Editing Tubular reabsorption & secretion

*started here on 03/23/11


==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<sub>reabs</sub> = 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<sub>reabs</sub> (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<sub>secr</sub> = 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<sub>secr</sub> (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<sub>reabs</sub> and T<sub>secr</sub> 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<sub>reabs</sub>?===
*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'''.
*There is a difference in the maximum glucose reabsorption rate in cortical and juxtamedullary glomeruli.

===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 concentration of inulin in the tubular fluid and in the plasma.
**The ratio of the tubular fluid concentration to plasma concentration is a function of the length of the nephron: the farther along the nephron, the higher the ratio; that is, the farther along the nephron, the more inulin found in the filtrate and the less found in the plasma.
**The huge change in concentration between the DCT and the urine represents that ability of the collecting duct to significantly concentrate the filtrate even in its already-pretty-concentrated form.
**This graph also shows us that the PCT is specifically designed for and is very capable of bulk reabsorption of solutes and water.
***This is evidenced by the rise in ratio over a short distance.

===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.


*Note that while many solutes and water molecules are being reabsorbed in the PCT, the osmolarity of the filtrate and plasma are similar throughout the PCT.
**This indicates that the PCT is capable of bulk transport but not of concentrating the filtrate (because then the osmolarity would increase).


*Recall that diabetes insipidus occurs when ADH is elevated such that water loss is large.

===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 (like glucose and aa) 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.
***Na is also moved via Na-aa channels on the apical surface.
*Note that K+ does not accumulate in the epithelial cell (from Na / K ATPase activity on the basal surface) because of K+ ions pores on the basal surface.

===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 (intracellular 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.
***The hydrostatic pressure of the blood in the peritubular capillaries is much lower than it was at the glomerular capillaries.
***The plasma colloid pressure is high because so much fluid was lost at the glomerulus yet the proteins remained.
**Hydrostatic pressure of the ICF and colloid osmotic pressure of the blood force solutes to move into the blood.
***Interstitial hydrostatic pressure is increased because of the active pumping of solutes with water following.

===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 (organic anion transporter) and OCT (organic cation transporter) 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 penecillin secretion, inhibits uric acid reabsorption)
***Was important back when penecillin was so expensive and we wanted to keep it in the pt's blood.
**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 [http://en.wikipedia.org/wiki/Loop_diuretic 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+) into the filtrate in order to trap these lipid soluble molecules in the filtrate.
**The idea here is that neutral, polar solutes will not remain in the filtrate, so the filtrate can be acidified (with H+) or alkalized (with HCO3-) to cause lipid-soluble substrates to adopt a charged or non-polar state so they will remain in the filtrate.
***For example, H+ reacts with ammonia to trap it in the filtrate.
**H+ reacts with acids to neutralize and reabsorb them into the blood.


*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 alkalinity 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===
*The loop of Henle serves to dilute the urine.
*Dilution occurs because the ascending loop is impermeable to water (that is, water from the filtrate cannot be reabsorbed) yet there is active reabsorption of solutes like Na and Cl.
**Thus, as the epithelial cells remove solutes but leave behind the water that would like to follow, '''the filtrate becomes more dilute'''.


*The loop of Henle is where '''loop diuretics''' work.
**Loop diuretics cause a loss of water and are therefore useful in treating hypertension (by decreasing the extracellular fluid compartment).
**Loop diuretics include furosemide, bumetanide, etc.
**'''Loop diuretics inhibit the Na / K / 2Cl cotransporter of the ascending limb''' that moves Na, K, and Cl from the filtrate to the interstitial fluid.
**One might think that this means water will not follow, but recall that the ascending limb is nearly impermeable to water anyhow.
**Thought loop diuretics have their pharmacological effect on the channels and cells of the ascending loop, '''loop diuretics have their physiological affect on the collecting duct'''.
**When loop diuretics decrease the Na / K / 2Cl content of the interstitial fluid between the (parallel) ascending loop and collecting duct, water reabsorption is decreased at the collecting duct because there is less osmotic force pulling water from the filtrate to the interstitial fluid.

===Distal terminology===
*To describe physiological functions, we talk about the "distal tubule" and the "distal nephron".
*The distal tubule is more than just the convoluted tubule (DCT).
**The distal tubule includes the distal convoluted tubule, the connecting tubule, and the initial cortical collecting duct.
*The distal nephron is the distal tubule plus all of the collecting duct.
**The distal nephrn is the DCT, connecting tubule, cortical collecting duct, outer medullary collecting duct, and inner medullary collecting duct.

