Proximal Tubule Non-Tm Mechanisms & Osmotic Diuresis PDF

WSUSOM Medical Physiology Rossi-Renal Physiology Page 1 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Learning Objectives: 1. Proximal tubule A. Non-Tm mechanisms 1) Recogniz...

WSUSOM Medical Physiology Rossi-Renal Physiology Page 1 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Learning Objectives: 1. Proximal tubule A. Non-Tm mechanisms 1) Recognize that quantitatively most of the transport in the proximal tubule occurs via non-Tm mechanisms 2) Identify the solutes that are handled by non-Tm transport a) Bicarbonate i. Recognize the mechanism whereby the proximal tubule reabsorbs bicarbonate (secretes hydrogen ion) ii. Appreciate the unique influence of carbonic anhydrase in this tubular segment iii. Demonstrate how bicarbonate concentration, partial pressure of carbon dioxide, volume status and other factors influence tubular reabsorptive rate of bicarbonate b) Chloride i. Understand the electrochemical forces that influence chloride reabsorption in the proximal tubule and how they change along the length of the tubule ii. Quantify (relatively) the sodium reabsorbed with chloride vs other solutes B. List the major extrinsic factors that influence proximal tubular reabsorptive rate C. Know the general handling of potassium by the proximal tubule D. Osmotic diuresis 1) Define diuresis 2) Understand the specific concept of osmotic diuresis 3) Learn the type of substances that produce osmotic diuresis and why they do so WSUSOM Medical Physiology Rossi-Renal Physiology Page 2 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Non Tm Mechanisms Given GFR = 125 ml/min and 80% reabsorption by proximal tubule, let’s look at an 8 minute snapshot of time: 1000 ml filtrate formed/8 min 80% * 300 mosm » 800 ml filtered water with ~ 240 mosm reabsorbed/8 min Tm mechanisms account for only 10-15 mosm of solute with Na co-transport mostly 1:1 (some 2:1) ® so only about 20-30 mosm solute and 66 - 100 ml of water are accounted for Thus, other mechanisms account for most reabsorption of filtered solutes (~210 mosm) and water (~700 ml) by the proximal tubule. 80% of filtered solute and H2O are reabsorbed by the end of the proximal tubule. Recall proximal tubules cells are very permeable to H2O. As solutes are reabsorbed, a small but significant osmotic gradient is set up. H2O is reabsorbed via the aquaporin 1 (AQP1) down its osmotic gradient. The substances reabsorbed by Tm mechanisms account for only a small fraction of the total solute that is reabsorbed. Carbohydrates, amino acids, metabolic intermediates, and inorganic phosphates contribute only 10-15 mosmoles to the total osmolality of the glomerular filtrate (300 mOsm/kgH2O). Even if these solutes are completely reabsorbed down to zero concentration in TF (and they are not), and if we assume the reabsorptive mechanisms involve co-transport of Na on a 1:1 basis (some are 2:1 or 3:1), this still accounts for a total of only 20-30 mosmoles out of the 240 mosmoles (80%) that must be reabsorbed every 8 minutes. Note that some solutes are secreted by Tm mechanisms, but the total mosmoles of secreted solutes is negligible. It follows that large amounts of the major filtered solutes *** Na+, Cl-, and HCO3- *** must be reabsorbed in order to account for the reabsorption of H2O in the proximal tubule. These solutes (and filtered K) are reabsorbed by non-Tm mechanisms, mechanisms for which the tubules do not appear to have a maximum reabsorptive capacity. WSUSOM Medical Physiology Rossi-Renal Physiology Page 3 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis HCO3 Reabsorption (H+ secretion) [HCO3-] in filtrate = 25 mM NET RESULT: for each [HCO3-] in filtrate, one Na+ is reabsorbed This occurs by Na+ exchange for H+ IF GFR is normal (125 ml/min), 1 L of plasma is filtered every 8 minutes, By the end of