Renal System: 10:00-11:00Scribe: Aimée Mcbride

Renal System: 10:00-11:00Scribe: Aimée McBride

Friday, May 8, 2009Proof: Andrew Treece

Dr. ShaferProximal Tubule and Nephron Beyond the Proximal TubulePage1 of 7

LOH = Loop of Henle

Introduction [S1]:
Today I’m going to be running through the nephron, telling you what transport processes go on where, and how those processes are regulated.
I spoke to you the last time about the proximal tubule, and talked about the process there of the active Na+ reabsorption of bicarbonate. We’ll talk more about that on Monday.
We also talked about the Tm limited processes that reabsorb metabolically useful substances like glucose and amino acids, dicarboxcylic acids, and tricarboxcylic acids, I gave you a whole list.
Renal Physiology - Lecture #4[S2]
We’re going to go on then today in the proximal tubule and talk about those substances that are reabsorbed passively, and the two primary examples I’ll give you are Cl- and urea.
We’ll then talk about active secretion, how substances are put into the tubular fluid, so they can actually be excreted at greater rates than they’re filtered at the glomerulus.
Then we’ll go along the nephron, the descending limb of the loop of Henle (LOH), going into the medulla, and we’ll find out that primary thing going on there is water reabsorption; there is very little in the way of solute transport there.
And then into the thin ascending limb of the LOH, and there’s a big change in the transport properties between the descending and thin ascending limb.
This segment is water impermeable, and there is passive NaCl reabsorption without water. In the thick ascending limb, there is active NaCl reabsorption, again without water. These two segments are pretty unique in being water impermeable under all conditions.
Major segments of the nephron [S3]
We’re going to talk first about the proximal tubule.
Remember when I say the proximal tubule, I include the proximal convoluted tubule and the proximal straight tubule.
The last part of the proximal straight tubule goes into the very outer region of the medulla called the outer stripe of the outer medulla.
I’m not making any differentiation between what goes on in the proximal convoluted tubule and the proximal straight tubule. The only distinction I’d make between the two that may be of importance is the fact that the proximal straight tubule is the primary region where active secretion goes on. It’s not a distinction I’m going to ask you to be responsible for though really.
Na+ and water reabsorption in the proximal tubule[S4]
Remember what’s going on along the proximal tubule.
You have this active reabsorption of Na+, accompanied by bicarbonate being preferentially reabsorbed.
You’ve also got the active reabsorption coupled to Na+ of glucose, amino acids, etc., these Tm limited systems.
This is what drives the water reabsorption; the water to go along with these solutes that are being resorbed.
But now as the Na+ is being resorbed, it leaves behind a slightly negative luminal potential. Not very big in the proximal tubule, because the proximal tubule is kind of leaky. The junctional complexes are fairly permeable to ions, and so not much of a voltage develops, but a little bit lumen-negative.
What that means is that the lumen-negative voltage, plus the fact that as bicarbonate is being reabsorbed preferentially with the Na+, Cl- is left behind. There will be both a small concentration driving force for the Cl- to just be reabsorbed passively (indicated by a dashed line). Also the lumen-negative voltage, just the electrical voltage, will tend to drive the Cl- reabsorption.
There are also other substances, for example, urea. As the water is being reabsorbed along the proximal tubule, the urea concentration is going to rise slightly. Now, this being a leaky epithelium in the proximal tubule, the urea can diffuse through the junctional complexes. Nevertheless, as the water is reabsorbed, its concentration rises slightly, and that gives a diffusional driving force for the urea to just passively be reabsorbed in the proximal tubule, to follow the water.
Passive reabsorption of Cl-, urea, and other solutes due to Na+ and water reabsorption [S5]
So put together in a schematic, as you get Na+ reabsorption as well as the other Tm limited solutes, that’s also driving water reabsorption.
In addition, that Na+reabsorption is making the lumen slightly negative.
As the water is reabsorbed, luminal concentration of these passively reabsorbed solutes like Cl- and urea goes up. So in the case of Cl-, two things are driving it to be passively reabsorbed. Both the lumen negative potential and the rise in its concentration as the water is being resorbed. That’ll lead to the passive Cl- reabsorption.
In the case of urea the negative voltage doesn’t matter because urea is a nonelectrolyte. It doesn’t have any charge, so the main thing is the rise in the luminal urea concentration. Not very much of rise, it goes from around 4 millimolar when it’s filtered to 6 or 7 millimolar at the end of the proximal tubule. That’s enough to drive the passive urea reabsorption along the proximal tubule.
