Tubular reabsorption & secretion

From Iusmphysiology

Contents 1 Tubular reabsorption and secretion 1.1 Three processes involved in urine formation 1.2 Calculating the reabsorption of a solute 1.3 Calculating the secretion of a solute 1.4 Fractional excretion differentiates between reabsorption or secretion 1.5 Glucose reabsorption 1.6 Why does glucose reach a maximal Treabs? 1.7 Micropuncture and microperfusion of nephrons in vivo 1.8 Proximal tubule fluid is essentially iso-osmotic to plasma 1.9 Perfusion of isolated tubules can teach us about the PCT 1.10 Na reabsorption in the PCT 1.11 Reabsorption: from tubular cell to blood 1.12 PCT and secretion of organic ions 1.13 Excretion of lipid soluble organics 1.14 Loop of Henle

Tubular reabsorption and secretion

Three processes involved in urine formation

• There are three major processes involved in forming urine: filtration, reabsorption, and secretion.
  • Filtration occurs at the glomerulus and is the movement of material from the blood to the filtrate
    • See Renal blood flow, glomerular filtration for more information
  • Reabsoprtion occurs at the tubule and is the movement of desired material from the filtrate into the ECF.
  • Secretion occurs at the tubule and is the movement of waste material from the ECF to the filtrate.

Calculating the reabsorption of a solute

• We can calculate how much of a solute is reabsorbed.
• We start with the filtered load: the amount presented to the tubule / min.
• T_reabs = Px * GFR - Ux * V, where:
  • Px is the plasma concentration of the substance (x) (mg / ml)
  • GFR is the glomerular filtration rate (ml / min)
  • Ux is the urinary concentration of the substance (x) (mg / ml)
  • V is the urine flow rate (ml / min, not the volume!)\
  • The units of T_reabs (the reabsorption rate) will come out to mg / ml / min.

Calculating the secretion of a solute

• We can calculate how much a substance is secreted at the tubule.
• As with the reabsorption rate, we start with the filtered load: the amount of a substance presented to the tubule (mg / min).
• Then T_secr = Ux * V - Px * GFR, where:
  • Ux is the urinary concentration of the substance x (mg / ml)
  • V is the urinary flow rate (ml / min)
  • Px is the plasma concentration of the substance x (mg / ml)
  • GFR is the glomerular flow rate (ml / min)
  • The units of T_secr (the secretion rate) will come out to be mg / ml / min.

Fractional excretion differentiates between reabsorption or secretion

• Notice that the variables of measuring T_reabs and T_secr are the same, we simply reverse the order of subtraction.
  • These two terms ("Px * GFR" and "Ux * V") have their own names: filtered load and urinary excretion.
  • Recall that the filtered load of substance x is the amount seen by the tubule and is defined as Px * GFR (which results in a mg/min term).
    • So the filtered load describes how much of the substance enters the tubule / minute.
  • Recall that the urinary excretion of substance x is the mount secreted and is defined as Ux * V (which results in a mg/min term).
    • So the urinary excretion describes how much of the substance exits the tubule / minute.
• When we ask if a substance is net reabsorbed or net secreted we are asking if the filtered load is greater or the urinary excretion is greater.
  • The ratio (fraction) of the amount of the substance that enters the tubule (filtered load) to the amount that exits the tubule (urinary excretion) tells us whether the substance was reabsorbed or secreted or neither.
    • When the ratio (of filtered load to urinary excretion) is over 1, the substance is reabsorbed (which makes sense because more entered the tubule than left the tubule so there must have been some reabsorption).
    • When the ratio is 1, the substance is secreted and reabsorbed equally.
    • when the ratio (of filtered load: Px * GFR, to urinary excretion: Ux * V) is less than 1, the substance is secreted (which makes sense because less is filtered than is excreted so there must have been some tubular secretion).
• So what is the FE for inulin?
  • We know that inulin shows little reabsorption so the filtered load will be equal to the urinary excretion.
  • The ratio will be 1:1; the fraction will be 1.
• What is the FE for glucose?
  • We know that glucose is reabsorbed very well in the tubule so the filtered load will be much higher than the urinary excretion.
  • The ratio will be 100:1 (as an example); the fraction will be 100 (as an example).

Glucose reabsorption

• Glucose reabsorption takes place in the renal proximal tubule.
• Glucose reabsorption (from the lumen into the proximal tubule cell) occurs through a Na-Glucose cotransporter (SGLT) on the apical surface of the proximal tubule cell by way of the Na gradient generated by Na / K ATPase on the basal surface of the proximal tubule cell.
• Second, glucose is moved from the proximal tubule cell to the ECF / blood by facilitated diffusion by way of a GLUT protein.

