VII. Osmoregulation
A. Gill Function
Basic Problem
The process of osmosis makes the blood of freshwater (FW) fishes have a higher osmotic pressure than the water in which they swim. Osmotic pressure is expressed in milliosmoles [An Example] and the blood of a FW fish has approximately 300 mOsmol/l while fresh water generally has less than 5 mOsmol/l. So, FW teleosts are hyperosmotic to their environment. That is, their bodily fluid (principally their blood) have a higher solute content than the environment. The result is a strong tendency for water to diffuse into the FW fish and salts out. Fish can resist this osmotic movement by having a relatively impermeable body covering, skin and scales help in this regard, however, the epithelial membrane of gill must be highly permeable for gas exchange to occur. So, water diffuses in and salts out across the gill and the fish tends to hydrate.
In saltwater (SW) fish, the problem is reversed. Saltwater fish are hypoosmotic to the sea, their blood has a lower solute content and, therefore, a lower osmotic pressure (about 400 mOsmol) than sea water (about 1000 mOsmol). SW fish suffer a passive loss of water at the gills, and a passive gain of salts. SW fish tend to dehydrate [ha!].
The Gill's Role in Osmoregulation in Freshwater Fish
In order to maintain 300 mOsmol/l in its blood despite the osmotic tendency to gain water and lose ions, a FW fish must actively scavenge ions from the environment and excrete water from its body. It accomplishes this by: 1) stopping the salt outflow at the gills and 2) producing a copious, dilute urine. FW fish never drink, as they gain all (and more) of the water they need passively over the gill [ha!].
The accepted theory of how FW fish reverse the flow of salt at the gills has evolved over time. Early research indicated enzymes (ATPase's) in gill tissue split ATP and moved salts against the osmotic gradient with the energy that was released. It was presumed that most of this activity occurred in specialized cells in the gill epithelium termed beta chloride cells (to differentiate them from the anatomically different alpha chloride cells found in saltwater fish). Chloride cells have many mitochondria, capable of producing a lot of potential energy in the form of ATP that can be used by ATPase's. This theory was intuitively appealing because the ATPase's could exchange Na+ for NH4+ and Cl-for HCO3-. The exchange of like charges would promote acid-base balance and both NH4+ and HCO3- are modified, waste by-products of metabolism. Chloride cells, like the epithelial cells (See Ch. IIF), contain the enzyme carbonic anhydrase. It seemed logical that carbon dioxide and ammonia would diffuse into the chloride cell from the blood and the carbon dioxide, catalyzed by carbonic anhydrase, would dissociate into bicarbonate and hydronium. The hydronium would ionize the ammonia and the ammonium (NH4+) and bicarbonate (HCO3-) would be excreted by Na+ -NH4+ ATPase and Cl--HCO3- ATPase as salt was pumped inward. Additionally, Na+-K+ ATPase located on the interior membrane would help move Na+ from the cytoplasm into the blood.
Further research, however, indicated that other enzymes and an electrochemical gradient were also involved. A newer theory holds that while all of the above described movements no doubt occur, Na+ also follows an electrical gradient (the cell being more negatively charged than the water) into the cell. The combined effect of the electrical gradient and the opposite osmotic gradient is termed the electrochemical gradient, and it is close to 0 for freshwater fish, stopping the outflow of salt at the gill. This electrical potential is maintained by a different ATPase that pumps H+ out of the cell into the water and Na+-K+ ATPase that exchanges 3 Na+ for 2 K+ at the interior membrane. It is unclear which of these movements may occur in the chloride cell and which may be located in epithelial cells.
In the illustrations above, CO2 is shown diffusing from the blood into the chloride cell where it reacts to form HCO3-. In fact, most of the carbon dioxide is already in the form of bicarbonate by the time it reaches the gill because at the pH of blood, HCO3- is the predominate ion and the reaction from CO2 to HCO3- is catalyzed by the enzyme carbonic anhydrase in the red blood cells. The chloride cell also contains carbonic anhydrase to catalyze any remaining CO2 to HCO3- and that is what is shown. Obviously, the HCO3- already in the blood simply diffuses directly to the ion pump and is exchanged for Cl-. Since there is no carbonic anhydrase in the water, the slow conversion back to CO2 does not occur until the water has exited the gill. Therefore, the CO2 does not diffuse back into the fish.
The Gill's Role in Osmoregulation in Saltwater Fish
In order to maintain 400 mOsmol despite a passive gain of salts and loss of water, SW fish must: 1) stop the inflow of salt and actively secrete it at the gill and 2) drink seawater and hydrate themselves with it. SW fish excrete very little urine [ha!], in fact, some saltwater fish do not even have glomeruli in their kidneys.
While FW and SW fish face similar osmotic gradients (300-400 mOsmol) between their blood and environment, the ATPase activity in the SW fish's gill is relatively higher. The reason for this is that salt must be actively pumped into the fish at the gut (discussed in detail later in this section) for hydration to occur. So, at the gill, SW fish must rid their body of both actively and passively gained salt. Additionally, it may be necessary for SW fish to employ ATPase pumping of Na+ in exchange for NH4+ to rid themselves of nitrogenous waste, at times. Obviously, Na+ -NH4+ ATPase and Cl--HCO3- ATPase are not going to aid in osmoregulation in SW fish as they are presumed to do in FW fish.
It is believed that electrical gradients in a SW fish's gill move the necessary ions. In this gradient, the salt water is negatively charged, the alpha chloride cell is more highly negative and the blood is positively charged. The electrical gradient is maintained by Na+ -K+ ATPase located in an extensive microtubule system schematically represented in the illustration as a single large indentation. The active pumping of Na+ out of the chloride cell results in a Na+ gradient (higher in the blood than in the chloride cell) that drives a Na+-Cl- linked carrier system increasing the Cl- content of the cell and thus the electronegativity of the chloride cell. The highly negative chloride cell causes Cl- to move to less negatively charged sea water along the electrical gradient. The positively charged Na+ will follow the gradient from positively charged blood to negatively charged seawater outside the chloride cell. Since this ion moving mechanism is powered entirely by Na+ -K+ ATPase, it is not surprising that this enzyme increases dramatically as euryhaline fish move from fresh to salt water.
Drinking in Saltwater Fish
Ingesting salt water will not hydrate an animal unless that animal has a mechanism to move the water molecules from the gut into the blood against an osmotic gradient [An Example]. Organisms cannot pump water directly. The only way they can move water is to move salts, thus creating an osmotic gradient that water will follow. The cells of the gut lining in SW fish use Na+ -K+ ATPase to create an electrochemical gradient similar to the chloride cells of the gill (but in the opposite direction) that move NaCl into the tissue surrounding the gut creating a localized area of where the osmoality is greater than sea water which causes water to flow into the blood of the fish. The final step in this hydration is the excretion of the salt by the chloride cell of the gill.
Assignment VIIIA
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