proximal tubule ® 40 mosmol of solute are reabsorbed by this mechanism per 8 min (80% of 25 mmol = 20 mmol; 1Na+ and 1 HCO3- totaling 20 + 20 = 40 mmol = 40 mosmol) CO2 + H2O Û H2CO3 Û H+ + HCO3- Both reactions can occur spontaneously, but the enzyme carbonic anhydrase catalyzes CO2 + H2O Û H2CO3, thereby speeding up the reaction. The dissociation into ions is itself very rapid and does not require a catalyst or enzyme. In the proximal tubule, carbonic anhydrase exists not only in ICF, but also on the apical brush border membrane of the proximal tubule cell. Thus, the reaction can proceed rapidly in the TF. This is necessary for reabsorption of the large quantities of HCO3- that are filtered. ~80% of filtered HCO3 is reclaimed by the proximal tubule. Filtered [HCO3-] = 25 mM. By the end of the second part of the proximal tubule, the [HCO3-] in the lumen is ~5 mM. A bit more HCO3- is reabsorbed in the proximal straight tubule, the last segment of the proximal tubule. The remaining HCO3- is reabsorbed by the thick ascending limb of Henle and distal nephron (see later lecture). Filtered load of HCO3: GFR * AHCO3 = 1 L/8 minutes * 25 mmol/L = 3.12 mmol/min or = 180 L/day * 25 mmol/L = 4500 mmol/d = 4.5 mol/day WSUSOM Medical Physiology Rossi-Renal Physiology Page 4 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Bicarbonate Reabsorption CA CO2 + H2O Û H2CO3 Û H+ + HCO3- Intracellular CO2 (from metabolism), H2O in the presence of carbonic anhydrase (CA type II) form H2CO3 which rapidly dissociates to H+ and HCO3-. H+ is secreted at the apical side into TF against its electrochemical gradient o non-Tm. o Na-H exchanger (known as NHE-3) = secondary active o H-ATPase (primary active transport) HCO3- moves into the ISF via a Na-3HCO3 co-transporter. Because the reaction products (H+ and HCO3) are removed from ICF, the reaction proceeds to the right. Intraluminal filtered HCO3- reacts with secreted H+ to form H2CO3 H2CO3 CO2 + H2O catalyzed by carbonic anhydrase in the brush border (rate limiting here too) H2CO3 dissociates into CO2 and H2O in the presence of carbonic anhydrase (CA IV) on the brush border The cell membranes are very permeable to H2O (remember AQP1) and CO2, so these substances can readily diffuse into the cell and are recycled/reabsorbed. In fact, CO2 can be transported through AQP1. reaction proceeds to the left (In case any of you are really up to date with newest findings, there are data coming out that it is not BICARBONATE that is reabsorbed on the basolateral side but CARBONATE. This will make things more complex. For now, the textbooks and exams are still reporting BICARBONATE.) WSUSOM Medical Physiology Rossi-Renal Physiology Page 5 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Intracellular CO2 combines with H2O catalyzed by carbonic anhydrase (CA) to form H2CO3 (rate limiting step) H2CO3 dissociates readily H+ is secreted into TF against its electrochemical gradient (non Tm) HCO3- moves into ISF down its gradient H+ and HCO3- removed from ICF reaction proceeds to the right (mass action) Net Effect 1. HCO3 is reabsorbed from the TF but HCO3 itself does not cross the apical membrane 2. H+ ions are secreted, but these H + are not excreted into the urine. Recall that the difference in the process of secretion and excretion: 1. secretion - transport of a substance from the plasma space into the tubular lumen (this does NOT necessarily mean that the substance will end up in the urine as it may be reabsorbed by the renal tubule at a site farther down the nephron. 