You might ask, “Why would you want to resorb urea? That’s not metabolically useful. You’d really like to get rid of urea as a by-product of metabolism.” Well, this is kind of an accident of nature, because the proximal tubule is so permeable to a lot of different substances. It’s leaky, and urea is a small molecule. It has a molecular weight of 60, which is very small, 1/3 the size of glucose. The urea just gets passively reabsorbed, even though that’s not exactly what you want to do in the overall scheme of things, you want to get rid of the urea.
As we’ll see, you can effectively do that later in the nephron. We’ll talk about that when we get to the urinary concentrating mechanism.
Active secretion of some metabolic byproducts, toxins, and drugs[S6]
Also in the proximal tubule along the length is active secretion of certain substances.
It turns out that quantitatively, the substances that are actively secreted don’t add a lot of osmoles to the lumen.
These are small processes that involve certain things that the body wants to get rid of.
These can be by-products of metabolism, like hippurate and uric acid to a certain extent, but they appear in very small concentrations in the plasma and also in the final urine.
More importantly, these active secretory processes are also effective ways that the body can get rid of toxins and drugs. It’s particularly the importance of the secretory mechanisms in clearing drugs.
Generally when you take in potential toxins or drugs, the body doesn’t make a distinction between what’s going to help it like a drug and what’s going to hurt it like a toxin. These substances are either broken down metabolically in the liver; we call that the clearance by the liver, the ability of the liver to get rid of the potential toxins and drugs by metabolizing them.
For other toxins and drugs, you may not have only liver metabolism, but the kidneys also operate to get rid of these substances by eliminating them in the urine. It turns out that there are transport processes that can actively secrete a lot of different drugs and toxins. Quantitatively it’s not a big amount, but these substances in low amounts can be potentially toxic in the body.
This is an important process to remember when prescribing drugs for patients, because if you give a person who may have kidney disease a normal dose of the drug, it may become toxic. Their kidneys aren’t operating to get rid of that drug as fast as a normal individual.
It’s very important to take a history and background, and asking a patient what drugs they’re taking to get an idea if they may be being treated for chronic kidney disease. If they are, there are tables by which you can calculate how much renal function they’ve lost and how much theirsecretory capacity for the drug may be diminished. You adjust the drug dose accordingly.
It’s very important to know about this mechanism and realize that many drugs are cleared from the bloodstream by kidney clearance filtration, but augmented by secretion, and when the kidney’s ability to filter and to secrete goes down in chronic kidney disease, you have to adjust your drug dosage down accordingly.
Examples of substances that are actively secreted [S7]
So what kinds of substances are actively secreted? I’ll give you a few examples.
This list is not one that I expect you to memorize. It is just to get a general idea. You can look up and find out what substances are secreted by the kidney.
They generally fall into two groups of organic anions and organic cations. Within each of these, they fall into ones that are endogenously present and those that are exogenous.
Exogenous anions:
One of the best examples of a substance that is very actively secreted the most of anything is called para-amino-hippurate (PAH). He talked about this previously when he gave an example of something with a clearance so high it’s equal to the total renal plasma flow. He talked about clearance of PAH as an extreme example of something that’s removed entirely from the plasma as it goes through the kidney. That’s the ultimate example, but it’s not endogenously present. It’s something that you would add to the plasma, it’s not normally excreted.
Other endogenous substances (anions) include:
cAMP
Salts of bile acids that appear in the plasma are actively secreted in the kidney and eliminated in the urine.
Prostaglandins are actively secreted into the tubular fluid. These can have a vasodilating role in the kidney, and they are produced locally in the kidney.
Urate, hipparte, oxalate.
Uric acid is actively secreted.
Hippurate is the normally appearing brand of PAH. It’s a similar compound without the para-amino group.
Oxalate
These are by-products of metabolism. The kidney is trying to get rid of these by-products because they cannot be further degraded and are not metabolically useful.
How about endogenous cations?
We already heard about creatinine, and he told us that there is a little bit ofcreatininesecretion.
It’s not a perfect substance like inulin that is not reabsorbed or secreted. Creatinineis secreted to a small extent.
Only about 10% of the excretion of creatinine is due to secretion.
Also among the endogenous cations are catecholamines like epinephrine, norepinephrine, and dopamine. Most adrenergic agonists and antagononists that you may use are actively secreted.
