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Na+ Transport 1 and 2 Linda Costanzo, Ph.D

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Na+ Transport 1 and 2 Linda Costanzo, Ph.D Na+ Transport 1 and 2 Linda Costanzo, Ph.D. OBJECTIVES: After studying this lecture, the student should understand: 1. The terminology applied to single nephron function, including the meaning of TF/P and the double ratio. 2. How to calculate fractional water reabsorption using TF/Pinulin 3. The pattern of sodium reabsorption along the nephron. 4. Features of and transporters involved in sodium reabsorption in the early proximal tubule, late proximal tubule, thick ascending limb, early distal tubule, and late distal tubule and collecting ducts. I. TERMS TO DESCRIBE SINGLE NEPHRON FUNCTION Up to this point, we have discussed primarily whole kidney function (e.g., GFR, urine, clearance, excretion). Now we will turn our attention to the nephron, which is the functional unit of the kidney. There is a specific vocabulary of the nephron, with terms analogous to that of the whole kidney (e.g., tubular fluid is analogous to urine). • [TF]x is the concentration of substance X in tubular fluid. (Tubular fluid is the fluid inside the nephron......also called luminal fluid.) • [P]x is the concentration of substance X in plasma and is considered to be constant. • SNGFR is the single nephron glomerular filtration rate. • [TF/P]x is the concentration of substance X in tubular fluid relative to the concentration in plasma. There are three possibilities for the value of this ratio, which are explained as follows: • [TF/P]X = 1.0. X has not been reabsorbed or secreted (all freely filtered substances in Bowman's space), or X is reabsorbed in proportion to water (e.g., Na in proximal tubule). For example, [TF/P]X =1.0 for all freely filtered substances in Bowman’s space (no reabsorption or secretion has taken place yet). For another example, [TF/P]Na = 1.0 throughout the proximal tubule because Na+ is reabsorbed in exact proportion to water. • [TF/P]X < 1.0. X is reabsorbed more than water, causing the concentration of X in tubular fluid to fall below that in plasma. For example, [TF/P]glucose starts at 1.0 in Bowman’s space, but then falls below 1.0 along the proximal tubule as glucose is reabsorbed more than water. • [TF/P]X > 1.0. X is reabsorbed less than water or X is secreted. If X is reabsorbed less than water (or if X is secreted into tubular fluid), the concentration of X in tubular fluid rises above that in plasma. For example, [TF/P]urea is > 1.0 in cortical collecting ducts in the presence of ADH because water is reabsorbed and urea is not. For another example, [TF/P]K is > 1.0 in cortical collecting ducts because this part of the nephron secretes K+. • [TF/P]inulin is the concentration of inulin in tubular fluid relative to the concentration of inulin in plasma. This specific [TF/P]X ratio is used to measure water reabsorption since inulin, once filtered, is "inert" (i.e., is neither reabsorbed or secreted). Thus, the amount of inulin in tubular fluid is constant along the nephron (because inulin is not reabsorbed or secreted) but the concentration of inulin in tubular fluid is determined by how much water remains; as water is reabsorbed, the tubular fluid concentration of inulin increases. For example, if 50% of the filtered water has been reabsorbed, then the tubular fluid inulin concentration doubles and the [TF/P]inulin = 2.0. (Don't forget that the "P" in TF/P is always assumed to be constant.) Calculate water reabsorption with this ratio as follows: Fraction of filtered water reabsorbed= 1 - 1 [TF/P]inulin For example, if tubular fluid is sampled at the end of the proximal tubule, and the [TF/P]in ratio is measured as 3.0, what fraction of the filtered water has been reabsorbed up to that point? What fraction of the filtered water remains in the lumen of the nephron? Fraction of filtered water reabsorbed = 1 - 1 [TF/P]in = 1 - 1/3 = 2/3, or 66.7% reabsorbed If 2/3 of the filtered water has been reabsorbed, then 1/3 of the filtered water remains in the lumen of the neprhon. • [TF/P]x [TF/P]inulin, or the "double ratio" is the fraction of the filtered load of a substance remaining in the nephron at any point. If the "double ratio" is 0.3, then 30% of the filtered load of the substance remains in tubular fluid, and 70% of the filtered load must have been reabsorbed. If you’re wondering why the double ratio corresponds to fraction of filtered load of a substance remaining in the nephron, here’s the derivation (in italics, just FYI...): % of filtered load remaining = excretion rate of x at any point in nephron divided by filtered load of x in nephron Excretion rate at any point in nephron = [TF]x x V SNGFR = single nephron GFR= [TF]inulin x V [P]inulin