2. excretion - the process whereby a substance is removed from the body In the case of the kidney, this entails having that substance in the final urine. (Excretion can also occur via the feces, saliva, respiratory tract, sweat, etc.) Factors Affecting HCO3 Reabsorption CA CO2 + H2O Û H2CO3 Û H+ + HCO3- 1. Carbonic anhydrase (CA) activity blockers of carbonic anhydrase (acetazolamide) ® ¯ carbonic anhydrase activity® ¯ H+ secretion ® ¯ reabsorption of HCO3- and, thereby, also of Na+ and H2O ® ­ urine flow rate (diuresis) drugs/diseases that inhibit/decrease carbonic anhydrase activity will decrease the rate of the reaction H2CO3 ® H2O + CO2 This will effectively decrease the rate of HCO3- reabsorption to a level consistent with the speed that the reaction proceeds spontaneously. WSUSOM Medical Physiology Rossi-Renal Physiology Page 6 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis 2. Partial pressure CO2 (pCO2) - by mass action ­ pCO2 ® ­ HCO3- reabsorption ¯ pCO2 ® ¯ HCO3- reabsorption 3. Amount filtered HCO3- the more HCO3- filtered, the more favorable the apical H+ gradient, so more H+ can be secreted (only against electrical gradient) H+ is secreted against its electrical gradient, thus H+ concentration gradient (pH gradient) that can develop is small. The more HCO3- is filtered, the more H+ can be secreted without developing a pH gradient as well. Time to check out our balance sheet… Reabsorption of Na+, Cl- & K+) Tm solute reabsorption = 20-30 mosmol HCO3- + Na+ reabsorption = 40 mosmol TOTAL = 70 mosmol H2O isotonically reabsorbed: 300 mosmol = 70 mosmol__ kgH2O 230 mL water By reabsorbing ~30 mosmoles solute (15 solute + 15 Na+) by Tm mechanisms and 80% of the filtered HCO3- (20 mosmoles HCO3- + 20 mosmoles Na+), the proximal tubule reabsorbs 70 mosmoles solute every eight minutes. To reabsorb this solute isotonically, 230 mL H2O must be reabsorbed by osmosis with the solute. But we said that 80% was to be reabsorbed…that would be 800 ml water! So how do we reabsorb the other 570 ml? Some other solute must be reabsorbed also: Cl- Cl- is not readily reabsorbed early in the proximal tubule. The electrochemical gradient is very unfavorable, due to the very unfavorable electrical gradient. As a result of the 230 ml of H2O reabsorption, [Cl-] rises in TF as fluid moves from the early proximal tubule down to the later portion of the proximal tubule. This results in a favorable concentration gradient for Cl- overcome the unfavorable electrical gradient so that the electrochemical gradient now permits Cl- to move passively from TF into the ICF and then into ISF. In summary 1. Cl- is not readily reabsorbed early in the proximal tubule. 2. TF [Cl-] concentration increases with distance along proximal tubule. 3. Cl- is reabsorbed down its concentration gradient into ICF and paracellularly (between the cells) into ISF in the later sections of the proximal tubule 4. Na+ is reabsorbed with the Cl- 5. More Na+ and Cl- are reabsorbed in proximal tubule than all other solutes combined. WSUSOM Medical Physiology Rossi-Renal Physiology Page 7 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Chloride Reabsorption: EARLY Proximal Tubule In the early proximal tubule, the electrical gradient is not favorable and the chemical gradient is insufficient to overcome this. Thus, the net electrochemical gradient is not favorable for Cl- to move from TF to the inside of the cell. As water is reabsorbed into and through the cell, the TF [Cl-] rises and is slightly higher than the [Cl-] in the interstitial fluid ISF (on the basolateral side). Therefore, in this early segment of the proximal tubule Cl- moves from TF to the ISF BETWEEN the cells. Chloride Reabsorption: LATE Proximal Tubule In the late proximal, the lumen potential is actually a bit positive. Yes, that WOULD make it harder for Cl to move into the cell…if that was all that was going on. 1. in the late proximal tubule, Cl- is reabsorbed mostly THROUGH the cell (transcellularly) 2. the slightly positive lumen potential makes it more favorable for an intracellular anion to move OUT of the cell in exchange for Cl- moving in. 3. the chemical gradient for Cl- is favorable to move in (the electrical is