Electrolyte secretion and absorption (partial)

Question 1

Mucosal protection in the duodenum is dependent on this duodenal secretion

Answer 1

HCO3

Question 2

Toward the lumenal pole of epithelial cells are found the apical junction complexes which comprises what kind of junctions (2)

Answer 2

Tight junctions and adherens junctions

Question 3

In the apical junction complexes, these are basally located structures and are characterized by a high level of E-cadherin

Answer 3

adherens junctions

Question 4

True or false

Tight junction permeability is constant along the length of the intestinal tract

Answer 4

False

• • Tight junction permeability is not constant along the length of the intestinal tract and generally decreases aborally
• • Thus, the epithelium of the proximal small intestine is considered to be leaky, whereas that of the colon is considered to be tight

Question 5

Transmembrane components of the tight junctions include (3)

Answer 5

occludin, claudins, and junctional adhesion molecules (JAMs)

Question 6

Constitute the barrier-forming components of tight junctions and are responsible for dictating the size and ionic permeability of the paracellular pores

Answer 6

Claudins

Question 7

These cells perform the absorptive and secretory functions of the intestine

Answer 7

Enterocytes

Question 8

Fluid secretion predominantly occurs in the _____region, whereas absorption mainly occurs across the _____

Answer 8

Crypt, villus cells

Question 9

These cells in the crypt region secrete mucus that lubricates and protects the epithelial surface

Answer 9

Goblet cells

Question 10

These cells play an important role in host defense by secreting antibacterial peptides, such as lysozyme and defensins and are located toward the base of the crypts

Answer 10

Paneth cells

Question 11

HCO3 secretion is highest in what part of the intestine?

Answer 11

Proximal small intestine

Question 12

Na+ absorption is mediated by electroneutral transport in what part of the intestine?

Answer 12

Proximal colon

Question 13

Electrogenic Na+ absorption through the epithelial Na+ channel predominates in what part of the intestine?

Answer 13

Distal colon

Question 14

Na+ absorption occurs predominantly through nutrient coupled pathways in what region of the intestine?

Answer 14

Proximal small intestine

Question 15

This kind of transport moves net charge across the membrane by the flux of either single ions or combinations of transport substrates whose charges are unequal

Answer 15

Electrogenic

Question 16

This kind of transport mediate a net flux of ions, but do so without moving net charge across the membrane during a transport cycle

Answer 16

Electroneutral
** Electroneutral transporters may mediate an exchange of equal charges across the membrane (Na+/H+ exchanger) or a cotransport of equal and opposite charges (Na+/K+/2Cl− cotransporter)

Question 17

This kind of transport is often a compensatory mechanism that opposes the electrical gradient established by the action of electrogenic transporters thereby maintaining a electrical neutrality

Answer 17

Paracellular transport

Question 18

Driving force affecting electroneutral carriers

Answer 18

Chemical gradient

**not ELECTRICAL gradient

Question 19

In the presence of Mg2+, catalyzes the efflux of three Na+ ions from the cell and the uptake of two K+ ions at the expense of hydrolyzing one ATP molecule per cycle

Answer 19

Na+,K+-ATPase

Question 20

True or false

Na+,K+-ATPase is electroneutral

Answer 20

False

** More cation is pumped out than is replaced thus electrogenic

Question 21

Na+,K+-ATPase is localized ______

Answer 21

Basolaterally

Question 22

Protons secreted by the colonic H+,K+-ATPase are important for stimulating uptake of what luminal content?

Answer 22

Short chain fatty acids (SCFAs)

Question 23

Many food-derived products including glucose, amino acids, and vitamins, as well as endogenous luminal solutes such as bile acids, are taken up by what transport protein?

Answer 23

Specific Na+-coupled cotransport proteins – in the apical membrane of enterocytes

Question 24

Each transport cycle of SGLT1 results in ____

Answer 24

Uptake of two Naions and one glucose molecule (electrogenic)

Question 25

Expressed in the apical membrane of cells in the small intestine and can transport a wide variety of dipeptides and tripeptides

Answer 25

Proton-coupled peptide cotransporter (PEPT1)

Question 26

PEPT1 transports uncharged peptide substrates from the lumen in together with protons (1:1), what transport mechanism is responsible for supplying the protons?