Filtered load of X = SNGFR x [P]x = [TF]inulin x V/[P]inulin x [P]x Substituting, and cancelling V: % of filtered load remaining = [TF/P]x/[TF/P]inulin We will use [TF/P]X and the double ratio together to describe how substances are handled in the nephron. For example, if, at the end of the + proximal tubule, the double ratio for Na is 0.33 and [TF/P]Na is 1.0, we would say that 33% of the filtered Na+ remains in the nephron at the end of the proximal tubule, that 67% of the filtered Na+ was reabsorbed by the proximal tubule, and that this Na+ reabsorption must have been in exact proportion to water reabsorption (since [TF/P]Na was 1.0). II. OVERALL NA+ BALANCE + - - Na is the major ECF cation and, with accompanying anions Cl and HCO3 , constitutes the major ECF solute. As we have already discussed, the amount of Na+ in ECF determines ECF volume and therefore also determines blood volume and blood pressure. Thus, regulation of Na+ balance is the most important function of the kidneys. On an average daily diet of 150 mEq of Na+ ingested, the kidneys must excrete 150 mEq of Na+ to keep us in Na+ balance (neglecting small non-renal losses such as via sweat). If the kidneys excrete less Na+ than is ingested, then we are in positive Na+ balance; if the kidneys excrete more Na+ than is ingested, we are in negative Na+ balance. Figure 1. Na+ handling in the nephron. Arrows show locations of Na+ reabsorption; numbers are percentages of the filtered load reabsorbed or excreted. Na+ is reabsorbed along the nephron as follows: 67% of the filtered load in the proximal tubule, 25% in the thick ascending limb of Henle, 5% in the early distal tubule, and 3% in late distal tubule and collecting duct. Cumulatively, this is more than 99% of the filtered load reabsorbed, leaving less than 1% of the filtered load to be excreted. (For a person in Na+ balance, 1% of the filtered load excreted corresponds to the daily Na+ excretion that would equal daily Na+ ingestion.) III. PROXIMAL CONVOLUTED TUBULE: EARLY AND LATE PROXIMAL The entire proximal convoluted tubule reabsorbs 67% or 2/3 of the filtered Na+. A major feature of proximal Na+ reabsorption (and total solute reabsorption as well) is that it is linked directly to water reabsorption. Thus, Na+ (and solute) reabsorption is proportional to water reabsorption and we call the process isosmotic. The basis for isosmotic reabsorption will be explained later in the lecture. Proximal tubule is divided between an "early" part (first half, nearest the glomerulus) and "late" part (second half). The cellular mechanisms for Na+ reabsorption are different in the two parts, so they will be discussed separately. A. Early proximal tubule Figure 2. Cellular mechanisms of Na+ reabsorption in the early proximal tubule. The transepithelial potential difference is the difference between the potential in the lumen and the potential in blood, -4 mV. ATP, Adenosine triphosphate. Early proximal tubule has the following features: • Na+-glucose, Na+-amino acid, and Na+-phosphate cotransporters in the luminal membrane • Na+-H+ exchange in the luminal membrane (linked to filtered - HCO3 reabsorption, which will be covered in the acid-base portion of the course) - - • Preferential reabsorption of HCO3 over Cl (as the anion accompanying Na+ reabsorption) • Na+- phosphate cotransport is inhibited by parathyroid hormone (PTH) and responsible for the “phosphaturic” effect of PTH. • Lumen-negative transepithelial potential difference due to Na+- glucose cotransport • Always isosmotic reabsorption • [TF/P]Na and [TF/P]osm = 1.0 B. Late proximal tubule Figure 3. Cellular mechanisms of Na+ reabsorption in the late proximal tubule. The transepithelial potential difference is +4 mV. ATP, Adenosine triphosphate. Late proximal tubule has the following features: • High luminal Cl- concentration (created by preferential - reabsorption of HCO3 in early proximal) • Cl- reabsorption by Cl--formate exchange in luminal membrane and by Cl- diffusion between cells (down Cl- concentration gradient) • Lumen positive transepithelial potential difference created by Cl- diffusion • Always isosmotic reabsorption • [TF/P]Na and [TF/P]osm = 1.0 IV. PROXIMAL TUBULE: ISOSMOTIC REABSORPTION, GLOMERULOTUBULAR BALANCE + - - Solute (mainly Na , HCO3 , Cl , glucose and amino acids) and water reabsorption are always proportional to each other in proximal tubule. They are linked mechanistically, so the reabsorption process is isosmotic. A consequence of this proportional, isosmotic process is that, along the entire proximal tubule, + [TF/P]Na = 1.0 and [TF/P]osm = 1.0. In other words, the concentration of Na and total osmolarity do not change in proximal tubule fluid, even though lots of sodium and total solute are reabsorbed.
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