BISC 307L 2nd Edition Lecture 38 Current LectureReabsoprtion of Na + , Glucose, HCO3- , etc.Protons that are extruded across the apical membrane combine with bicarbonate ions in thetubular fluid (yellow). These bicarbonate ions are filtered bicarbonate ions – they came out in the filtrate along with everything else. They create carbonic acid, which dissociates to CO2 and water. CO2 is very permeable, so it easily permeates through the apical membrane, basolateral membrane, and between adjacent cells. There is always CO2 coming out of the tubular fluid. In the epithelial cell, some of the CO2 produced in the tubular fluid enters the epithelial cell, where it combines with water. And there are high levels of carbonic anhydrase ions inside the epithelial cell, and that forms carbonic acid, which dissociates to form protons and bicarbonate. Its these protons, formed by the dissociation of carbonic acid, that provide the protons to be extruded across the apical membrane which is necessary for it to have sodium come in. The bicarbonate leaves the cell – in the basolateral membrane there is a sodium bicarbonate symport (one Na for 3 bicarbonate per cycle of the symporter). This symporter is driven by the bicarbonate gradient. The bicarbonate in the tubular fluid disappears as it combines with a proton, and another bicarbonate appears in the epithelium (H+ + HCO3). Almost all of the filtered bicarbonate gets reabsorbed by becoming H2CO3 and then H2O and CO2, which combines with water inside the epithelium to form HCO3, which is extruded from the epithelium with the Na.This is not only a mechanism for the reabsorption for sodium, but it is also one of the main mechanisms for reabsorption of bicarbonate. Because this is a 3 bicarbonate/1 sodium symport,this is an electrogenic transporter and makes the inside of the cell more negative. So far, we have movement of sodium and bicarbonate to the right (reabsorption). All the movement of these solutes will cause water to follow by osmosis. And things that are not transported will build up in concentration because water is moving to the right. There is a buildup in concentration of Cl-, for example, you can see the conc of 140mM in the tubular fluid but only 105 in the ECF. So the concentration of chloride is because water has left, and Cl is left behind. And that creates a gradient for Chloride to move across the epithelium. About 2/3 of the chloride goes through the paracellular route, going between the cells. And one third goes through chloride ion channels in both membranes. So the chloride movement is driven by the concentration gradient for chloride, which is created by the osmotic movement of water. Just like chloride got concentrated by the movement of water out of the lumen, many other solutes have been affected as well. Urea gets concentrated in the tubular fluid and in the proximal tubule and the distal tubule and the upper part of the collecting duct, urea is not permeable so as the water gets reabsorbed, the urea gets left behind and its concentration goesup. This creates a strong gradient and in the areas where urea is permeable (mainly in the collecting duct), it gets reabsorbed due to this gradient.Not much protein gets filtered because in a healthy kidney, proteins should not be able to leak out. Proteins that do get out are re-uptaken by endocytosis through the epithelium, where it gets degraded or transcytosedback across the epithelium allthe way back to the lumen. Secretion of H+ ionsHere are hydrogen ions – sincethey are produced in excess bymetabolism, and since ourfood and drink is more acidicthan it is basic, H ions have tobe consistently excreted fromthe body to preventacidification of the body fluids.There are three mechanismsfor this:1. Proton secretion through the proximate tubule. Proton secretion occurs together with the reabsorption of bicarbonate (seen on previous page).In the distal tubule and collecting duct, the next two sodium independent mechanisms occur.2. It involves a proton ATPase that pumps protons out into the tubular fluid. The protons inside the cell come from the dissociation of water molecules. And the hydroxide ions from water combine with CO2 to form bicarbonate. And the bicarbonate ion is removed from the cell by a bicarbonate/chloride exchanger.3. Shown on bottom. Another ATPase, a proton/potassium one. It pumps out protons and potassium in. The protons come from the same dissociation of water. But it also pumps potassium ions in, and they leave the cell through a potassium channel in the basolateral membrane. This is a mechanism for potassium reabsorption. This transporter, in getting rid of acid, by reabsorbing potassium. There are some clinical conditions of acidosis when this mechanism works in high gear, and these are associated with hyperkalemia. So acidosis resulting in reabsorption of K, mayresult in hyperkalemia. Fate of Secreted H+ IonsThe secreted protons don’t justbuild up – would be dangerous ifthey did because active H+ transportcan concentrate 1,000 times, tourine pH of 4.5. There are buffers in the tubular fluid and urine that prevent this from happening. These buffering reactions take up the protons, convert them to something else and renew the proton gradient. The first of these buffers is the bicarbonate buffer. CO2 produced here when the protons in the tubular fluid combine with the filtered bicarbonate gets reabsorbed back into theepithelial cell to form the bicarbonate. The second buffer is a phosphate buffer, and involves potassium reacting with dibasic sodium phosphate. And this dibasic sodium phosphate can accept a proton to become monobasic. And it releases a sodium ion which gets reabsorbed. Third buffering reaction involves ammonia. It is a gas molecule that is quite soluble, it diffuses into the tubular fluid and combines with protons to form ammonium ion. And that is a bufferingreaction, it takes out the proton to renew the gradient. NH4+, being charged, is not permeablethrough membranes like NH3 is, and that traps the protons in the lumen and they get excreted as ammonium ion in the urine. K+ HomeostasisYou have to keep your potassium in the bodyconstant.Potassium is the most abundant cation in thebody. Membrane potential of most cells isdetermined by this potassium concentrationgradient and the high resting potential ofpotassium. So it would wreak havoc with themembrane potential of many cells if theexternal potassium was different. Figure to