still not) 4. the base that came out in exchange for the Cl- is actually coupled to the Na/H exchanger. The H+ that moves out (as Na+ goes downhill) actually neutralizes the base that is favored to come out. Voila….this is tertiary active transport! (See what happens when I try to simplify it…) EVERYTHING IS LINKED IN SOME WAY TO SODIUM. NOW YOU KNOW WHY WE SAY “THE SALT OF THE EARTH!” Example If GFR = 125 ml/min and prox reabsorption =80% then, every 8 min 1000 ml filtered with 300 mosm Of these Tm solute reabsorption = 20-30 mosmol HCO3 + Na reabsorption = 40 mosmol SubTOTAL = 70 mosmol Reabsorption of remaining 170 mosmol must be mostly Na+, Cl- (some K+, Ca2+, etc. too). And a total of 800 ml water is reabsorbed via AQP1. Some may visualize chloride reabsorption better with a picture: 1. Early PCT: Cl- moves by solvent drag paracellularly (between the cells) 2. Late proximal tubule: Cl- moves down electrochemical gradient thru the cell WSUSOM Medical Physiology Rossi-Renal Physiology Page 8 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Control of Proximal Na+ Reabsorption: the Big 3 1. Catecholamines: renal sympathetic nerves & adrenal medulla adrenergic receptors ® ­ proximal Na+ reabsorption 2. Angiotensin II ­ angiotensin II ® ­ proximal Na+ reabsorption 3. Atrial natriuretic hormone ­ atrial natriuretic hormone ® ¯ proximal Na+ reabsorption Three major factors that influence proximal Na+ reabsorption are listed above. In the distal tubule and collecting duct, aldosterone is the major factor. We will discuss aldosterone later when we take on the distal nephron. Aldosterone does not work on the proximal tubule. K+ Reabsorption Filtered K+ is reabsorbed passively in proximal tubule Recall cell membranes are more permeable to K+, so that electrochemical equilibrium occurs. In the proximal tubule, filtered K is reabsorbed primarily paracellularly (between the cells) by two mechanisms: 1. Early proximal tubule - K+ reabsorption is due to H2O reabsorption, via solvent drag. 2. Late proximal tubule – K+ reabsorption due to electrodiffusion. Recall that the luminal potential is ~ +3mV so that this sufficient to provide a favorable electrical gradient for paracellular K+ reabsorption from lumen (+3 mV) to interstitial fluid (0 mV, ground). If K+ gains entry into the cell (recall proximal cell membranes are more permeable to K+ than Na+ or Cl-), the concentration gradient for K+ favors K+ moving from the cell to ISF. The Na,K-ATPase prevents this. There are K+ channels and transporters that move K+ out of the intracellular space when needed, but of little significance to us here. TF [K+] must be » 4 mM in order for K+ to be in electrochemical equilibrium across the apical membrane. Measurements of TF [K+] indicate that this is very nearly correct. The distal nephron handles K+ very differently as we will see later. WSUSOM Medical Physiology Rossi-Renal Physiology Page 9 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Osmotic Diuresis GFR = 125 ml/min 300 mosm filtered / 8 min 80% (240 mosmol / 8 min) isotonically reabsorbed 20% (60 mosmol) mainly Na and Cl remain at end of proximal tubule GFR 125 ml/min filtrate with Na, Cl, HCO3 PLUS 100 mosm/kgH2O mannitol 400 mosmol filtered / 8min Mannitol is confined to the TF Less H2O reabsorbed (due to the osmotic force of the mannitol in TF) Rate of Na reabsorbed decreases (due to Decrease in [Na] in the TF NET diffusional flux of Na+ into the lumen) Note that the osmolality of TF is still 400 mosm/kgH2O but that only 50% of solute and H2O is reabsorbed Definition of DIURESIS: increases excretion of both salt and water (hence “DI” meaning two). Normally, 60-80% of filtered solutes and H2O are reabsorbed (we are using 80% for our illustrations). Fluid is reabsorbed isotonically throughout the proximal tubule. Osmotic diuresis A non-reabsorbable substance such as