Among the actively secreted endogenous anions, quite interestingly the major diuretics, both the negatively charged thiazide, furosemide, bumetanide,and acetazolamide are actively secreted, and also another diuretic amiloride is a positively charged organic ion (Note: it is on the exogenous list).
All of these diuretics are actively secreted in the proximal tubule.
The importance of this is because they’re actively secreted, their concentrations in the more distal regions of the nephron are going to be even higher than they would be due to filtration. It’s very useful that the diuretics are actively secreted because it gives them a much higher concentration in the sites where they act along the nephron. We’ll talk about these sites later.
What that means is that the diuretic concentrations that you give don’t have to produce a very high plasma level, because the kidney will secrete these substances, and they can act more effectively by having a higher concentration at their sites of action.
(He skipped back to exogenous anions): Look at other drugs; most of the antibiotics are actively secreted in the proximal tubule. This is important because antibiotics at higher dosages, at higher levels in the plasma be toxic and produce toxic side effects. A patient with chronic kidney disease can’t secrete the penicillin or other antibiotics, so you have to adjust the dosage downward.
Probenecid is another example, as are salicylates like aspirin and some of the other NSAIDS are actively secreted in the proximal tubule.
Among the exogenous cations, atropine, isoproterenol, morphine, quinine, cimetedine.
Cimetedine is an ACE inhibitor.
A lot of different drugs are actively secreted, as are a lot of potential toxins. One of the real mysteries of this is how the kidney knows to synthesize all of these transporters that it may never need. It’s like it has to be prepared for anything, and it looks like there are relatively nonspecific transporters of these organic ions and cations that have a wide degree of specificity for what they can transport and what they can secrete.
Just like in the case of reabsorptionof organic solutes, these are Tm limited processes. You don’t have a lot of theses transporters to secrete these substances but enough to do the job for the normal levels that are ingested or given as drugs.
It’s an important process that goes on in the kidney, but quantitatively it doesn’t make a big contribution to the final amount of solutes that the kidney is excreting. It’s important to get rid of the traces of these potential toxins and other substances.
Key points - Proximal Tubule[S8]
To summarize what we’ve talked about in the proximal tubule.
General epithelial structure
The proximal tubule, like all of the segments of the nephron, is a single cell layer of cells connected by junctional complexes, with a luminal membrane and a basolateral membrane, a basement membrane. They’re right next to the capillaries, there’s no real serosal tissue layers. The nephrons are right in contact with the capillaries.
General mechanism of active Na+ reabsorption
Keep this in mind as we go along the nephron. The Na+ is entering cells passively across the luminal membrane, and that’s being driven by the active transport of the Na/K ATPase which is located ONLY on the basolateral membrane. That’ll hold through all along the rest of the nephron.
Isosmotic reabsorption of 2/3 of filtered solutes and water
Primarily Na+, bicarbonate, which are reabsorbed isosmotically. The majority of the filtered solutes and water are resorbed in the proximal tubule. This process is isosmotic because the tubular fluid stays at about the same osmolality as the plasma.
Tm-limited reabsorption of glucose and other metabolicallyuseful solutes
Glucose titration curve
In the case of glucose, Tm limitation means that it is normally completely reabsorbed, but once you reach a plasma threshold, the glucose begins to appear in the urine and the reabsorptive rate falls off below the rate of filtration until it reaches a Tm, a transport maximum.
Passive reabsorption: urea, Cl-, K+
Active secretion of metabolic by-products, drugs, &potential toxins
What remains after the proximal tubule?[S9]
Going on along the rest of the nephron, think first about what’s left over.
The tubular fluid has 1/3 of the filtered solute and water, and has an osmolality that’s nearly the same as the plasma.
We’re going to be focusing on as we move along the segments beyond the proximal tubule is the changes in the osmolality of the tubular fluid, and what regions of the nephron are water impermeable verses water permeable, and what happens to water reabsorption. There are solutes that we’re really interested in at this point: Na+, Cl-, K+, and urea are going to be the focus. On Monday he’ll talk about magnesium and calcium.
Costanzo Fig. 6-3 [S10]
After the proximal tubule, the fluid enters the descending limb of the LOH.
This segment is called thin because the epithelium is very attenuated here; it’s very flat with few mitochondria. The nucleus takes up most of the intracellular space.
