Distal Renal Sodium, Potassium Handling and Acidification PDF

WSUSOM Medical Physiology Rossi-Renal Physiology Page 1 of 12 Distal Renal Sodium, Potassium Handling and Acidification Distal Renal Sodium, Potassium Handling and Acidification Learning Objectives: 1. Distal tubule and collecting duct A. Identify the t...

WSUSOM Medical Physiology Rossi-Renal Physiology Page 1 of 12 Distal Renal Sodium, Potassium Handling and Acidification Distal Renal Sodium, Potassium Handling and Acidification Learning Objectives: 1. Distal tubule and collecting duct A. Identify the transporters and their function in sodium chloride reabsorption in the distal tubule cell B. Identify the channels responsible for sodium and potassium transport by the principal cell C. Recognize how inhibition of transporters for sodium at each segment may lead to a diuresis D. List the mechanisms/factors involved in modulating distal nephron sodium reabsorption and potassium secretion and/or reabsorption 2. Acid handling by the distal nephron segments A. Identify the cell type(s) involved in hydrogen ion secretion and reabsorption in the distal tubule and collecting duct (a and b) intercalated cells B. Distinguish between titratable acid and ammonium excretion C. Distinguish between the concepts of acid excretion and secretion. D. Be able to calculate net acid excretion E. Understand the source of renal ammonium and its processing by the nephron segments F. Learn the factors that influence hydrogen ion secretion and excretion along all nephron segments, including the role of pCO2, pH, carbonic anhydrase, filtered buffers and electrical potential. 3. Relationship between potassium balance and hydrogen ion secretion A. Know the effect of potassium secretion/reabsorption on hydrogen ion secretion B. Learn the concept of total body potassium deficit and excess, its effects on hydrogen ion handling by the nephron and final body pH C. Learn how body pH (hydrogen ion content) impacts on total body potassium content and plasma potassium concentration WSUSOM Medical Physiology Rossi-Renal Physiology Page 2 of 12 Distal Renal Sodium, Potassium Handling and Acidification Overview of Tubular Solute Handling Na+ reabsorption and K+ secretion or reabsorption quantitatively - early nephron segments regulation - distal tubule and collecting duct Water quantitatively - early nephron segments regulation - distal tubule and collecting duct H+ quantitative reclamation of HCO3 in proximal tubule excretion of H+ & regulation of urinary pH - distal nephron In general, the proximal tubule handles the bulk of solute and water reabsorption and secretory function. The distal nephron segments are responsible for the fine tuning of solute and water reabsorption and secretion. The final excretion UxV of any given substance X is dependent upon the proper function of the whole nephron. We will now focus on the areas immediately after the macula densa: 1. Distal convoluted tubules 2. Collecting tubules and ducts There are three major cell types in this part of the nephron: 1. Distal tubule cells: reabsorb Na+ and Cl- 2. Principal cells: reabsorb Na+ and H2O; secrete K+ 3. Intercalated cells (three subtypes: a, b, and g): secrete H+ and K+ (some types reabsorb K+) Distal Convoluted Cell Main function reabsorption of Na+ and Cl- impermeable to H2O Membrane potential Basolateral -80 mV (K+ potential) Apical -55 mV (net, Na+ and K+ potentials) Apical membrane NaCl co-transporter (NCC) also known as “thiazide sensitive” co- transporter (thiazide diuretics work here) Basolateral membrane Na,K ATPase Cl- channel The distal convoluted tubule cell’s function is primarily to reabsorb Na+ and Cl-. WSUSOM Medical Physiology Rossi-Renal Physiology Page 3 of 12 Distal Renal Sodium, Potassium Handling and Acidification The Na,K ATPase on the basolateral membrane maintains intracellular Na+ low. Na+ moves from TF into the cell down its electrochemical gradient and then out the basolateral side via the Na,K ATPase into the ISF and plasma. Cl- is reabsorbed passively down its transcellular electrical gradient. The Na,Cl co-transporter is also known as the thiazide sensitive co-transporter. Thiazides are a class of diuretics that inhibit this co-transporter and are used in treating hypertension. Principal Cell Main function reabsorb Na+, Cl- and H2O secrete K+ Basolateral Membrane Na,K-ATPase (again!!!) K+ channels Apical Membrane Na+ enters via channels (ENaC) (amiloride sensitive channel) Low Naicf and negative PD inside cell facilitates Na+ entry Hi