Transport energetics and organic solute transport

Question 1

What are the 2 types of mediated transport?

Answer 1

Passive and active

Question 2

What is passive transport?

Answer 2

1. facilitated diffusion through pores and channels
2. fluxes are always passive and usually uncoupled
3. Substrates move down their chemical (if uncharged), or net electrochemical gradient (if charged) –> Dissipates gradients (equilibrates)
4. Can saturate

Question 3

What is active transport?

Answer 3

• Transport of a particular solute is active when net flux occurs against an opposing gradient of electrochemical potential (Δμ)

Question 4

What is the electrochemical potential?

Answer 4

If there is a solute in solution which is more concentrated on inside vs. outside membrane, there is a chemical potential.
The (passive) net flux of a solute between two compartments is driven by the difference in chemical potential on each side of the membrane.

Question 5

How can we calculate the chemical potential?

Answer 5

μ~i = μ~’ + RTlnCi + zFi + RT lnfi

RTlnCi = chemical work
zFi = electrical work
RT lnfi = work of interractions between solute molecules

Question 6

How can we change the formula so that all of the parameters can be measured?

Answer 6

Although the chemical work and work of interactions cannot be determined experimentally, these terms can be combined by replacing solute concentrations (Co) with activities (Ao), which are measurable using selective electrodes. They will measure the activity of the solute (sum of concentration and interaction terms).

μ~i =μ~’+RTlnAi+zFi

Question 7

What happens to the formula when an ion with z = 1 is at equilibrium?

Answer 7

then eqn. (6) is easily rearranged to the familiar Nernst equation.
Nernst equation: relationship between the ion concentration gradient across the membrane and the voltage.
At equilibrium, the above equation becomes the Nernst equation

Vm = (- RT / zF) ln (Ai/Ao)

Question 8

Name the 2 types of active transport

Answer 8

1. Primary

2. Secondary

Question 9

What is primary active transport? Give examples

Answer 9

• The energy for “uphill” transport is provided directly by ATP hydrolysis (energy input).
• Ion transporters (“pumps”) use ATP
• Examples: 3Na/2K ATPase exchange pump, H+-ATPase pump, H+/K+ ATPase pump

Question 10

What is the Na/K ATPase? Where is it located?

Answer 10

uses 1 ATP; found in virtually all cells of the body; on basolateral membrane of kidney

Question 11

What is the H+ ATPase? Where is it located?

Answer 11

vacuolar proton pump; generally on apical membrane or intracellular vesicles; important for proton secretion in collecting duct and proximal tubule (to a lesser extent)

Question 12

What is the H+/K+ ATPase? Where is it located?

Answer 12

exchanger; also important in extruding protons and bringing potassium in cells; important in the gut, stomach; take inhibitors of that in case of gastric ulcers) in the outer medullary collecting duct

Question 13

What is secondary active transport? Give examples

Answer 13

“Uphill” flux of one solute driven by the “downhill” flux of another solute.

E.g. Na+/H+ exchanger, Na-Glucose cotransport

Question 14

What is the Na+/H+ exchanger? Where is it located?

Answer 14

secretes protons into the lumen of the proximal tubule. The large inward concentration gradient favoring Na+ influx into the cell through the apical membrane provides the driving force for proton secretion from the cell to the lumen. Occurs by transporters that couple solute movements. Energy from the downhill flux of one molecule or ion drives the uphill transport of another

Question 15

In which part of the kidney are most solutes reabsorbed?

Answer 15

The proximal tubules

Question 16

What are gap junctions? What is their function? where are they located in the kidneys?

Answer 16

Gap junctions are between epithelial cells of the tubules.
Gap junctions between the cells allow large solutes or signalling molecules to pass from one cell to the next within the tube. All cells around are connected. If ATP is used in one cell then it can pass through and be used in another cell.

Question 17

Where is the basement membrane of the tubules and what is its main function?

Answer 17

Basement membrane on the outside of the renal tubule is continuous with the renal capsule. (forms “sheet” over bowman’s capsule and around tubule). High pressure in tubules; is a supporting structure. Keeps epithelium intact despite high pressure.