Why does glucose reach a maximal T_reabs?

• Recall that doctors of old used to taste the urine of patients to diagnose diabetes; the glucose levels of a diabetic patient can be so high that the tubule cannot reabsorb it all making the urine taste sweet.
• At normal levels, glucose enters the filtrate but is very well reabsorbed such that urinary excretion (Ux * V) values are very low.
• When glucose reaches a very high level in the blood, the epithelial cells of the tubule don't have enough time to reabsorb all the glucose in the filtrate (glycosuria).
• Recall that reabsorption of glucose occurs in the proximal tubule via SGLT (active) and then GLUT (passive).
• Because not all the glucose can be reabsorbed at these high levels, there is increased osmotic pressure in the filtrate and less water is reabsorbed causing polyuria.

Micropuncture and microperfusion of nephrons in vivo

• A nephron can be punctured and perfused with well handled micropipettes; this allows for experimental determination of nephron function.
• For example, we can sample filtrate along the tubule and measure the concentration of inulin at each location.
  • Concentrations of inulin increase as one travels distally.
  • Since we know that inulin doesn't get reabsorbed, we can determine that the concentration is increasing because water is being reabsorbed.

• We can also measure the tubular fluid and the plasma.

Are these volumes or concentrations or what?

• • The ratio of the tubular fluid to plasma is a function of the length of the nephron: the farther along the nephron, the higher the tubular fluid concentration.

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Proximal tubule fluid is essentially iso-osmotic to plasma

• In general, water is reabsorbed along the tubule by way of an osmotic gradient between the ECF and the filtrate.
  • This reabsorption occurs through the cells via aquaporin channels (AQP).
    • Recall that ADH causes increased expression of AQP (aquaporin) at the distal convoluted tubule and the collecting duct, thus increasing water reabsorption (and increasing blood pressure).
• In the proximal tubule, there is no such osmotic gradient between the ECF and the filtrate.
  • This means the proximal tubule is not responsible for much water reabsorption.
  • There is no gradient because of the high water permeability of the epithelium.
• We can observe how the body acts to conserve water by measuring the ratio of urine and plasma osmolarity when rats are infused with waste molecules:
  • Inducing water loss by administering lots of a waste product is called diuresis as in "mannitol diuresis".
  • When urea, glucose, saline, and mannitol are given, the osmolarity of urine rises more rapidly than plasma (causing the U / P ratio to increase); this indicates the ability of the kidney to concentrate urine.
  • The highest urine osmolarity to plasma osmolarity (U / P ratio) is seen when rats are dehydrated.

Perfusion of isolated tubules can teach us about the PCT

• PCT = proximal convoluted tubule.
• Experimentally, we can perfuse a short segment of a tubule and collect the filtrate at the other end.
  • This allows us to measure what is being reabsorbed and secreted in this small area.
  • Note that in perfusion / puncture studies we expect reabsorbed things to decrease in TF / P ratio and secreted molecules to increase in TF / P ratio.
    • This makes sense because....

• Through these puncture studies we have learned about the PCT:
  • Glucose and aas are nearly 100% reabsorbed in the PCT.
  • Na, H20, and K are 70% reabsorbed in the PCT.
    • HCO3- and Cl- accompany Na reabsorption in the PCT.
    • HCO3- transport is favored over Cl transport.
    • It makes sense that HCO3- is reabsorbed over Cl- in the PCT because Cl- can be reabsorbed with Na later in the tubule (ascending loop and distal convoluted tubule).
  • Urea is 50% reabsorbed in the PCT.
  • Inulin becomes more concentrated in the PCT.
    • This means that water is reabsorbed in the PCT.
    • This provides a measure of how much water is reabsorbed.
  • Organic ions (like PAH) are secreted.
    • Recall that PAH is secreted "so vigorously" that nearly all of it is removed from the blood in a single pass.
    • Because nearly all PAH is removed in a single pass, PAH is a good indicator of renal plasma flow (when Hct is taken into account).

• Note that PCT filtrate remains iso-osmotic (relative to plasma) because there is lots of water permeability.
  • That is, while lots of glucose, aas, Na, K, and urea are being reabsorbed so you might think the urine would be starting to concentrate, in actuality, water is following the many of these molecules into the blood (because the epithelium is leaky) so the osmolarity of the filtrate and plasma remain about the same.

