decreases as the Michaelis constant increases

Migratory fish and other euryhaline fish are dramatic and scientifically important e.g.s of hyper-hyposmotic regulators

-fish that migrate between freshwater and seawater breed in one habitat and grow and mature in the other: - 2 categories of species: 1-anadromous(running upward): >ascend rivers and streams from oceans to breed >e.g. salmon and certain smelts, shad and lampreys 2-catadromous(running downward): >grow in freshwater and descend to oceans for breeding >e.g. freshwater eels of North America, Europe and East Asia -superb osmoregulators: >function as hypersmotic regulators when in freshwater and hyposmotic regulators when in seawater>effective in both habitats>blood osmotic pressure only changes a little between two >can reverse AT: •outward in seawater •inward in freshwater

Water-salt relations in a fresh water animal-Problems

-freshwater animal e.g. a crayfish faces challenges because of passive water and salt exchange: >freshwater animals tend to gain water continuously by osmosis>water gain dilutes their body fluids >high conc.s of ions in their blood suggest that the net diffusion of ions tend to be from blood into ambient water >ion diffusion depends on electrical gradients also >Na+ and CL- diffuse from the blood into environmental water >loss of major ions by diffusion> dilution of body fluids of a freshwater animal >ions and water are also lost by feces -# of generalized>approximate values for the osmotic pressure and Na+ conc. found in blood of a crayfish and the ambient water

Gills as ion regulatory organs

-gills assume their uptake function their Gas exchange function -images of single gill filament emphasize two major functions of gills in adult teleosts -coloured diagram=overall gill structure A(microscopic structure of the filament): >consists of a thin, principal lamellar element(shaped like the blade of a feather) and bears many folds=secondary lamellae >blood flows through all parts >secondary lamellae greatly increase the SA across which O2 can diffuse inward from ambient water into blood B(same filament visualized by confocal microscopy and stained with immunocytochemistry): -increased sodium/K+ ATPase concentration means more energy being made to provide for the AT -shows presence and location of membrane proteins>needed in ion transport between blood and ambient water -cells containing ion-transport proteins are located in parts of the filament other than the secondary lamellae -Na+-ATPase labelled red -NKCC-1 labelled green -cell nuclei labelled blue -yellow=places where both transport proteins occurred together

Extra renal NaCl excretion by the gills

-gills of adult marine teleost are responsible for excreting excess major ions Na+ and Cl- from blood plasma into the surrounding ocean -excretion of Cl- is active and is carried out by seawater type MRCs in gill epithelium=chloride cells -chloride cells excrete Cl- -soon after hatching MRCs are found in general integument but they become localized to the gills -creates -ve electrical gradient -excretion of Na+ occurs by mixed mechanisms>passive in some species and active in 1/2 species studied -elimination of Cl- and Na+ by the gills of marine teleost fish =1st e.g of extra renal salt excretion=excretion of inorganic ions by structures other than the kidneys

Marine teleost fish are hyposmotic to seawater

-hyposmotic regulators: blood osmotic pressure are far lower than osmotic pressure of seawater in which they swim -lower ion concentration in blood compared to water> lower water potential in seawater •evolutionary : dilute body fluids of marine teleosts are an evolutionary vestige>fish are descended from ancient ancestors that lived in freshwater •seawater is 600mOsm greater than blood plasma> desiccating (drying)effect>water removed •less permeable to water •still permeable but less than freshwater fish

contrasting water-salt relations-marine teleost

-hyposmotic to ambient seawater Problems: •water loss by osmosis •salt gain by diffusion Solutions: •remove excess salt taken up by active transport system •salts are also lost in feces •water lost is replaced by ingested water •water loss is reduced by reducing amounts of urine and making it isosmotic to plasma but rich in Mg2+ and SO42-

What does effect does osmotic influx have on the animal?

-increases urine production -increases ion excretion

Permeabilities characteristics in freshwater animals

-integument has low permeability: >reduces rate of passive exchange >reduces energy cost of requilibrating ion content in blood plasma -some permeability is still required: >cant have totally impermeable exchange membranes >particularly gills>where most osmosis and diffusion occurs>ion and water loss >high metabolic rate requires O2>gills are even more permeable

example of the concept of compatible solutes

-interplay of urea and methylamines in body fluids of sharks, skates, rays and a few other grps of marine fish: >urea has strong destabilizing and inhibiting effects on enzymes and other macromolecules >certain methylamine compounds e.g. TMAO,glycine, betaine and sarcosine tend to stabilize and activate enzymes>can counteract effects of urea -in animals that employ urea as an osmolyte one or more methylamines are usually present in quantities that more or less exactly "titrate away" the effects of urea

How is ion loss replaced?

