3.1 - **ICPP** pH + Cell Volume Regulation Flashcards
cell pH features
- Limits of human tissue survival are from 6.8 to 7.8
- A change in [H+] by a factor of 2 causes pH change of 0.3
- At pH 7.4 the [H+] is 40nM and a pH change of 0.3 either doubles or halves [H+]
- Normal plasma pH is 7.35-7.45 (extracellular pH)
- Cytoplasmic pH is 7.2 (but varies depending on organelle)
Why is pH tightly regulated
- pH causes change in net electrical charge on proteins + molecules
- disrupts electrostatic interactions + hydrogen bonding (not covalent)
- alters protein structure + function → alters binding of substrates + ligands
- normal cellular metabolism occurs at extracellular pH of 7.35-7.45
- lungs + kidneys are responsible for this control
- equilibrium is present: changing amount of free H+ will shift equilibrium one way
what are the pH extracellularly and intracellularly
- extracellular pH is 7.35-7.45
- cytoplasmic pH is 7.2 (but this varies with other organelles depending on their function)
- the difference between intracellular + extracellular pH allows for pH gradient
pH dysregulation: tissue ischaemia
- eg cardiac ischaemia + stroke (blockage of an artery)
- reduction in blood flow → decreased O2
- causes switch to anaerobic glycolysis
- this causes cytoplasmic acidification
- causes overactivity of Na-H exchanger 1
- this causes overload of intracellular sodium
- this also increases intracellular Ca2+ overload die to Na-Ca exchanger
- causes altered cellular function, apoptosis/necrosis
- in the heart ☞ arrhythmogenic
pH dysregulation: Dent’s disease
- characterised by proximal tubule dysfunction + progressive renal failure
- due to mutations in CLC5 ☞ 2Cl-/H+ exchanger + defects in endocytosis
- alteration of pH
Sources of protons
protons drawn in via electrochemical gradient
- electrochemical gradient favours inward movement of H+ and outward movement of HCO3-
- consists of the transmembrane pH gradient + electrical gradient
- protons accumulate intracellularly
metabolism causes CO2 production (where ‘=’ is equilibrium symbol)
CO2 + H2O = H2CO3 = H+ + HCO3-
Therefore when CO2 is produced, more H+ will be produced
anaerobic glycolysis
Glucose is metabolised → lactic acid
The production of H+ therefore depends on how metabolically active tissues are
Buffer systems
☞ buffers immobilise H+ and reduce its destructive effects
☞ buffers only reduce impact of acute changes
☞ need dynamically regulated transport proteins to manage pH better and prevent acidosis
bicarbonate buffer system
CO2 + H2O = H2CO3 = H+ + HCO3 (where = is equilibrium)
By far the most physiologically important
Equilibrium shifts depending on concentration of free protons
phosphate buffer system
H+ = HPO4(2-) = H2PO4(-)
proteins as these contain numerous H+ binding sites
What are the ion transporters involved in cellular pH regulation
primary transport
☞ Sodium-potassium ATPase as this maintains the Na+ gradient to provide energy for secondary/tertiary transport (3Na+ out, 2K+ in, ATP used)
alkalinises cell – these all rely on Na+ gradient
☞ Na+/H- exchanger (Na+ in, H+ out)
☞ Na+-Cl—HCO3—H+ co-transport (Na+ and HCO3- in, and H+ and Cl- out)
☞ Na+-HCO3- co-transport (Na+ and 2HCO3- in)
acidifies cell
☞ anion exchange (HCO3- out and Cl- in)
Na+/H+ exchanger (NHE)
- important as there is a drive for H+ to enter into cell
- for every Na+ into cell, one H+ out of cell
- electroneutral due to 1:1 ratio (so doesn’t change membrane potential)
- regulates intracellular pH
- also regulates cell volume
- activated by pH and growth factors
- inhibited by amiloride
bicarbonate transporters (AE and NBC)
- these are the NBC (Na+ bicarbonate chloride cotransporter) and the anion exchanger
- both are involved in pH and cell volume regulation
sodium bicarbonate chloride co-transporter
Na+ and HCO3- in
H+ and Cl- out
Alkalinises cell
anion exchanger
HCO3- in and Cl- out
Acidifies cell
Co-ordination of intracellular pH regulation
- this is all done by negative feedback
- pH is held at set point
- any change is corrected by increased activity of the ion transporters
- acidification activates NHE and NBC
- alkalinisation activates AE
pH sensitivity of NHE and AE2
- very sensitive to small changes in pH → small change in pH will cause large change in activity
- they are very active at ideal pH – tight regulation
- NHE is most active at lower pH (more acidic)
- AE is most active at higher pH (more alkaline)
Cell volume regulation – why is it important
- cells must avoid large changes in cell volume to survive
- excessive swelling → changes in membrane integrity
- swelling /shrinking → interferes with cell cytoskeleton
- cellular functions depend on correct hydration of proteins
- some cellular functions rely on localised volume control (eg if they send out cellular processes)
how is cell volume controlled
- no ‘standard’ method for cell volume regulation
- mainly by H2O diffusing across the membrane
- this depends on the osmotic gradient (hypo/hypertonic)
- osmosis is passive diffusion across plasma membranes
- intracellular volume is sensitive to extracellular osmolarity
- therefore, cell volume regulation is by the transport of osmotically active ions, such as Na+, K+, amino acids and Cl- etc
☞ some patients are osmotically challenged due to changes in the extracellular environment by hypo/hypernatraemia etc
Hypo/hypertonic and osmosis
hypertonic solution
Water moves out cell
Greater solute concentration outside cell
Causes cell shrinkage
hypotonic solution
Water moves into cell via osmosis
Greater solute concentration inside cell
Causes cell swell → lysis
isotonic solution
No net movement of water