Na reabsorption in the PCT

• Recall that most of the oxygen used by the kidneys goes to generating ATP for the Na / K ATPase.
  • The consistent, large Na gradient allows tubule cells to couple transport of many molecules to Na.
• Na reabsorption is the main driving force for reabsorption of solutes and water.
  • Recall that the epithelium of the PCT is leaky, so as solutes are moved via Na gradient, H20 can and will follow.
  • Recall that glucose is moved via the Na-Glucose symporter SGLT on the apical surface of the tubule cells and then via the passive GLUT transporter on the basal membrane.

Reabsorption: from tubular cell to blood

• The Na gradient produced by the tubular cell and Na / K ATPase moves solutes and water into the cell's cytoplasm.
• Mostly passive movement of solutes and water down their concentration gradients moves them from the tubular cell into the interstitial fluid between the tubular cells and the endothelial cells of the peritubular capillaries.
  • Recall that there are two sets of capillaries in the kidney: glomerular and peritubular.
• Movement of these reabsorbed solutes and water molecules from the ICF (intercellular fluid) to the blood is (of course) determined by the four Starling forces:
  • Hydrostatic pressure of the blood and colloid osmotic pressure of the ICF force solutes to stay in the ICF.
  • Hydrostatic pressure of the ICF and colloid osmotic pressure of the blood force solutes to move into the blood.

PCT and secretion of organic ions

• Recall that the PCT is responsible for secretion of organic ions in addition to reabsorption of glucose, aa, Na, K, Cl, and HCO3.
• Secreting organic ions is a two step process: the organics must get through the basal and apical membranes of the tubule epithelial cell.
• Anions must be accompanied by carrier proteins as they cross the tubular cells (from basal to apical surface).
  • This is one reason for drug interactions: when one drug (anion) preferentially binds the carrier protein, another drug (anion) may be not be secreted as fast, causing an elevated effect at a normal dosage.
• OAT1 and OCT are two important transport proteins for ions and are found on the basal membrane of the tubular cells.
  • OAT1 is the transporter that so effectively secretes PAH.

• Here are some organic anions secreted at the PCT:
  • Phenol red (a pH indicator dye)
  • PAH (used for measurement of renal plasma flow)
  • Penicillin (an antibiotic)
  • Probenecid = benemid (inhibits penecillini secretion, inhibits uric acid reabsorption)
  • Furosemide = lasix (a loop diuretic drug)
    • "Loop diuretics act on the Na+-K+-2Cl- symporter (cotransporter) in the thick ascending limb of the loop of Henle to inhibit sodium and chloride reabsorption. This is achieved by competing for the Cl- binding site." per wikipedia
  • Acetazolamide = Diamox (Carbonic anhydrase inhibitor)
    • Recall that carbonic anhydrase converts between CO2+H20 and H + HCO3, thus controlling blood pH.
  • Creatinine (normal end product of muscle metabolism)

• Here are some organic cations secreted at the PCT:
  • Histamine (vasodilator, stimulator of gastric acid secretion)
  • Cimetidine (drug for treatment of gastric and duodenal ulcers)
  • Cisplatin (cancer therapy drug)
  • Norepi (neurotransmitter)
  • Quinine (antimalarial drug)
  • Tetraethylammonium = TEA (ganglion blocking drug)
  • Creatinine (normal end product of muscle metabolism; models GFR)

Excretion of lipid soluble organics

• Lipid soluble organics get into the filtrate through non-ionic diffusion through the tubular cell membranes.
• The lipid soluble molecules would simply diffuse back out if they are not trapped in the filtrate.
• The tubule cells pump hydrogen (H+) and ammonia into the filtrate in order to trap these lipid soluble molecules in the filtrate.

Are both H+ and NH3 secreted?  If so, what does each do?

• • H+ reacts with acids to neutralize and reabsorb them into the blood.
  • H+ reacts with ammonia to trap it in the filtrate.
  • Ammonia reacts with H

• Phenobarbital is used as a sedative and is a lipid-soluble weak oranic acid (A-).
  • In order to neutralize phenobarbital when you no longer want your patient sedated, you give NaHCO3-.
  • NaHCO3- dissociates into Na and HCO3- and increases the HCO3- concentration of the blood.
  • Increased plasma HCO3- will result in less reabsorption of HCO3- in the proximal tubule and thus an increased alkalinization of urine
  • At higher alkalinity (that is, fewer H+), phenobarbital will remain as an acid (and not bind H+) in the filtrate and thus be secreted (instead of binding H+, neutralizing, and being reabsorbed).

Loop of Henle