-ion loss is replaced through AT -major ions=Na+ and Cl- -AT back into blood plasma against very high conc. gradient to replace ions lost -Ca+ is also AT into the blood plasma -requires energy -site of uptake and AT is the gills-have 2 functions

Marine teleost ionic regulation

-ionic concentrations in blood plasma are far lower than seawater -gradients of Na+ and Cl- are large compared to freshwater fish -influenced by: •electrical gradient- overall potential difference>more positive on one side or other •gill permeability •ion concentration gradients •Na+ uptake is not problematic for marine animals(may not even occur)- positive charge set up across membrane>inside positive •movement of sodium is reduced- Na+ uptake is reduced by +vely charged gill epithelium inside gill>repels Na+ •Cl- remains a problem>more attractive>can move into animal easily>tends to diffuse into the blood plasma of marine teleost from seawater at substantial rates>concentrates the body fluids of the fish

What is the second condition that has been demonstrated to lead to increased #s of MRCs in freshwater fish?

-life in very "soft" water=water of exponentially low Ca2+

What effect does the process of volume regulation have on ionic regulation in marine teleosts?

-like freshwater teleosts process of volume regulation worsens the problems of ionic regulation -divalent ions like NaCl are handled differently to monovalent ions Na+ and Cl- -gut epithelium is poorly permeable to these ions: •e.g. Mg2+ and SO42- •diffuse into blood to a small extent as seawater passes through the gut •or most of the time they stay in the gut and are excreted by the gills

Estivation

-metabolic depression >reduced metabolic rate>reduced respiratory water loss >kidneys no longer produce urine >nitrogenous waste changes from NH3 to urea> ammonia is more toxic than urea >animals survive more than one year

Molecular phenotypic plasticity in gills of trout transferred between freshwater and seawater

-modern research aims to understand molecular mechanisms of successful transitions between freshwater and seawater -e.g. studies of gill function -researches found that gills of individual fish undergo extensive molecular remodelling during such transitions>distinctive freshwater and seawater gill phenotypes -phenotypic adjustments include critical changes in cell morphology and the suites of ion-transport proteins in MRCs Procedure: 1-used monoclonal antibodies to assay defined cell-membrane proteins of MRCs e.g. concentrations and types of Na+-K+ ATPase and NKCC cotrasnporter during freshwater to seawater transition 2-predicted that both ion transport proteins will increase in individuals transferred from freshwater to seawater 3- quantitative changes in proteins follow the prediction in brown trout>proteins increase in gill MRCs when trout is transferred to seawater and decrease when fish is returned to freshwater 4-research on other species reveals that the molecular form of Na+-K+ATPase also changes between freshwater and seawater>detailed function of ATPase is modulated

Animals in the ocean

-most marine invertebrates are isosmotic to seawater >isosmotic=blood plasma composition same as that of ambient water environment >osmotic pressure 1000 mOsm >no osmotic regulation required>no need to transport water

Osmotic and ionic gradients

-most types of freshwater animals have far less concentrated body fluids than their marine relatives -e.g. decapod crustaceans -most marine decapods are isosmotic to seawater(~1000 mOsm) while most freshwater decapods have blood osmotic pressures of 500 mOsm or less(~440 mOsm in crayfish) -marine molluscs are isosmotic to seawater while freshwater molluscs have far lower osmotic pressures(~40-80 mOsm) -lower blood concentrations seen in freshwater animals>smaller osmotic and ionic gradients between their blood and the freshwater environment. -Osmotic difference between the blood and the surrounding water in freshwater decapod crustaceans and molluscs is lower>blood is less concentrated than that of marine progenitors e.g. in crayfish the osmotic difference between blood and surrounding water is ~440 mOsm