Just as it looks, the descending limb of the LOH has a very limited transport capacity. It doesn’t really actively reabsorb any solutes. It’s largely passive with regard to solute transport, but the main thing that’s happening here is a consequence of the environment that surrounds it. It’s entered the medulla, and particularly down in the inner regions the osmolality of the interstitial fluid, which is the extracellular fluid surrounding the tubules and capillaries, is hyperosmotic.
At the junction between the cortex and the medulla, and all through the cortex, the interstitial fluid surrounding the cells is isotonic, like it is everywhere else in the body. The unique thing about the kidney is, as you go into the medulla, the interstitial fluid becomes increasingly concentrated. Na+ and Cl- concentration rise, and the urea concentration rises, so when you get down here toward the tip of the papilla, in the human when a person is in antidiuresis (when they’re forming a concentrated urine), the osmolality of the interstitial fluid is hypertonic here.
The hypertonicity is about 1200 (which is about 4x the osmolality of the plasma) down in the medullar interstium at the tip. About ½ of it is due to urea; 600 milliosmoles of urea, and about ½ is due to Na+ and Cl-. The NaCl concentration down here is about 300 millimolar, but remember that osmolality for Na+ and Cl- is double the molality, because you have Na+ plus Cl-, so the two osmotically give double the molal concentration, which is about 600 milliosmols due to Na and Cl.
The gradient of osmolality going from isotonic at the corticomedullary junction, and progressively increases to reach the high point at the tip of the papilla. He’ll explain why later in the lecture.
As this isosmotic tubular fluid flows down the thin descending limb of the LOH and is surrounded by this hypertonic environment, water is osmotically taken out of the tubular fluid and is put into the medullar interstitium and is taken up by the capillary loops that go through the medullar interstitium.
Major regions of the kidney[S11]
The important part here is that all of the cortex is isotonic, but as you go down into the medulla in particularly the inner medulla down by the papilla, there is a gradient of osmolality going from nearly isotonic at the junction to as high as 1200 in the human at the tip of the papilla.
Thin descending limb of the loop of Henle[S12]
What does that do? This structure is very water permeable, so the primary thing that happens in the thin descending LOH is that water gets reabsorbed, primarily just water. The high water permeability is due to the same thing as in the proximal tubule - the water channel aquaporin-1 is present in the luminal and basolateral membrane of the thin descending limb of the LOH. It’s very water permeable, the main thing happening is that water is being reabsorbed. By the time you get down to the tip of the LOH, you’re going to have reabsorbed water. As you reach the tip of the longest loops of Henle, 90% of the water that was filtered is resorbed.
How about solutes? As the water is reabsorbed, the Na+ and Cl- concentration rises, as does the urea concentration. The urea concentration out in the medullary interstitium is really high, so urea can diffuse into the descending limb of the LOH to a small extent and add to the amount of urea present.
Na+ and Cl-aren’t very permeable here, so they tend to get concentrated in the tubular fluid as the water is reabsorbed. That means at the tip that the tubular fluid is the same osmolality as the interstitial fluid, at about 1200. Most of the osmolality in the tubular fluid is due to Na+ and Cl- that have been concentration by the water reabsorption, and to a lesser extent urea, whereas out in the interstitial fluid surrounding the nephron, urea and NaCl are present in about equal amounts.
Ascending limb of the loop of Henle [S13]
As you turn the bend and begin to go up the thin ascending limp of the LOH, you have a much lower urea permeability, so urea tends not to leak into the lumen so much, and there is a higher Na+ and Cl- permeability.
This segment has changed its permeability properties to be more permeable to Na+ and Cl-so that they diffuse out of the tubular fluid into the interstitial fluid down a favorable concentration gradient.
It’s also important that the characteristics of this epithelium are different in terms of water permeability. The thin and thick ascending limbs of the LOH are always water IMPERMEABLE. As the salt is reabsorbed, the tubular fluid becomes more and more dilute.
As you go up into the thick ascending limb, there is active Na+ and Cl-reabsorption, so even more salt is taken out. Up in the cortex, the tubular fluid is more dilute than the plasma, down to about 1/3 the osmolality of the plasma. The thin ascending limb and the thick ascending limb of the LOH is called the diluting segment. Both structures are water impermeable, there’s no aquaporin-1 present there. The thin ascending is fairly permeable to Na+ and Cl-so it can diffuse down a concentration gradient into the medullar interstitium.
From this point on, the urea is impermeable, which means it is trapped in the tubular fluid as it goes back up to the cortex, it can’t be reabsorbed, even though its concentration is higher as it goes back up to the cortex.

SQ: Question about the NaCl that is being reabsorbed.