Answer 26

Na+/H+ exchanger NHE3 * *Na inside the cell, H outside the cell * *H coupled with peptides goes inside the cell * *expressed preferentially on the villus in the small intestine and on the surface and in the upper one-third of the colonic crypts

Question 27

This cotransporter is part of transcellular SCFA absorption, in which it is the route for basolateral SCFA flux from cells to the blood stream

Answer 27

Proton-coupled monocarboxylate cotransporter | MCT1

Question 28

This cotransporter plays a key role in Cl− secretion as it provides the predominant route for basolateral Cl− uptake, and it also participates in K+ secretion by providing a second route for basolateral K+ uptake in addition to the activity of the Na+,K+-ATPase

Answer 28

The Na+/K+/2Cl− * * This cotransport is electroneutral because each transport cycle moves equal numbers of cations (one Na+ plus one K+) and anions (two Cl−) into the cell * *predominantly in the colon rather than the small intestine

Question 29

This isoform of Na+/H+ exchanger is present in virtually all cells of the body and is believed to be responsible for cellular pH regulation

Answer 29

NHE1

Question 30

This isoform of Na+/H+ exchanger is found in the basolateral membrane of intestinal epithelial cells

Answer 30

NHE1 ** NHE2 and NHE3 are predominantly epithelial isoforms of the exchanger that are expressed in the apical membranes of intestinal cells

Question 31

These 2 transport mechanisms mediate electroneutral NaCl uptake in colonocytes and small intestinal cells

Answer 31

Cl−/HCO3− exchange and Na+/H+ exchange (NHE3) in the apical membrane (both electroneutral) * *Na goes in; H goes out * *Cl goes in; HCO3 goes out * *net formation of NaCl from the 2 transporters

Question 32

Plays a key role in intestinal Cl− absorption and HCO3− secretion as mutations in this transporter are responsible for congenital chloride diarrhea

Answer 32

Down-regulated in adenoma (DRA)

Question 33

cAMP-activated channel in the apical membrane known to participate in hormone-stimulated Cl− secretion is encoded by ____

Answer 33

Cystic fibrosis transmembrane conductance regulator gene (CFTR) * * nonrectifying or linear (it conducts ions with equal efficiency in either an inward or an outward direction) * * The CFTR protein is also able to mediate the conductance of other ions (notably HCO3−)

Question 34

Potassium channels in the apical membrane of epithelial cells are involved in ______, and K+ channels in the basolateral membrane are involved in ______

Answer 34

K+ secretion and Cl− secretion

Question 35

The small intestine performs what kind of electrolyte absorption?

Answer 35

electroneutral NaCl absorption and Na+-coupled nutrient absorption

Question 36

The large intestine avidly absorbs Na+ through what mechanisms? (2)

Answer 36

electrogenic mechanism involving apical Na+ channels and an electroneutral NaCl absorptive mechanism similar to that in the small intestine

Question 37

Responsible for SCFA absorption and some K+ absorption, and it is essential for conservation of fluid and electrolytes

Answer 37

The colon

Question 38

True or false In the electroneutral absorption of NaCl by the intestine, Na+ absorption requires the presence of Cl and vice versa

Answer 38

True | **This describes the model of electroneutral NaCL absorption wherein the mechanism is Na+/Cl- cotransport

Question 39

This mechanism of Na absorption accounts for about half of the Na+ reabsorbed in the distal colon

Answer 39

Electrogenic amiloride-sensitive Na+ absorption (ENaC)

Question 40

In the electroneutral absorption of NaCl, what kind of Na+/H+ exchanger is expressed in the apical cells?

Answer 40

NHE 2 or NHE 3

Question 41

In the electroneutral absorption of NaCl, what kind of Cl/HCO3 exchanger is expressed in the apical cells?

Answer 41

Down-regulated Adenoma (DRA)

Question 42

A mineralocorticoid well known for its Na+-retaining and K+-wasting effects on the distal nephron

Answer 42

Aldosterone | **also exerts profound effects in the distal colon

Question 43

A crypt cell function which involves an apical K+ conductance

Answer 43

K+ secretion

Question 44

In electrogenic absorption of Na in the colon, it is a compensatory mechanism to the charge imbalance as a result of Na+,K+,-ATPase pump in the basolateral membrane

Answer 44

electrogenic efflux of K+ through K+ channels in the basolateral membrane

Question 45

Aside from the compensatory mechanism of K channels in the basolateral membrane, this is another mechanism which restores electroneutrality between lumenal and serosal compartments

Answer 45

Paracellular flux of Cl from lumen into the serosal compartments

Question 46

This is where bile absorption takes place

Answer 46

Terminal ileum

Question 47

In the small intestine, the absorption of many nutrients is mediated by what type of transport mechanism?

Answer 47

Na+-coupled cotransport * *occurs at villus cells * *example: Na/glucose cotransporter

Question 48

Efflux of solute down its steep concentration gradient across the basolateral membrane is efficiently mediated by ______

Answer 48

Facilitated diffusion carriers (GLUT2)

Question 49

SCFAs are produced in the colon by bacterial catabolism of unabsorbed carbohydrate and protein. What are the predominant lumenal SCFAs in the colon?