mannitol 100 mosm/kgH2O is injected into plasma to increase Posm to 400 mosm/kgH2O. Mannitol is freely filtered at the glomerulus into tubular fluid, so the filtrate contains Na+, Cl-, HCO3- etc. at normal concentrations plus and mannitol. Thus, TF osmolality at the beginning of the proximal tubule will be 400 mosm/kgH2O. Na+ will be actively reabsorbed. Typically, Na reabsorption leads to water reabsorption and the [Na+] does not change. BUT in osmotic diuresis... Mannitol cannot be reabsorbed. Passive water reabsorption (via AQP1) is prevented from keeping pace with active Na+ reabsorption by the osmotic force exerted by mannitol. Since the proximal tubule must reabsorb iso-osmotic Na+ salts from a mixture of Na+ salts and non-reabsorbable solutes, the [Na+] falls in the tubular fluid while the [mannitol] rises. Thus, the rate of Na+ and fluid reabsorption falls as the tubular fluid [Na+] falls. Less Na+ (and Cl-) and water are reabsorbed along the tubule, leaving a larger volume of tubular fluid with a greater net AMOUNT (though not concentration) of Na+ and Cl-. Thus, less Na+, Cl- and H2O are WSUSOM Medical Physiology Rossi-Renal Physiology Page 10 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis reabsorbed…that is, the definition of a diuretic and in this case an OSMOTIC DIURETIC leading to an OSMOTIC DIURESIS (ultimately more EXcretion of Na+, Cl- and H2O). One might think that this would increase water excretion ONLY. Although the concentration of Na+ may be lower in tubular fluid, the absolute amount of Na+ left behind in the tubular fluid is greater. Thus, osmotic diuretics increase BOTH Na+ and H2O excretion (and Cl- and K+ too). High concentrations of non-reabsorbable solutes impair proximal tubule Na+, Cl-, H2O reabsorption Examples mannitol radiologic contrast high glucose concentration (diabetics) In addition, the proximal tubule is “leaky,” that is, the intercellular tight junctions are not very tight. Normally, the [Na+] tubular fluid (TF) and interstitial fluid (ISF) are so close that NO NET diffusional flux occurs. HOWEVER, when the Na+ concentration of the tubular fluid decreases as water reabsorption is retarded by the presence of mannitol, there is a significant net diffusional flux of Na+ INTO the tubular lumen, the only condition when Na+ is “secreted.” RESULT: the OVERALL NET Na+ removal from the proximal tubular fluid is less. Na and water reabsorption are BOTH decreased (excretion increases »10% overall). K+ reabsorption (distal) is also decreased…mostly due to increased flow. Above are agents that commonly cause osmotic diuresis. High plasma concentration of glucose that exceed the threshold and hence, the reabsorptive maximum of the tubule Tm, can act as an osmotic diuretic. Thi leads to the polyuria (frequent large quantities of urine) seen with uncontrolled diabetes mellitus. The loss of solute and water, in turn, leads to thirst and polydipsia (ingestion of large quantities of fluid). WSUSOM Medical Physiology Rossi-Renal Physiology Page 11 of 11 Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis Basic Question Using a laboratory micropuncture technique, samples of renal tubular fluid are collected from the end portion of the proximal tubule. Compared to the blood plasma entering the glomerulus, which of the following has the lowest tubular fluid to plasma concentration ratio? A. bicarbonate B. chloride C. glucose D. potassium E. sodium Clinical Question A 30 year old otherwise healthy man has an auto accident and sustains severe head trauma. His kidney function is normal but the CT scan of his brain shows significant swelling. Knowing what you do about the distribution of substances between ECF and ICF and excretion, which of the following would you administer intravenously to decrease his brain edema and why? A. Glucose B. Mannitol C. Phosphate D. Bicarbonate E. Chloride