Kicf facilitates K+ secretion H2O enters via AQP2 (not shown) Paracellular (between the cells) Pathway Cl- driven by lumen negative transcellular voltage The basolateral membrane is more permeable to K+ than Na+, so that the potential across this membrane approximates the Nernst potential for K+: 80 mV (similar to the proximal tubule) The apical membrane is equally permeable to Na+ and K+, so that the potential difference across this membrane is halfway between the Nernst potentials for K+ (-80 mV) and for Na+ (+30 mV) = -25 mV negative inside. (Conventionally, ISF is set at 0 mV). The principal cell is the same cell that is responsive to ADH and is so important for H2O reabsorption. Here we deal with its function to reabsorb Na+ and secrete K+. Again, Na,K-ATPase is absolutely KEY. By keeping intracellular [Na+] low, this drives Na+ into the cell from the TF through the Na+ channel on the apical membrane. This channel is known as the epithelial Na channel, ENaC. K+ enters the cell via the Na,K-ATPase and “leaks” out into the TF and the ISF via channels, one of which is known as ROMK (renal outer medullary K+ channel). The lumen negative potential favors K+ secretion into the tubule. (This negative lumen potential is important - see below). Quite a bit of Cl- is reabsorbed. It is currently thought that it travels between the cells driven by the negative TF potential relative to ISF. WSUSOM Medical Physiology Rossi-Renal Physiology Page 4 of 12 Distal Renal Sodium, Potassium Handling and Acidification Factors Affecting Distal Na+ reabsorption and K+ secretion 1. Aldosterone 2. Na+ load 3. Non-reabsorbable anions 4. Distal tubular fluid flow rate How do these factors work? 1. Aldosterone Synthesized and secreted by adrenal cortex aldosterone secretion increases in response to o angiotensin II o ­ plasma [K+] o ¯ plasma [Na+] ­ aldosterone ® ­ Na,K ATPase, ENaC and ROMK ® ­ Na+ reabsorption and ­ K+ secretion -55 ¯ UNaV and ­ UKV mV ¯ aldosterone ® opposite effects -80 0 - mV Cl mV - Fascinating FACT - HSDH2: The mineralocorticoid receptor that binds aldosterone binds the glucocorticoid hormone, cortisol, with the same affinity. Since every morning our cortisol levels are 1000 times higher than aldosterone, we would ALL reabsorb salt very avidly every day. This does not happen. Hydroxysteroid dehydrogenase type 2 (HSDH2) is located in the principal cell. It conveniently converts cortisol to corticosterone that does NOT bind very well to the mineralocorticoid receptor. Thus, the principal cell responds only to the ambient aldosterone and not cortisol! This is a slick way to get around high levels of cortisol, but not slick enough to avoid Na+ reabsorption when doctors give people synthetic corticosteroids, like prednisone. These cannot be metabolized by HSDH2 and DO cause Na+ retention, swelling and higher blood pressure. 2. Na+ Load (or Na+ delivery) Na+ load = TF flow rate * [Na]tubule ­ Na+ load ® ­ Na+ reabsorption and ­ K+ secretion ¯ Na+ load ® ¯ Na+ reabsorption and ¯ K+ secretion ­ GFR or ¯ Na+ reabsorption upstream ® ­ Na+ load ® ­ Na+ reabsorption distally BUT UNaV still ­ (urinary [Na] high) UKV increased (can lead to severe K+ loss in the urine) WSUSOM Medical Physiology Rossi-Renal Physiology Page 5 of 12 Distal Renal Sodium, Potassium Handling and Acidification The effect of Na+ load is independent of Renin-Ang II-Aldo system!!! The Na+ load to the distal tubule is the product of the TF fluid flow rate and the Na+ concentration of the TF. Thus, either a fast flow rate and/or high [Na+] in the tubular fluid can increase the “load” or delivery to the principal cell. The effect of Na+ load is independent of the renin-Ang II-aldosterone system. Na+ load and aldosterone can act simultaneously upon the tubule cell. At times stimuli may give opposite signals to the tubular cells. For example, in hemorrhage the GFR may be low and so Na+ load to the distal nephron will be low and decrease Na+ reabsorption. Aldosterone would be high which would increase Na + reabsorption. The net result may be difficult to predict. BUT recall that the kidney is designed to protect blood pressure and volume, so reabsorption of Na+ usually wins even at the expense of significant loss of K+. When GFR is high or proximal Na+ reabsorption is decreased, there is more Na+ delivered distally (increased load). Na+ reabsorption increases, but the proportion of Na+ left behind in the TF may still be high. K+ secretion is increased and K+ losses occur. This is why some of the effect of osmotic