===Thiazide diuretics===
*Thiazide diuretics cause a decrease in water reabsorption and are thus useful for treating hypertension (by decreasing volume of the extracellular compartment).
*Thiazides include chlorothiazide and metolazone.
*'''Thiazides inhibit the Na / Cl cotransporter at the distal convolunted tubule''' (DCT) causing a decrease in Na / Cl reabsorption.
*Decreased reabsorption means there is more Na and Cl in the filtrate; that is, the filtrate will be less dilute.
*Recall that the job of the collecting duct is to reabsorb water and that the collecting duct achieves water reabsorption by the osmotic gradient between the medullary area and the filtrate.
**So when solutes of the filtrate are not reabsorbed, the osmotic pull of the filtrate increases and there is a smaller difference in the medullary and filtrate osmotic forces, resulting in decreased water reabsorption.
**More explicitly, when thiazide diuretics decrease Na / Cl reabsorption, they increase filtrate osmotic force, they decrease the medulla-filtrate osmotic gradient, and they decrease water reabsorption at the collecting duct.


*Note that loop diuretics decrease solute reabsorption at the ascending loop and thiazides decrease reabsorption at the DCT, but '''both loop diuretics and thiazides achieve their physiological effect by decreasing water reabsortpion at the collecting duct'''.

===Collecting tubule===
*The collecting tubule is the last segment of the nephron.
*The collecting tubule is a '''tight epithelium''' and contains '''two cell types''': principle cells and intercalated cells.
**2/3 of the cells of the collecting duct are principal cells and 1/3 of the collecting duct cells are intercalated cells.
*'''The collecting duct affects Na, K, and H+ secretion / reabsorption.'''


*Principal cells help the rest of the epithelial cells of the nephron regulate Na, K, and water.
**Principle cells reabsorb Na and water while excreting K.
**K secretion make sense because the Na / K ATPase on the basal surface of the principal epithelial cell generates a flow of Na into the blood (reabsorption, which H20 follows) and a flow of K out into the filtrate.
 "K+ secretion is increased when urine flow increased due to diuretic action (problem of K+ wasting)."
 What is causing the diuretic action?
 Why is K+ secretion increased?
**Note: the more concentrated the urine, the more K+ lost.
***This makes sense because concentrated urine occurs when Na reabsorption is high and when Na reabsorption is high, we know that lots of K+ is being exchanged for Na+.


*'''Intercalated cells help maintain pH homeostasis.'''
**There are two types of intercalated cells: one to move the pH up (alkalinate) and one to move the pH downward (acidify).
**'''Intercalated cells function to maintain blood pH by secreting what is high and reabsorbing what is low.'''
**The only difference in intercalated cells is which of H+ and HCO3- they secrete and which they absorb.
**Alpha cells (think "a for aaaahh! too much acid") secrete H+ and reabsorb HCO3-, thus raising the pH of the blood.
***Alpha cells secrete H+ via an apical H+ATPase and an apical H+/K+ ATPase.
***'''AE1 is the protein of alpha cells that exchanges HCO3- into the blood for Cl- into the alpha intercalated cell.'''
***Note that H+ secretion is an active process taking place on the apical surface of alpha intercalated cells of the collecting duct.
**Beta cells secrete HCO3- and reabsorb H+, thus decreasing the pH of the blood.
***'''Pendrin is the protein of the beta cells that exchanges HCO3- into the lumen for Cl- into the beta intercalated cell.'''
**Movement of H+ (in either direction: secretion or reabsorption) is achieved by antiporting with K+.
***Alpha cells secrete H+ into the filtrate and thus reabsorb K+ (because to move H+ into the filtrate we have move K+ into the blood).
***Therefore '''alpha intercalated cells are activated when dietary K+ is low such that K+ reabsorption becomes a high priority'''.
**AE1 and pendrin are both HCO3- / Cl- exchangers; AE1 is used by alpha cells to move HCO3 into the blood while pendrin is used by beta cells to move HCO3 into the filtrate.
**Note that '''via intercalated cells of the collecting duct, the kidney can help regulate blood pH'''.

===Aldosterone's effects on the kidney===
*Aldosterone acts on the principal and intercalated cells of the collecting duct.
*Recall that aldosterone is released from the zona glomerulosa of the adrenal gland in response to angiotensin 2 signaling.
*Aldosterone acts on the principal cells in the collecting duct of the nephron to increase water reabsorption.
**Aldosterone increases expression of ENaC on the apical surface.
**Aldosterone also increases the expression of Na / K ATPase on the basal surface of the principal cells.
**Both of these increases in protein expression / function will cause an increase in Na / water reabsorption from the filtrate.
*Aldosterone acts on the intercalated cells of the collecting dcut of the nephron to decrease blood pH.
**Aldosterone increases alpha intercalated cell activity which causes increased secretion of HCO3- into the blood and H+ into the filtrate.


