Transport Across Membranes Flashcards
What are some examples of facilitated diffusion transporters?
Ligand-gated ion channels e.g. nACHR, ATP-sensitive K+ channel
Voltage-gated ion channels e.g. Na+ channel
List some functions of transport processes across membranes.
- maintenance of ionic composition
- maintenance of intracellular pH
- regulation of cell volume
- concentration of metabolic fuels and building blocks
- extrusion of waste products of metabolism & toxic substances
- generation of ionic gradients necessary for electrical excitability
Define cotransport and name each subtype.
COTRANSPORT = more than one type of ion or molecule may be transported on a membrane transporter per reaction cycle
- uniport: one molecule transported in one direction only (therefore no cotransport)
- symport: both molecules transported in same direction
- antiport: both molecules transported in opposite direction
What is the function of Na+/K+-ATPase?
Maintains Na+ & K+ concentration gradients (NOT THE RESTING MEMBRANE POTENTIAL/AP GENERATION!!! - only generates ~ 5-10mV)
Drives secondary active transport
Inhibited by ouabain
How is control of resting calcium controlled by membrane transporters?
- Ca2+/Mg2+ATPase: PMCA or SERCA (high affinity, low capacity)
- Na+/Ca2+-exchanger (NCX) (low affinity, high capacity)
- Ca2+ uniporter (mitochondrial - activated by hypercalcaemia)
What is the effect of ischaemia on calcium transporters?
ATP depleted -> increased [Na+]i (Na+/K+-ATPASE cannot function)
-> depolarisation
NCX reverses -> extrudes calcium
How is the pH of a cell controlled by membrane transporters?
Acid extruders:
- Na+/H+ exchanger (NHE) (inhibited by amiloride)
- Na+ dependent Cl-/HCO3- exchanger (NBC)
Base extruders:
Cl-/HCO3- anion exchanger
How is glucose coupled to ion transport, and in what organs?
Na+/glucose cotransport
Gut/kidneys
How is cell volume regulated by membrane transporters?
Cell swelling resisted by:
- extrusion of K+ & Cl- (uniport & symport)
- extrusion of amino acids
Cell shrinking resisted by:
- intrusion of K+, Cl-, Na+, & Ca2+
- intrusion of amino acids
- entry of CO2 -> H2CO3
How is bicarbonate reabsorbed in the kidneys?
Na+/H+ antiporter provides proximal tubule lumen with H+ (& Na+ reabsorbed into capillary)
H+ combines with HCO3- to form H2CO3
H2CO3 converted to H2O & CO2 by carbonic anhydrase in lumen and reformed in cell (diffusion of CO2 from capillary)
HCO3-/Cl- antiporter transports HCO3- into capillary
How is Na+ reabsorbed in the thick ascending limb of the kidney?
NKCC2: Na+/K+/2Cl- symporter (inhibited by loop diuretics)
Na+ then reabsorbed by Na+/K+-ATPase
ROMK: K+ uniporter (removes excess potassium)
KCICT: K+/Cl- symporter (maintains electroneutrality)
CIC-Kb: Cl- uniporter (maintains electroneutrality)
How is Na+ & Ca2+ reabsorbed in the distal convoluted tubule?
Sodium:
NCCT: Na+/Cl- symporter (inhibited by thiazides)
ENaC: Na+ uniporter (inhibited by amiloride)
Calcium:
TRPM6: Ca2+/Mg2+ symporter, then NCX
CIC-Kb & KCICT maintain electroneutrality
How is Na+, Cl-, K+, & H2O reabsorbed in the cortical collecting duct?
Sodium:
ENaC, then Na+/K+-ATPase (stimulated by ADH & aldosterone)
Chloride:
CIC: chloride uniporter in both membranes
Potassium:
ROMK: K+ uniporter in both membranes (stimulated by ADH, aldosterone; inhibited by spironolactone)
Water:
Aquaporins (stimulated by ADH & aldosterone)
How is glucose transported into adipose, brain, liver, skeletal muscle, kidney, and intestinal tissue?
Adipose & resting skeletal muscle tissue = GLUT 4 (insulin required to up-regulate and insert GLUT 4 into the membrane via tyrosine kinase - therefore no efflux)
Liver & brain = GLUT 1,2, & 3 (liver - insulin required, brain - insulin insensitive)
Working skeletal muscle = GLUT 4 (no insulin required)
Intestine & kidneys = Na+-glucose cotransporter (apical membrane) and GLUT 2 (basement membrane -> bloodstream)