diuretics (as seen with diabetics with uncontrolled glucose) is counteracted by the distal tubule and why K+ depletion is observed. Cellular Dilemma INCREASED NaCl load to macula densa DECREASES RENIN (so lower Ang II and lower Aldo and decreased Na reabsorption by principal cell) INCREASED NaCl load to principal cell INCREASES Na+ reabsorption…independent -55 mV of renin-angiotensin-aldosterone system -80 mV YES the principal cell may get opposing - 0 mV messages Cl The principal cell may get opposing messages. Which one wins out? Consider yourself confronted with the decision to stay home and study or go to a movie. The strength of one or the other stimulus may be the determining factor. Is there a test on Monday? The kidney is not different. The cells process conflicting inputs and the integrated response to all the inputs determines the final course of action. Do not be dismayed…in the exam or boards the questions have to be clear as to which stimulus is being considered in THAT instant. You may then assume all other factors are stable. (This is not always true in real life and real clinical practice). WSUSOM Medical Physiology Rossi-Renal Physiology Page 6 of 12 Distal Renal Sodium, Potassium Handling and Acidification 3. Non-reabsorbable Anions Non-reabsorbable anions phosphates ketones penicillins bicarbonate ­ transtubular potential difference (PD) tubule more NEGATIVE ¯ Na+ reabsorption ­ K+ secretion ­ H+ secretion In the slide, the P- indicates negative ions in the tubular fluid (penicillin, phosphate, etc). Note that phosphate is ONLY reabsorbed in the proximal tubule and cannot be reabsorbed here. Penicillin is an anion and is filtered and also secreted into the proximal tubule. In metabolic alkalosis where there is a LOT of HCO3- that exceeds what the proximal tubule and thick limb can reabsorb, HCO3- also can be an anion here. The more of these anions that are present, the more negative the TF becomes (e.g, from -55 mV to -70 mV). This decreases the electrical component of the electrochemical gradient favoring Na+ reabsorption. So, less Na+ is reabsorbed. This also favors K+ secretion (less positive electrical gradient from cell to tubular fluid), and as we shall see later also favors H+ secretion by the intercalated cells. 4. Distal Tubular Fluid Flow Rate One cilium is on each distal tubule and principal cell Increased flow ® bends cilium ® opens Ca2+ sensing K-channels More flow, more bending, more K secretion The higher the plasma [K] the more K is secreted For any given plasma [K] more K is secreted if the tubular flow rate is higher Each distal tubule or principal cell has one cilium (proximal tubules do too). The figure shows the cilium (arrow). When the tubular fluid flow rate is high, the cilium bends in the direction of flow (the cell sort of dips its toe, the cilium, in the water). This leads to a series of signals including ATP and Ca2+ entry into the cell. Some of the K+ channels are Ca2+ sensitive and open when intracellular Ca2+ increases. These channels are on the apical membrane. WSUSOM Medical Physiology Rossi-Renal Physiology Page 7 of 12 Distal Renal Sodium, Potassium Handling and Acidification Since intracellular [K] is high compared with plasma or tubular fluid [K], K+ will move out through its channels into the tubular fluid and be secreted….and eventually excreted into the urine. Two important points (see graph above): 1. At a given TF flow rate, more K+ is secreted (and excreted) if plasma [K] is higher 2. At a given plasma [K], higher the TF flow → more K+ is secreted and excreted. Interestingly, one of the major components of the cilium is coded by the gene that is responsible for polycystic kidney disease. This is thought to contribute to the formation of cysts rather than tubules. The protein, polycystin 1, is also an anchoring protein on the basolateral side. Basically, in polycystic kidney disease, the cell does not know which way is up…or even which way the flow goes. Aldosterone PARADOX Low ECF volume (hypotension) or high plasma [K+] (hyperkalemia) both can increase aldosterone. In low ECF volume urinary K+ excretion does not increase, but in hyperkalemia urinary K+ excretion goes up to restore the plasma [K] to normal. How? Although aldosterone increases in both conditions, Ang II increases only in low ECF volume (left panel of figure below). Ang II also increases NaCl reabsorption by the proximal tubule, distal tubule (thiazide sensitive) NaCl co-transporter, and ENaC, but Ang II inhibits K+ secretion by ROMK channel