Question 18

What is present in the apical membrane of the proximal tubule but not other parts of the tubules?

Answer 18

The apical membrane of the proximal tubule (ONLY PROXIMAL TUBULE) forms tightly packed microvilli, the brush border –> increased surface area for absorption, covered with transporters

Question 19

Where is the dividing line between apical and basolateral membrane?

Answer 19

At the tight junctions

Question 20

How are the tight junctions in the proximal tubules vs. the more distal segments?

Answer 20

The tight junctions are leaky to ions and water in the proximal tubule but are tight in more distal segments.
This trend is reflected in the transepithelial electrical resistance, which is 6 ohms •cm2 in the proximal tubule and 200 ohms•cm2 in the collecting duct.

Makes sense: in proximal tubule, absorbing liquid around that is practically isotonic. Small gradient, so need a lot of active transport. As we move further, gradient develops; thus, need to close gaps to maintain gradient.

–> tightness of epithelium varies tremendously.

Question 21

Name an example of heterogenous cell types in the collecting duct

Answer 21

the collecting duct consists of potassium-secreting “principal” cells interspersed with acid- or bicarbonate secreting “intercalated” cells.

Question 22

How does inulin concentration vary in the tubule as you go through the proximal tubule

Answer 22

It increases almost linearly because it is not secreted nor reabsorbed, but water is reabsorbed. So there is half the amount of water there was at the beginning after filtration.

Question 23

How does concentration if Na and Cl ions vary in the tubule as you go through the proximal tubule

Answer 23

Ions (Na, Cl): Very little change in Na concentration despite water reabsorption, meaning the sodium was also reabsorbed.
Chloride increases then reaches a plateau; because Na starts to be reabsorbed with other solutes so the Cl is left behind in the lumen, then concentration builds up as fluid is reabsorbed.

Question 24

The leaky pathway between epithelial cells is selective to which ion?

Answer 24

Chloride: gradient of chloride from lumen to interstitium, an electrical potential is developed. This is why the polarity in the lumen switches and lumen becomes positive. Cl concentration has become bigger in the lumen, Cl wants to diffuse out.

Question 25

How does concentration bicarbonate vary in the tubule as you go through the proximal tubule

Answer 25

Bicarb: starts normal at glomerulus but then decrease in concentration, meaning it is reabsorbed faster than water is.

Question 26

How does the net osmotic pressure change along the proximal tubule?

Answer 26

Net effect –> only slight change in osmotic pressure along tubule (water is absorbed but along with many other solutes, leaving a relatively constant pressure).

Question 27

What is splay of the glucose titration curve?

Answer 27

The Tm of glucose is not reached abruptly but is reached gradually, causing splay of the titration curve

Question 28

What is the glucose threshold?

Answer 28

The level of plasma glucose at which glucose begins to “spill” into the urine is called the glucose “threshold”.
In normal man, the threshold is at a plasma glucose concentration of 180-200 mg% (about 10 mM).

Question 29

In what cases might splai be exaggerated?

Answer 29

splay has clinical importance because it may be exaggerated in some hereditary tubular diseases. Such patients excrete glucose at lower than normal plasma glucose levels but have a normal Tm at high glucose loads.

Question 30

In what cases might splay be exaggerated?

Answer 30

splay has clinical importance because it may be exaggerated in some hereditary tubular diseases. Such patients excrete glucose at lower than normal plasma glucose levels but have a normal Tm at high glucose loads.

Question 31

Where does glucose reabsorption occur?

Answer 31

Reabsorption takes place in the proximal tubule (most avidly in the early convoluted or “S1” portion) where the concentration of glucose in the tubular fluid is reduced far below that present in the peritubular interstitium.

Question 32

What type of transport takes glucose out of the lumen of the proximal tubule?

Answer 32

Secondary active transport

Question 33

What happens to the transporters of glucose when sodium is too high or too low in the tubules

Answer 33

Brushborder membrane forms vesicles in the cell

Question 34

What process takes care of maintaining the gradient for glucose reabsorption?