Water-salt relations in a marine shark

-protein rich foods are required for adequate urea synthesis -possible advantages of elasmobranch strategy over teleost strategy of water-salt regulation in the sea: •costs less energy as marine elasmobranchs are able to obtain H2O by cost free osmosis while marine teleosts must drink seawater and pump NaCL out of it to get H2O >error in view: •elasmobranch strategy is not cost free> elasmobranch must synthesize urea to keep blood hyperosmotic to seawater>costs more ATP than making ammonia from nitrogen •elasmorbanch might also need to pay ATP costs to recover urea from its urine and intercept urea diffusing outward across its gills Conclusion: costs of the elasmobranch and teleost strategies are essentially the same>strategies are the same but equal

Osmoregulators

-regulation is often limited to certain ranges of ambient osmotic pressure -2 major different categories of regulators recognized: 1-hyper-isosmotic regulation 2-hyper-hyposmotic regulation

What causes doubling of diffusion distance?what does it interfere with? and how is it resolved?

-replacement of pavement cells by MRCs in the secondary lamellae can double the average diffusion distance between blood and water in gills>MRCs are thicker than pavement cells they replace -doubling of diffusion distance interfere with O2 uptake -This is resolved by the freshwater fish exhibiting a trade-off between their ability to take up Ca2+ and their ability to take up O2 -increasing one ability decreases another -trade off concept is major theme in modern ecology and evolutionary biology

Active uptake of ions and role of pumps

-requires high levels of local ATP-energy resources -Na+ and Cl- pumps are independent-one pump moves sodium and the other moves chloride=exchange pumps -Electoneutral: >Cl- pump exchanges with HCO3- >Na+ exchanges with H+(or NH4+) >no electrical difference across membrane>electrical gradient makes it more difficult for diffusion -pumps remove metabolic waste>NH4+ and HCO3-=metabolic waste products -pumps play a role in acid-base physiology>able to remove protons

Solutes differ in some seawater invertebrates from seawater

-solute concentration of blood differs -solutes in blood plasma of marine invertebrates are mostly inorganic ions -ionic composition of their blood plasma tends to be grossly similar to that of seawater -ionic composition of the blood plasma seems to differ in detail from the ionic composition of seawater -particular ions can prove to be relatively concentrated in some animal species but not in others e.g. Mg2+ is relatively high in concentration in blood plasma of squid but low in that of crab -two ions that change in seawater invertebrates are magnesium and sulfur

Ion exchanges mediated by active Na+ and Cl- transport in the gill epithelium of freshwater teleost fish

-the Cl- pump exchanges bicarbonate ions HCO3- for Cl-ions>remains electroneutral -Na+ pump exchanges H+ protons for Na+ ions(or possible exchanges ammonium ions NH4+) in some animal grps>remains electroneutral -mechanisms of AT exist within single epithelial cells>view shows whole epithelium> does not specify cell membrane mechanisms -Source of waste products is respiration -as there is AT there needs to be an energy input -Bicarbonate and protons are exchanged for Cl- and Na+ by AT mechanisms that require ATP

What do some marine fishes do to keep their body fluids more dilute than water?

-these marine fishes expend energy to keep their body fluids more dilute than seawater. -major questions raised=why they do this and what mechanisms they employ -water has high ionic concentration and low water potential -evolved to live in diff. environments at an energy cost

How do marine teleosts eliminate excess ions?

-through 2 organs 1-Kidney: >isosmotic pressure of urine matches that of blood plasma(U/P=1)>isosmotic to blood>water is not lost >no osmotic regulation needed >ionic composition of urine is different >divalent ions are removed by kidneys e.g. Mg2+,SO42-,Ca2+ •U/P ratio>>1 •U/P Na+ and Cl-<1 •urine volume is very low and urine is concentrated 2-Gills: >monovalent ions •Na+ and Cl-

Cellular acclimation to living in 2 types of water(normal/soft) in the gill epithelium of freshwater fish

-tissue sections of secondary lamellae in gills of rainbow trout were viewed using LM and stained to show MRCs in: •A(fish living in normal freshwater(Ca2+ conc. 0.4mmol/L) •B(fish living in soft freshwater(Ca2+ conc. 0.05mmol/L) -freshwater fish get most of calcium ions from water in which they live>rather than from food -MRCs are the sites of active calcium ion uptake -when fish are living in Ca2+ poor water>increase #s of MRCs>help them acquire sufficient Ca2+>but can also interfere with uptake of O2

