Membranes and gases Flashcards
The small intestine - why is it unusual? Features that allow for high h2o reabs?
Apical side has NO AQPS (which allow for the drive for the movement of water)
Large reabs of water occurs at the level of the small intestine due to the hypertonic conditions created by the enzymatic activity in the stomach.
BUT CAN STILL ABSORB WATER
Wet transport proteins - what is meant but this?
Co transport proteins present in the small intestine - allow movement of water along with other solutes against the osmotic gradient
Examples of wet transport proteins
GLUT2 - glucose transporter
hSGLT1 - Na+/glucose transporter
KCC4 - K+/CL- contransporter
NKCC1 - Na+/Cl-/2Cl- contransporter
Transport models - examples and how do they operate?
AQPs - provide a pore in the membrane
KCC - K+/Cl- moved along with H20
SGLT1/EAAT1 - mixed mode (use pore type mechanism and cotransport process via microgradients to allow absorption of nutrients)
Molecular turnover model - outline
- Na+ binds to the transporter.
- Glucose surrounding the protein also enters cleft (h2o is trapped along with glucose)
- a confirmational change occurs -> H20 and glucose released on other side of membrane into the IC environement
Small intestine model - proteins on apical membrane? what does this happen off the back of
GLUT2 - brings glucose and water into cell from SI lumen - NOTE this protein is inserted into the apical membrane during digestion
SGLT2 - Na+, glucose and H20 brought into cell from lumen of SI
Eating food contains hypertonic conditions in the lumen
Overton’s law for membranes and solutes?
‘The permeability of membranes to solutes is proportional to the oil:water partition coefficient for that molecule’
What dictates the permeability of a membrane to gases? - what about to Co2 and o2?
The amount of lipid bilayer - gases can freely diffuse across bilayer at the membrane (so where transport proteins are present perm to gases reduced)
Gases such as oxygen and carbon dioxide have a high solubility in oil, so a natural extension of this law was that all biological membranes were freely permeable to gases.
What happens when NH3 moves into a cell?
Combines with H+ to create NH4+, causing alkylisation of the cell
What happens when NH4+ moves into a cell?
Dissociates into H+, NH3 and acidifies the cell
Conditions wrt NH4+/NH3 at lower pH?
Low pH = more NH4 moving in which is a weak base
Isotonic drinks - how do they work?
High glucose environment created in the small intestine which creates hypertonic environment, stimulating water to be reabsorbed, giving the hydrating effect
Effect of Co2 diffusing into cell?
Co2 moves in and dissoc into HCO3- and H+ (acidification)
Effect of HCo3- diffusing into cell?
Combined with H+ to produce Co2 and H20 giving alklysating effect (process slower than acification)
Thick ascending limb - impact of adding NH4Cl on the basolateral side
Causes alkalysation of cells and this membrane has high permeability to NH4 and NH3
Thick ascending limb - impact of adding NH4Cl on the apical side
Causes acidification and has a high perm to NH4 but extremely low to NH3
The gastric gland - perm to Co2 on the basolateral membrane - how is this shown (experiment)
NORMALLY permeable to Co2 on the basolateral membrane - seen by adding to blood on baso side Co2 causing a decrease in pH of the cell (this is done using perfusion pipettes of isolated gastric gland)
The gastric gland - perm to Co2 by lumenal changes - how is this shown (experiment)
By changing the conc of Co2 of the lumen to the PK for co2 (gassed with 100% co2) attempting to create pH 6.1 –> NO CHANGE IN PH SEEN!!!! Apical side not permeable!
Why would the apical membrane of the epithlia lining the stomach be impermeable to gases?
Allows the maintenance of a tough, tight membrane so that the acidic conditions of the stomach can be maintained –> stops stomach from allowing enzymes to function so that the stomach doesnt digest itself!
Colonic crypt cells - composition of membrane and why important?
77% cholesterol so tight membrane –> acts as a barrier for gases by having low gas permeability. High level of transport proteins found to decrease proportion of lipid bilayer which limits gas movement
MDCK cells - what was impact of stripping cholesterol from the membranes?
Increase co2 permeability
Relationship between cholesterol in membrane and perm to gases?
What challenges this?
Low perm membrane = high cholesterol content (decreased membrane fluidity)
RBCs challenge this –> 45% cholesterol content of membrane however impermeable to gases
Cancer cell line – evidence for cholesterol/permeability relationship?
Low membrane cholesterol in cancerous cells and cells are metabolically active –> high co2 perm!!
Oocytes exposed to co2 and hco3- then water. Measured how long lysis took and corellated this with Co2 permeability. What did the results show?
Acidification seen.
Acidification proportional to the co2 perm of the membrane
In this case, co2 perm was negatively proportional to the time taken for lysis to occur –> less acidification = more time taken for lysis to occur –> MORE AQPS = FASTER ACIDIFICATION
what is the action of PCMB and what was seen in regards to AQP1 / cs189 residue?
PCMB is a mercurial agent which binds to the C189 residue of AQP1 -> adding causes decrease in co2 perm.
Therefore, pCMBS inhibits the AQP1 dependent increase in CO2 permeability
This was shown because n c189S mutant, increase in CO2 perm back to WT level when PCMB was added
COLTON NULL INDIVIDUALS- whats wrong with them and what was shown wrt co2 perm
Colton null individs have no AQPs in their RBCs.
- Causes a 50% decrease in permability to CO2
- PCMB doesn’t impact permability in colton null individuals
Impact of DIDS in WT and colton null individuals
CO2 permeability in Colton null and WT is inhibited by DIDS.
AQP1 - structure
Made up of hydrophobic and philic residues Central pore (mainly hydrophobic, gated by phobic residues, prevents H20 access)
AQP molecular dynamics - h2o and co2 movement?
Co2 - 6 molecules through 4 individ monomer water pores, 4 through central pore
O2 - 1 mol through 4 water pores and 6 through central pore
Other AQPs - what is different about them? Couple of examples of where they are found?
AQP4 - 2 isoforms –> M1 (kidney) and M23 (astrocytes in the BBB) - high permability to gases!
Differing H20/Co2 and NH3 permeabilities in different AQPs
Impact of cGMP on AQP1?
Binds and switches it to a cation (Na+) channel
Oocytes expressing AQP1 and added cGMP - impact seen?
Loss of perm to Co2 - due to change in conformation/shutting down of CO2 pore/allowing through cations?
Oocytes expressing AQP5 added cGMP - impact seen?
inhibition of Co2 perm but had no impact on H20 transport –> useful for transport of gas but without impacting water secretion (AQP5 found in the salivary glands)