Answer 34

1. Active Na+ transport from the cell to the interstitial fluid across the basolateral cell surface maintains the cell Na+ concentration below that in the tubular lumen. (Na/K ATPase)
2. the cell interior is electrically negative with respect to the tubular fluid. Thus, there is a steep net electrochemical gradient favoring entry of Na+ from the lumen into the cell.

Question 35

What is the stoichiometry of transport of Na and Glucose by the Na/G cotransporters?

Answer 35

In the early parts of the proximal tubule (S1 and S2) the stoichiometry of transport is 1 Na : 1 glucose. This ratio increases to 2:1 in the S3 portion, presumably so that luminal glucose can be reduced to very low concentrations

Question 36

Besides the stoichiometry of the transport, what is the difference between the transporters in S1 and S2 vs. the one in S3?

Answer 36

The predominant cotransporter in S1 and S2 carries D-glucose but not D-galactose whereas D-glucose and D-galactose are both carried by the transporters in S3.

*By contrast, D- mannose, D-ribose or L-glucose are not transported in any of these segments

Question 37

Name a molecule that is used to study the kinetics of cotransport

Answer 37

The kinetics are often studied using non-metabolizable analogs such as alpha-methyl-glucoside

Question 38

What is the Km of 1:1 cotransport of glucose and Na?

Answer 38

The KM of 1:1 cotransport is approximately 15 mM for sodium and 6 mM for glucose

Question 39

What inhibits glucose transport by the Na/G cotransporter?

Answer 39

Glucose transport is competitively inhibited by phlorizin (Ki ~ 10 M)

Question 40

How does glucose get out of the basolateral membrane of the tubular cell?

Answer 40

Glucose diffuses out of the cell through the basolateral cell membrane by facilitated diffusion to complete the process of trans-tubular reabsorption. The basolateral carrier that mediates this exit is found in most cells of the body and is sensitive to the inhibitor phloritin, an analog of phlorizin.

Question 41

How does glucose get out of the basolateral membrane of the tubular cell? What molecules inhibits it?

Answer 41

Glucose diffuses out of the cell through the basolateral cell membrane by facilitated diffusion to complete the process of trans-tubular reabsorption (GLUT2). The basolateral carrier that mediates this exit is found in most cells of the body and is sensitive to the inhibitor phloritin, an analog of phlorizin.

Question 42

Which SGLT isomer is present in the kidney?

Answer 42

SGLT2

Question 43

Describe the structure of SGLT1. Where is it located?

Answer 43

SGLT1 has 14 membrane spanning regions and probably functions as a heterodimer with another protein called RS1.
Found in the intestine and in the proximal straight tubule
13-14 transmembrane segments, alpha-helices
Some syndromes of glucose malabsorption are mutations of the SGLT1 gene
An example of mutation is D–>N mutation at the membrane. Causes glucose/galactose malabsorption

Question 44

Is SGLT2 coded by the same gene as SGLT1?

Answer 44

Found SGLT2 in the kidney

SGLT2 is coded by a different gene but is homologous to SGLT1

Question 45

Explain differences between SGLT1 and 2

Answer 45

Both SGLT1 and SGLT2 are found in the kidney but SGLT2 is unique to the kidney
SGLT1 brings 2 Na in the cell for 1 glucose. Not a big role in kidney (luminal glucose conc. can be reduced to < 1/10,000 of that in the cell)
SGLT2 carries 1 Na in the cell for 1 glucose (luminal glucose conc. can be reduced to ~ 1/100 of that in the cell)
SGLT1 = glucose and galactose; found in late proximal tubule
SGLT2 = only glucose; in early proximal tubule

Question 46

What does a mutation in SGLT1 lead to? SGLT2?

Answer 46

SGLT1: Glucose-galactose malabsorption syndrome (autosomal recessive; severe diarrhea and dehydration, has little effect on renal glucose reabsorpton)

SGLT2: Familial renal glucosuria (benign; leads to low plasma glucose or a decreased tm)

Question 47

Where is GLUT 2 vs GLUT 1 in the tubule?

Answer 47

GLUT2; early proximal tubule (S1) with SGLT2

GLUT1; late proximal tubule (S3) with SGLT1