Marine elasmobranch

-utilise a unique method of survival in seawater -e.g. sharks, skates and rays -Hyperosmotic to seawater> small osmotic influx(higher ionic concentration in blood plasma)>don't experience dessication problem -hypoionic(lower ionic con.) blood plasma compared to ambient water -Blood plasma contains organic solutes>increase conc. in blood>increase in osmotic potential -e.g. of organic solutes: •Urea: >product of protein catabolism>waste product of protein metabolism >reabsorbed through gills >low permeability in gills >denaturing agent •trimethylamine oxide(TMAO): >counteracting solute>damage that urea does is reduced by presence of TMAO in blood plasma -hyperosmotic -specific adaptation that these animals have come up with to adapt to marine environments

Water-salt relations in a freshwater animal-Solutions

-void excess water by : >making copious(abundant) urine>more dilute urine >e.g. a goldfish/frog might excrete urine equivalent to 1/3 body weight every day >rate of urinary water excretion provides a measure of the rate of osmotic water influx -Replace ions lost by: >taking up Na+ and Cl- by AT from ambient water >requires ATP -ions can also be replaced by: >ingestion of food which contain water and salts

What does the rate of exchange of water and ions depend on?

1-Magnitude of gradients: >higher gradients=faster rate of exchange/diffusion 2-Permeability of outer body covering: >not possible to decrease permeability as animal needs to take in oxygen >reduce permeability to respiratory gases 3-Surface area across which exchange occurs: >will have impact on rate of diffusion >large SA=increased rate of diffusion

2 categories of animals that face changes in salinity

1-Stenohaline: >narrow range of salinities>able to survive in narrow changes in environment/salinities 2-Eurhaline: >broader range of salinities> greater capacity to survive in a broader range of salinities

Gills have two functions

1-ion regulatory organ 2-gas-exchange organ

2 types of cells that gill epithelium in fish consists of

1-mitochondria-rich cells(MRCs)/ chloride cells: >principle sites of active ion transport(Na+ and Cl-) in gills >have high amounts of mitochondria >central focus of research on ion transport in both freshwater and marine fish >thicker than pavement cells 2-pavement cells: >uptake of O2 during breathing to occur principally across pavement cells >thinner than MRCs>ensure efficient gas exchange >occupy more than 90% of the gill epithelium>exchange of gases needs a large SA -in freshwater fish: >CL-uptake occurs across MRCs >Na+ uptake occurs across both the MRCs an pavement cells/just pavement cells >research research no evidence/mechanisms found yet

What features give gills more permeability?

1-very thin 2-large surface area

hyper-isosmotic regulation

=a species that keeps its blood more concentrated than the environmental water at low environmental salinities -allows its blood osmotic pressure to match ambient osmotic pressure(isosmotic) at higher salinities -species that are predominantly freshwater animals but venture into brackish waters and many coastal marine invertebrates show this pattern -this type of regulation is exhibited when the animals possess mechanisms of hyperosmotic regulation but lack mechanisms of hyposmotic regulation

Hyper-hyposmotic regulation

=those that keep their blood more concentrated than the environmental water at low environmental salinities -but make blood more dilute than environmental water at high environmental salinities -able to maintain osmotic pressure regardless of osmotic pressure of water -pattern requires mechanisms of both hyperosmotic and hyposmotic regulation -observed in salmon, eels, and other migratory fish and in a variety of crustaceans

Regulatory mechanisms for osmotic-ion regulation

Urine: >copious amounts produced >hyposmotic to blood (U/P<1) >Raises plasma osmotic pressure of blood maintaining it at a hyperosmotic position >reduced blood volume -increase plasma ion concentration: >Na+ and Cl- >regulated by kidneys

ionic concentration and osmotic pressure of freshwater animals

freshwater animals have higher ion concentration and higher osmotic pressure than the ambient water pressure.

role of Outer body covering

means of protecting the animal from hyperosmotic environment

MRCs

•variable: -responsible for exchange of chloride and sodium •adaptive(under regulation): -expression regulated by some conditions: >partly hormonal >alkalosis: respond to changes in increased bicarbonate production >low Ca2+: if there is low Ca2+ in water conditions>levels change >disadvantages: -if we increase MRC expression> reducing SA for pavement cells and hence uptake of oxygen -balance between responding to conditions and uptake of oxygen