*'''Amiloride is an anti-ENaC drug''' that is used to decrease Na / H20 reabsorption and thus decrease water retention and blood pressure.
**Recall that ENaC is used by epithelium of the collecuting tuble.
**Recall that aldosterone causes increased ENaC expression on the principal cells of the collecting duct.

===ADH's effects on the kidney===
*ADH affects principal cells of the collecting duct (as well as epithelial cells of the DCT).
*Recall that ADH is released from the posterior pituitary upon sensation of low blood pressure at the hypothalamus.
*ADH causes increased aquaporin-2 on the apical surface of the DCT and collecting duct epithelial cells.
**Recall that aquaporin-2 is a water channel.
**ADH binds the g-coupled V2 receptor which causes and increase in cAMP, an increase in Ca, and PKA activation which all lead to '''increased aquaporin-2 expression and exocytosis of vesicles holding aquaporin-2 channels'''.

===Potassium sparing diuretics===
*Potassium-sparing diuretics cause a decrease in water reabsorption and are thus useful for treating hypertension (as they decrease the volume of the extracellular compartment).
**Recall that principal cells of the collecting duct reabsorb Na and water while secreting K.
**'''Potassium-sparing diuretics act on the ENaC channels of principal cells to decrease Na reabsorption through ENaC.'''
**It makes sense that these diuretics are called "potassium-sparing" because they will ''allow the body to keep its potassium stores''; when you inhibit the flow of Na from filtrate to blood, you will also arrest the flow of K from blood to filtrate (because of electrochemical gradients across the epithelial principal cell).
**Note that '''potassium-sparing diuretics act on cells of the collecting tubule and have their physiological effect at the collecting tubule'''.

===Comparison of PCT and Collecting duct===
*The PCT and DCT differ in their inherent water permeability, precision of reabsorption control, transport capacity, and transepithelial gradients.
*Inherent water permeability:
**The PCT's inherent water permeability is high while the collecting duct's is low.
**Recall that the PCT reabsorbs most of the solutes and water of the filtrate.
*Transport capacity:
**The PCT has a very high transport capacity while the collecting duct's transport capacity is smaller.
**Recall that the PCT reabsorbs most of the solutes and water of the filtrate.
*Precision of reabsorption control:
**The PCT's control is course while the collecting duct's control is fine.
**Recall that diuretics, ADH, and aldosterone all work at the collecting duct (or at least the latter parts of the nephron).
*Transepithelial gradient
**The PCT has a low transepithelial gradient while the collecting duct has a high transepithelial gradent.
**Recall that the DCT generates a high transepithelial gradient so the collecting duct can reabsorb lots of water.


*Water and Na reabsorption by percent and nephron location:
**Na reabsorption:
***PCT: 70%
***Loop of Henle: 20%
***DCT and CD: 9%
**Water reabsorption:
***PCT: 70%
***Loop of Henle: 10%
***DCT and CD: 19%

===Inherited defects in kidney tubule epithelial cells===
*There are many diseases that affect the epithelial cells of the kidney tubule:
**Renal glucosuria (SGLT – Na / Glucose cotransporter)
***A faulty SGLT protein would caused decreased Na / Glucose reabsorption, increased filtrate osmolarity, and decreased water reabsorption, thus causing increased urination and water loss.
**Cystinuria (amino acid transporter)
***A faulty amino acid transporter would caused decreased aa reabsorption, increased filtrate osmolarity, and decreased water reabsorption, thus causing increased urination and water loss.
***Can also cause kidney stones.
**Bartter syndrome (Na / K / 2Cl cotransporter)
***...see above
**Gitelman syndrome (Na / Cl cotransporter)
***...see above
**Liddle syndrome (ENaC)
***Severe hypertension
***...see above
**Nephrogenic diabetes insipidus (V2 receptor or AQP2)
***A defective V2 receptor would cause decreased ADH signaling, decreased AQ2 (aquaporin-2) protein expression / release on principal cells, and decreased water reabsorption.
**Nephrogenic syndrome of inappropriate antidiuresis (increased V2 receptor activity)
***Increased V2 expression would cause an elevated response to ADH at the principal cells, and increased expression / release of AQ2 and increased water reabsorption.