Acidification of Urine.

Transcription

1 Acidification of Urine. Learning Objectives. After reading this chapter, the student should be able to: Outline the processes involved in the secretion of H+ into the tubules. Discuss the regulation of acid base balance. Define acidosis and alkalosis, and give the normal mean and the range of H+ concentrations in blood. using the Henderson Hasselbalch equation, describe what is unique about the bicarbonate buffer system. Describe the changes in blood chemistry that occur during the development of metabolic acidosis and metabolic alkalosis. Renal H+ Secretion: Proximal and distal tubular cells secrete hydrogen ions. Acidification also occurs in collecting ducts. Na H exchange is responsible for H+ secretion in proximal tubules. Extrusion of Na+ from cells into the interstitium by Na, K ATPase lowers intracellular Na+, and this causes Na+ to enter the cell with extrusion of H+. H+ comes from intracellular dissociation of H 2 CO 3, and the HCO3 that is formed diffuses into the interstitial fluid. For each H+ ion secreted, one Na+ ion and one HCO3 ion enter interstitial fluid. Secretion of acid by proximal tubular cells in the kidney. H + is transported into the tubular lumen by an antiport in exchange for Na +. Active transport by Na, K ATPase is indicated by arrows in the circle. Dashed arrows indicate diffusion.

2 Carbonic anhydrase catalyzes the formation of H 2 CO 3. Drugs that inhibit carbonic anhydrase depress both secretion of acid by the proximal tubules and the reactions which depend on it. Most H+ is secreted by an ATP-driven proton pump in distal tubules and collecting ducts. Aldosterone acts on this pump to increase distal H+ secretion. The I cells in this part of the renal tubule secrete acid. Fate of H+ in the Urine: Amount of acid secreted depends upon subsequent events in tubular urine. Maximal H+ gradient against which transport mechanisms can secrete in humans corresponds to a urine ph of about 4.5, the limiting ph. Three important reactions in the tubular fluid remove free H+, permitting more acid to be secreted. These are reactions with HCO 3 to form CO 2 and H 2 O, with HPO 4 2 to form H 2 PO 4, and with NH 3 to form NH 4 +. Fate of H+ secreted into a tubule in exchange for Na+. Top: Reabsorption of filtered bicarbonate via CO2. Middle: Formation of monobasic phosphate. Bottom: Ammonium formation. Note that in each instance one Na+ ion and one HCO3 ion enter the bloodstream for each H+ ion secreted. A, anion.

3 Reaction with Buffers: Concentration of HCO3 in plasma, and consequently in glomerular filtrate, is normally about 24 meq/l, whereas that of phosphate is only 1.5 meq/l. Most of the secreted H+ reacts with HCO3 to form H 2 CO 3 in proximal tubule. The H 2 CO 3 breaks down to form CO2 and H2O. In proximal (but not in the distal) tubule, carbonic anhydrase facilitates the formation of CO2 and H2O in the tubular fluid. CO 2 enters tubular cells. HCO3 is reabsorbed by this mechanism. For each mole of HCO3 removed from the tubular fluid, 1 mol of HCO3 diffuses from the tubular cells into the blood. Secreted H+ also reacts with dibasic phosphate (HPO42 ) to form monobasic phosphate (H2PO4 ). This happens to greatest extent in distal tubules and collecting ducts, The reaction with NH3 occurs in proximal and distal tubules. H+ also combines to a minor degree with other buffer anions. Each H+ ion that reacts with buffers contributes to the urinary titratable acidity. Ammonia Secretion: Reactions in renal tubular cells produce NH4+ and HCO3. NH4+ is in equilibrium with NH3 and H+ in cells. Because the pk' of this reaction is 9.0, the ratio of NH3 to NH4+ at ph 7.0 is 1:100. NH 3 is lipid-soluble and diffuses across the cell membranes down its concentration gradient into the interstitial fluid and tubular urine. In the urine it reacts with H+ to form NH4+, and the NH4+ remains in the urine. Principal reaction producing NH4+ in cells is conversion of glutamine to glutamate. Glutamic dehydrogenase catalyzes conversion of glutamate to -ketoglutarate, with the production of more NH4+. Subsequent metabolism of - ketoglutarate utilizes 2H+, freeing 2HCO3. Major reactions involved in ammonia production in the kidneys. In chronic acidosis, NH4+ excreted at any given urine ph also increases.

4 NH3 secretion is a further removal of H+ from the tubular fluid and consequently a further enhancement of H+ secretion. The process by which NH3 is secreted into the urine and then changed to NH4+, maintaining the concentration gradient for diffusion of NH3, is called nonionic diffusion. ph Changes Along the Nephrons: A moderate drop in ph occurs in the proximal tubular fluid. Most secreted H+ has little effect on luminal ph. Distal tubule has less capacity to secrete H+, but secretion in this segment has a greater effect on urinary ph. Factors Affecting Acid Secretion: Renal acid secretion is altered by changes in the intracellular PCO 2, K+ concentration, carbonic anhydrase level, and adrenocortical hormone concentration. When the PCO 2 is high (respiratory acidosis), more intracellular H2CO3 is available to buffer the hydroxyl ions and acid secretion is enhanced, whereas the reverse is true when the PCO2 falls. K+ depletion enhances acid secretion, apparently because the loss of K+ causes intracellular acidosis even though the plasma ph may be elevated. K+ excess in the cells inhibits acid secretion. When carbonic anhydrase is inhibited, acid secretion is inhibited because the formation of H2CO3 is decreased. Aldosterone and the other adrenocortical steroids that enhance tubular reabsorption of Na+ also increase the secretion of H+ and K+. Bicarbonate Excretion: HCO3 Reabsorption is proportional to the amount filtered over a relatively wide range. HCO3 reabsorption is decreased by an unknown mechanism when the extracellular fluid (ECF) volume is expanded. When the plasma HCO3 concentration is low, all the filtered HCO3 is reabsorbed; but when the plasma HCO3 concentration is high; that is, above

