Absorption of electrolytes

Question 1

Composition of intestinal fluid

Answer 1

Na, K, Cl, HCO3-

Question 2

Na

Answer 2

-concentration in small intestine is relatively constant
-concentration decreases in the colon as the K concentration increases
-normal concentration: ~90-100mM in intestine
>normal extracellular is 145-150mM therefore takes energy to maintain concentration

Question 3

K

Answer 3

-concentration in small intestine relatively constant
-concentration is higher than plasma but lower than Na concentration
-concentration increases in colon as Na decreases

Question 4

Normal intestinal K concentration

Answer 4

~20-40mM
Extracellular is 4-5mM

Question 5

Cl and bicarbonate concentrations

Answer 5

-relatively uniform
-Cl and HCO3 have reciprocal relationship in the small intestine (aboral anion exchange increases)= Cl high proximally, HCO3- high distally

Question 6

Ion concentration of intestinal fluid during fasting

Answer 6

-succus entericus (duodenum secretions) is isotonic
-decrease in K, increase in Na

Question 7

Ion concentration of intestinal fluid during meal

Answer 7

-Succus entericus (duodenum secretion) is hypertonic
-increase in K, decrease in Na =Na absorption which allows for uptake of sugars and amino acids

Question 8

pH of intestinal fluid in stomach

Answer 8

3.5

Question 9

pH of intestinal fluid in small intestine

Answer 9

-neutralized quickly in the small intestine due to pancreatic and intestinal bicarbonate secretion
-5.5-7.0 pH

Question 10

Overall changes in pH in GIT

Answer 10

-generally pH will increase aborally

Question 11

pH of intestinal fluid in cecum and proximal colon

Answer 11

-see a decrease followed by an increase in pH
>due to microbial production of Volatile fatty acids AND post gastric fermenters (horse)

Question 12

Canine intestinal fluid composition

Answer 12

-smaller gastric fluid volume relative to humans
-within species/across breeds (range in gastric pH)
-bile acids conjugated primarily to taurine and relative to humans, higher intestinal bile salt and phospholipid content

Question 13

Cat intestinal fluid composition

Answer 13

-lower gastric pH relative to the canine
-large variability between cats transit time

Question 14

Recapturing the fluid and electrolytes in proximal small intestine

Answer 14

-increased excretion from the small intestinal, pancreatic, and biliary secretion
-highly permeable to water allowing it to make rapid adjustments to osmotic pressure in response to meals

Question 15

Recapturing the fluid and electrolytes in distal small intestine and colon

Answer 15

-water moves across the mucosa by a passive process down its osmotic pressure gradient
-strong correlation between Na and H2O

Question 16

Absorptive Cell role in Proximal absorption in small intestines

Answer 16

-cells found at apical portion of villus
-involved in absorption of Na and glucose in the proximal small intestine. Water follows through the leaky tight junctions
-occurs rapidly once free glucose is released
-requires energy

Question 17

Mechanism of proximal absorption in small intestines (Glucose enhanced transport)

Answer 17

-driven by Na/K ATP pump creating a charge separation and concentration differences
-glucose driven into cell through SGLT 1 (sodium glucose co-transporter 1) along with 2 Na ions
-glucose either metabolized or exits the basolateral side via facilitated diffusion through GLUT 2
-once a strong gradient is established, the water follows through aquaporins or paracellularly
-the NHE (sodium hydrogen exchange) and AE (anion exchange) play a minimal role (more important with isosmotic absorption in jejunum)
-GLUT 2 can translocate to the apical membrane under high ~300mM glucose concentration

Question 18

Distal small intestine absorptive cell (isosmotic drive absorption)

Answer 18

-SGLT 1 expression decreases in aboral direction and isosmotic absorption becomes more important
-there are slightly tighter junctions than the proximal small intestine
-minimal amount of energy needed

Question 19

Colon fluid and electrolyte absorption

Answer 19

-strong Na absorption (epithelial Na channel)
-tighter tight junctions than small intestine (high resistance tissue)
-K secretion (apical K channels)- lowers membrane potential
-K/H ATPase present apically (K retention in distal colon to maintain homeostasis)
-SCFA (transport through SCFA/HCO3 exchange and nonionic diffusion (drives fluid absorption)

Question 20

Colon absorptive cell function

Answer 20

-found on apical portion of colon
-involved in the absorption of Na, Cl, and VFA/SCFA
>when Na absorbed, water follows
-cells have very tight junctions (so less water flow through them) allowing for a large osmotic gradient to pull water into body

Question 21

Mechanism of colon absorptive cells

Answer 21

-driven by Na/K ATPase
-Na enters colonocyte through epithelial sodium channel and is pumped out the basolateral side creating a large gradient RESULTING in water pulling in through aquaporin
-Anion exchanger and sodium hydrogen exchanger pull in Na and Cl resulting in pulling in water

Question 22

Where is the energy, HCO3-, H+ needed to drive colon absorption coming from?