Answer 49

Acetate, propionate, and butyrate | **Energy source for colonic epithelial cells

Question 50

Condition that activates Na+/H+ exchangers

Answer 50

Intracellular acidification and lumenal alkalinization

Question 51

about 85% of ingested Kis absorbed in this region of the intestine, with passive absorption driven by prevailing electrochemical gradients being sufficient to explain this uptake

Answer 51

Small intestine

Question 52

Active electroneutral Kabsorption occurs in this region of the intestine

Answer 52

Distal colon

Question 53

Mechanism involve in electroneutral uptake of Kfrom the lumen in the distal colon

Answer 53

Apical H, K-ATPase

Question 54

An apical H, K-ATPase promotes uptake of Kfrom the lumen to the cell. Once inside the cell, K+ exits the cell through what transport mechanism?

Answer 54

K+/Cl− cotransporter (KCC1) in the basolateral membrane

Question 55

This mechanism provides a source of intracellular Cl− to drive the basolateral K+/Cl− cotransporter

Answer 55

Cl−/HCO3 − exchange

Question 56

Produces profound Cl−-rich, acidic diarrhea commencing at birth, resulting in systemic hypochloremic, hypokalemic acidosis with volume depletion

Answer 56

Congenital chloride diarrhea

Question 57

Gene that is responsible for congenital chloride diarrhea

Answer 57

Down-regulated adenoma (DRA)

Question 58

It is caused by mutations in the amiloride-sensitive Nachannel that is present in both distal colonic and renal epithelia

Answer 58

Liddle syndrome | ** autosomal dominant disorder leading to salt-sensitive hypertension

Question 59

This condition removes the inhibitory influence on the amiloride-sensitive Nachannel thus there is an increase in electrogenic Na+ absorption

Answer 59

Cystic Fibrosis | ** An activated CFTR Cl− channel is required to mediate the inhibitory regulation

Question 60

True or false | Electrogenic Cl− secretion is found in stomach to proximal part of intestine

Answer 60

False – Electrogenic Cl− secretion is found in all segments of the gastrointestinal tract from the duodenum to the distal colon ** reflecting the common need for a mechanism to maintain hydration of the lumenal contents

Question 61

In Cl- secretion, what mechanism is involved in the uptake of Cl- from the blood?

Answer 61

Electroneutral Na+/K+/2Cl− cotransporter (NKCC1) in the BASOLATERAL MEMBRANE * *Cl− will exit the cell across the APICAL MEMBRANE when Cl− channels are opened (CFTR) * *Na+,K+-ATPase provides energy for this overall mechanism and recycles Na+ across the basolateral membrane. * *K+ channels in the basolateral membrane allow for K+ recycling, maintain cellular electroneutrality by compensating for Cl− efflux, and keep the cell hyperpolarized so that Cl− efflux across the apical membrane can be sustained

Question 62

True or false In Cl- secretion, Nafollows passively by paracellular flux through tight junctions from the lumen to the cell compartments

Answer 62

False – From cell compartments to the lumen | ** To maintain electroneutrality

Question 63

In HCO3- secretion in the duodenum, secreted HCO3− anions can be provided by carbonic anhydrase (there are two ways HCO3 is provided), which hydrates CO2 to produce intracellular HCO3− and a proton. The proton is eliminated from the cell by what mechanisms (2)?

Answer 63

1. Basolateral Na+/H+ exchanger (NHE1) – from the cell to the bloodstream 2. Apical Na+/H+ exchanger (NHE2/NHE3) – from the cell to the lumen

Question 64

In HCO3- secretion in the duodenum, HCO3− can be derived from the bloodstream (there are two ways HCO3 is provided) by what mechanism?

Answer 64

Basolateral Na+/HCO3− cotransporter NBC

Question 65

In HCO3- secretion in the duodenum, what are the two mechanisms in which HCO3 exits the cells?

Answer 65

1. Cl- together with HCO3 is secreted by CFTR into the lumen (electrogenic) 2. The Cl- secreted by the CFTR establishes a concentration gradient that drives the action of down-regulated adenoma, secreting HCO3 in exchange for Cl (electroneutral)

Question 66

True or false | Only the transverse colon up to the proximal part of descending colon can perform active K+ secretion

Answer 66

False – All portions of the mammalian colon

Question 67

K+ secretion is enhanced by what kind of diet?

Answer 67

Low-sodium diet – increased plasma level of aldosterone

Question 68

K+ secretion in the colon. K+ uptake at the basolateral membrane is mediated by either ______ or _____, depending on the species and intestinal segment.

Answer 68

Na+,K+-ATPase or Na+/K+/2Cl−cotransport

Question 69

K+ secretion in the colon K+ efflux at the apical membrane is through _____

Answer 69

K+ channels