via WNK4 (with no lysine kinase). Loss of K+ into the urine is prevented despite the high aldosterone. Thus, in volume depletion (low ECF volume), Na+ is reabsorbed but K+ is not depleted. With high plasma [K+], only aldosterone goes up (direct action on adrenal gland). Aldosterone does not act on the distal convoluted cell, so both NaCl cotransporter and ROMK are not stimulated there. In the principal cell, aldosterone stimulates ENaC, the Na channel, to reabsorb Na+ but stimulates the WNK4 to stimulate the K+ channel, ROMK, to secrete K+. Ang II is not elevated in hyperkalemia (in fact high plasma K+ inhibits renin)! Thus, in hyperkalemia, K+ is excreted. Hypovolemia (Hypotension) (high Ang II high aldosterone) Ang II inhibits K+ secretion so primarily Na+ reabsorption with aldosterone. Hyperkalemia (High plasma [K+]) (High aldosterone only) Aldosterone stimulates both Na+ reabsorption and K+ secretion – unopposed since Ang II is low. WSUSOM Medical Physiology Rossi-Renal Physiology Page 8 of 12 Distal Renal Sodium, Potassium Handling and Acidification Two more Duties for the Kidney Reclaim ALL the filtered HCO3- Excrete H+ generated by metabolism H+ secretion luminal - active transport via H+-ATPase o note smaller electrical gradient luminal - exchange for K+ (H,K-ATPase) note large pH gradient can be established o allows urinary ACIDIFICATION o urine pH as low as 4.4 Major distal nephron buffers Bicarb, phosphate, ammonia The a intercalated cell is shown above. This cell secretes H+ ion via an H+-ATPase in the apical membrane. When H+ is secreted into the tubular fluid, it meets with a “buffer” anion. Three major buffers occur in the distal nephron: - 1. residual HCO3 (left over from the proximal tubule and TALH) 2. phosphates, HPO42- and H2PO4- 3. ammonia, NH3 Remember the distal nephron (distal tubule and collecting duct) is responsible for fine tuning and regulation! Reclaim that last HCO3 that was filtered… the one that escaped from the proximal tubule and TALH reabsorptive processes! The cell above is reclaiming the last of the HCO3- that escaped reabsorption by the proximal tubule and thick ascending limb of Henle. Scant carbonic anhydrase on brush border, mostly in ICF Secreted H+ meets HCO3- and forms CO2 and H2O HCO3 is reabsorbed by the basolateral membrane in exchange for Cl- 1 HCO3 in tubular fluid is thereby reclaimed into the plasma The reaction here is very much like that in the proximal tubule…except the transporter(s) are ATPases. Any HCO3 that may be left in the tubular fluid is reclaimed by H+ secreted by the H- ATPase or the K,H-ATPase on the apical membrane and the HCO3- formed in the cell is extruded via the HCO3-/Cl- exchanger on the basolateral membrane. WSUSOM Medical Physiology Rossi-Renal Physiology Page 9 of 12 Distal Renal Sodium, Potassium Handling and Acidification Thus, the very last HCO3- from the tubular fluid is reclaimed without HCO3- itself traversing the apical membrane. So we have RECLAIMED the FILTERED HCO3- but have not yet REPLACED the HCO3- that was used up in buffering the acid formed in the body by metabolism of proteins. (We will speak more of this in the acid/base section.) For now, let us see how that bicarbonate is replaced. To put it another way, how can the kidney get rid of acid (H+). HCl + Na HCO3- « NaCl + CO2 + H2O; As shown here if acid is added to ECF, some will be buffered by HCO3- and lead to the formation of CO2 and H2O. The CO2 will be expelled by the lungs and the water into the urine. But that leaves the need to replace the HCO3- that was used up! In a nutshell - HCO3- is consumed in the BODY to buffer the H+ generated by metabolism of proteins. - How to get rid of H+? - Rephrase: How to replace the HCO3- lost is the buffering process? - This is done by two processes o excreting acid in the form of phosphates (& sulfates) and ammonium o while reabsorbing HCO3- Titratable Acid and Ammonium H+ is secreted H+ meets either phosphates or ammonia The resultant acid (phos or NH4+) is trapped in tubular fluid HCO3- is transported out basolateral membrane 1 NEW HCO3- enters the bloodstream to replace the one consumed in buffering o (This is a “new” HCO3- in the sense that this HCO3- was not filtered!) The basic process here is the same. The cell is the same, the a intercalated cell. Instead of the secreted H+ ion meeting a HCO3-, the H+ meets a different species, either phosphates or ammonia. When these are protonated, they are trapped in TF. WSUSOM Medical Physiology Rossi-Renal Physiology Page 10 of 12 Distal