Answer 22

-metabolism of SCFAs produced by digestion/fermentation of fibre

Question 23

SCFA role in colon absorption

Answer 23

-SCFA stimulate electroneutral uptake of Na through the acidification of colonocytes and activation of apical Na/H exchangers
-Metabolism of SCFA via TCA cycle and carbonic anhydrase results in H+ and HCO3- production

Question 24

Cl uptake in colon

Answer 24

-stimulated by increased HCO3- production during SCFA metabolism and stimulation of apical Cl/HCO3- exchanger

Question 25

Alternative colon absorptive model

Answer 25

-butyrate taken up by nonionic diffusion or SCFA/HCO3- exchanger or monocarboxylate transporter

Question 26

Why is resistant starch added to oral rehydration?

Answer 26

-colonic bacteria use it and produce short chain fatty acids which results in increases fluid absorption

Question 27

Absorption abilities of colon aborally

Answer 27

-increase aborally

Question 28

Electrolyte levels and net fluid and electrolyte movements in GI tract of swine

Answer 28

-used PEG (will not be absorbed by gut) and observed change in ratio between PEG and other nutrients.
>If PEG increases, then assume other nutrients decrease
-pH increased dramatically in colon (due to SCFA being synthesized by microbiome)
*more gradual increase in nutrient absorption (start absorbing earlier) in humans, dogs, horses, cattle compared to pigs that absorb more later on, closer to colon

Question 29

Pig water and electrolyte absorption

Answer 29

-unlike other species, the weaning pig relies largely on its large intestine for absorption of fluid and electrolytes with small changes in net fluid movement occurring along jejunal and ileal segments

Question 30

Water absorption in dog

Answer 30

-Water in= 3050
-Water out (absorbed)=3015
**small intestine absorbs more water than colon

Question 31

Water absorption in horse

Answer 31

-water in:144
-water out (absorbed)= 137
*similar water absorption in both small intestine and colon… small intestine slightly higher

Question 32

Forces used to absorb water

Answer 32

1.osmotic pressure
2.hydrostatic pressure (from increase in pressure due to water in a confined area)

Question 33

Absorption of water steps

Answer 33

1.in villous tip, countercurrent flow of arteriole and venule causes solute in the descending venule to diffuse into the ascending arteriole to be carried up to tip
2. water uses aquaporin channels and osmotic gradient created by Na and AA absorption into the extracellular fluid
3. Hydrostatic pressure drive water into capillaries which have lower resistance than the tight junctions

Question 34

Thirst

Answer 34

-excessive water excretion without intake
-extracellular fluid hyperosmolarity- causing either an increase in secretion of ADH (causes decreased excretion of urine) and sensation of thirst
*regions of brain identified

Question 35

Oral rehydration therapy

Answer 35

-used extensively in human and vet med
*diarrhea-associated hypovolemia (dehydration) is the second leading cause of death among children under 5 yrs
-this therapy first introduced in 1960s and is gold standard for treating fluid loss as a result of acute diarrhea

Question 36

Early Oral rehydration therapy success

Answer 36

-fluids in 1980s used hyper-osmotic/hypertonic glucose solution ; now we use hypo-osmotic/hypotonic solution
>important for the SGLT 1 glucose mediated active transportation (which is generally not significantly affected during diarrhea states)
-advised glucose to be used as an organic solvent carrier for Na transport. Other options include probiotics, zinc, prebiotics, glucose polymers, amino acids

Question 37

Current oral rehydration therapy

Answer 37

-hypoosmotic/hypotonic solutions recommended
-results in reduced fecal fluid loss, need for IV therapy, duration of illness

Question 38

Hyper vs. hypoosmotic fluid therapy solutions for calf vs. pigs

Answer 38

• Hyperosmotic for calves because less access to them during the day/can only give them 3-4x a day
• Hypoosmotic for pigs because can access them throughout the day/all day

Question 39

Oral rehydration therapy in vet med

Answer 39

-contains Na, K, Cl; typically isotonic, or hypertonic if animal handling is an issue
-NaHCO3- often added to help combat metabolic acidosis which comes with diarrhea
-Glucose and AA added to increase Na/glucose and Na/AA transporter mechanisms and provide energy
-SCFA and probiotics used to increase water reabsorption in colon