5 26 to 28 meq/l (the renal threshold for HCO3 ), HCO3 appears in the urine and the urine becomes alkaline. When the plasma HCO3 falls below about 26 meq/l, the value at which all the secreted H+ is being used to reabsorb HCO3, more H+ becomes available to combine with other buffer anions. Therefore, the lower the plasma HCO3 concentration drops, the more acidic the urine becomes and the greater its NH4+ content. Implications of Urinary ph Changes : ph of the urine in humans varies from 4.5 to 8.0. Acids are buffered in the plasma and cells, the overall reaction being HA + NaH3 NaA + H2CO3. The H2CO3 forms CO2 and H2O, and the CO2 is expired, while the NaA appears in the glomerular filtrate. To the extent that the Na+ is replaced by H+ in the urine, Na+ is conserved in the body. For each H+ ion excreted with phosphate or as NH4+, there is a net gain of one HCO3 ion in the blood, replenishing the supply of this important buffer anion. When base is added to body fluids, the OH ions are buffered, raising the plasma HCO3. When the plasma level exceeds 28 meq/l, the urine becomes alkaline and the extra HCO3 is excreted in the urine. Because the rate of maximal H+ secretion by the tubules varies directly with the arterial PCO2, HCO3 reabsorption also is affected by the PCO2. Defense of H+ Concentration: Cells are very sensitive to changes in H+ concentration. Intracellular H+ concentration is different from extracellular ph and appears to be regulated by a variety of intracellular processes. It is sensitive to changes in ECF H+ concentration. The ph notation is a useful means of expressing H+ concentrations in the body. Normal Na+ concentration of arterial plasma that has been equilibrated with red blood cells is about 140 meq/l, whereas the H+ concentration is meq/l. ph is therefore 7.4.

6 A decrease in ph of 1 unit, for example, from 7.0 to 6.0, represents a 10-fold increase in H+ concentration. I ph of blood is the ph of true plasma plasma that has been in equilibrium with red cells because the red cells contain hemoglobin, which is quantitatively one of the most important blood buffers. H+ Balance : ph of the arterial plasma is normally 7.40 and that of venous plasma slightly lower. Acidosis is arterial ph below 7.40, and Alkalosis is above The H+ concentrations in ECF that are compatible with life cover an approximately fivefold range, from meq/l (ph 7.70) to meq/l (ph 7.00). Amino acids are utilized in the liver for gluconeogenesis, leaving NH4+ and HCO3 as products from their amino and carboxyl groups. NH4+ is incorporated into urea and the protons that are formed are buffered intracellularly by HCO3, so little NH4+ and HCO3 escape into the circulation. Metabolism of sulfur-containing amino acids produces H2SO4, and metabolism of phosphorylated amino acids such as phosphoserine produces H3PO4. These strong acids enter the circulation and present a major H+ load to the buffers in the ECF. Sources of extra acid loads are: Strenuous exercise (lactic acid): Diabetic ketosis (acetoacetic acid and -hydroxybutyric acid), and Ingestion of acidifying salts such as NH4Cl and CaCl2, which in effect add HCl to the body. Failure of diseased kidneys to excrete normal amounts of acid Fruits are main dietary source of alkali. NaHCO3 and other alkalinizing salts.

7 Role of the liver and kidneys in the handling of metabolically produced acid loads. Buffering: Carbonic anhydrase is also found in high concentration in gastric acidsecreting cells and in renal tubular cells. Carbonic anhydrase is a protein with a molecular weight of 30,000 that contains an atom of zinc in each molecule. It is inhibited by cyanide and sulfide. The sulfonamides also inhibit this enzyme, and sulfonamide derivatives have been used clinically as diuretics because of their inhibitory effects on carbonic anhydrase in the kidney. The principal buffers in cerebrospinal fluid (CSF) and urine are the bicarbonate and phosphate systems. In metabolic acidosis, only 15 20% of the acid load is buffered by the H2CO3 HCO3 system in the ECF,most of the remainder is buffered in cells. In metabolic alkalosis, about 30 35% of the OH load is buffered in cells, whereas in respiratory acidosis and alkalosis, almost all the buffering is intracellular.

8 Summary: When a strong acid is added to the blood, the major buffer reactions are driven to the left. The blood levels of the three "buffer anions" Hb (hemoglobin), Prot (protein), and HCO3 consequently drop. The anions of the added acid are filtered into the renal tubules. They are accompanied ("covered") by cations, particularly Na+, because electrochemical neutrality is maintained. By processes that have been discussed above, the tubules replace the Na+ with H+ and in so doing reabsorb equimolar amounts of Na+ and HCO3, thus conserving the cations, eliminating the acid, and restoring the supply of buffer anions to normal. When CO2 is added to the blood, similar reactions occur, except that since it is H2CO3 that is formed, the plasma HCO3 rises rather than falls. References: Ganong s Review of Medical Physiology. 23rd Edition. THE END