Renal Sodium, Potassium Handling and Acidification Here is the difference!!! The reaction INSIDE the cell is the same. Carbonic anhydrase is present inside the cell, so that HCO3- is formed and reabsorbed via transport at the basolateral membrane. This is NOT the “presto chango” of now you see the HCO3- in tubular fluid and now it is in plasma. In this situation the cell has gotten rid of H+ and a new HCO3- that was NOT filtered has been formed and put into the blood. This function is very important if the kidney is to get rid of the acid (H+) produced by metabolism of proteins, otherwise we would accumulate more and more acid in our bodies. (This is distinct from catabolism of carbohydrates and fats that results in CO2 and water. CO2 is then eliminated by the lungs not the kidney.) Source of Phosphates in the Nephron Source of phosphate in distal TF is from filtered phosphates. (Remember the phosphate that was not reabsorbed by Tm mechanisms since phosphate is AT the threshold concentration. So the more phosphate you eat, usually from meat protein, the more acid you generate. If you exceed the threshold for phosphate reabsorption, more phosphate will be available downstream for titration of secreted H+.) Source of Ammonium in the Nephron Source of ammonia in distal TF is more complex (refer to slide): - Glutamine in proximal tubule is deamidated to 2 ammonium ions. - NH4+ is secreted by the proximal tubule. NH4+ substitutes for H+ on the Na/H exchanger (counter-transporter) - In the TALH, NH4+ substitutes for K+ on the NaK2Cl co-transporter and is reabsorbed. - Current model says NH4+ in equilibrium with NH3 in the interstitium and - NH3 diffuses into the distal tubule and collecting duct, where - NH3 is protonated and trapped as NH4+ and finally excreted. WSUSOM Medical Physiology Rossi-Renal Physiology Page 11 of 12 Distal Renal Sodium, Potassium Handling and Acidification a Intercalated cell (A-type) secretes H+ reabsorbs HCO3- b intercalated cell (B-type) secretes HCO3- reabsorbs H+ mirror image of a IC cell a There are far fewer b intercalated cells in the distal nephron. On a day to day basis, the organism has greater need for secreting acid than for getting rid of base, especially in Western culture where we eat lots of meat protein. On occasion of excess base or in very strict vegans, however, the nephron is capable of secreting HCO3-. b Five Factors Affecting H Secretion along Nephron 1. Partial pressure of CO2 (pCO2) - high pCO2 (respiratory acidosis), mass action predicts more HCO3 and H+ thus, more H+ secreted and more HCO3- reabsorbed - low pCO2 (respiratory alkalosis), mass action predicts opposite 2. Cell pH (independent of pCO2) - low cell pH (acidosis) --- more H+ secreted and more HCO3- reabsorbed - high cell pH (alkalosis) ---- less H+ secreted and less HCO3- reabsorbed 3. Carbonic anhydrase activity - inhibition by drugs will slow the reaction and thus slow the formation of H+ H+ secretion will be decreased leading to acidosis - scant amount on apical membrane, not really important since flow (and delivery) are so low 4. Amounts of filtered/secreted buffers - proximal tubule, little if any pH gradient - primary active transport of H+ in distal tubule can establish a big pH gradient with minimal urine pH at 4.4, three logs units lower than plasma (pH 7.41) !!! 5. Electrical potential difference - increased negativity of TF favors H+ secretion WSUSOM Medical Physiology Rossi-Renal Physiology Page 12 of 12 Distal Renal Sodium, Potassium Handling and Acidification (although K is reabsorbed by the intercalated cell; K+ secretion increases, too, because the principal cell secretes more than the intercalated cell reabsorbs when luminal potential is negative) Total H+ secretion = H+HCO3 reabs+ H+TA+ H+NH4 Total H+ excretion = H+TA+ H+NH4 A common error is the confusion between H+ excretion and H+ ion secretion. Note that much more H+ is secreted than excreted in the final urine. That is because most of the H+ secreted along the whole nephron (proximal tubule to end of collecting duct) never makes it into the urine as acid. Basically, H+ ions that are secreted into the lumen encounter one of the following in TF: 1. HCO3- , in which case the reaction proceeds to CO2 and water and no H+ appears as such in the urine 2. HPO42- and H2PO4- which forms titratable acid H2PO4- H3PO4 (TA in the formula) 3. NH3, which then forms NH4+ In #2 and #3, H+ ions are actually excreted into the urine. In #1 the filtered HCO3- is “reclaimed” and no new H+ occurs in the urine.