Renal_transport (L4-L6) Flashcards
What is the equation of the Fick’s Law?
J = -D dc/dx
Φ = D/X (C2-C1) = molescm-2sec-1
c = concentration gradient
x = thickness
D = diffusion constant
Φ = flux = rate of solute movement per unit of cross sectional area
what is the partition coefficient?
Kmw = Cmembrane/Cwater
Higher (0.1) for less polar molecules
Lower (10^-7) for more polar partition
*partition coefficient is directly proportional to permeability (smaller particles, for same K, have higher permeability)
What actually/in practice determines simple diffusion through a membrane?
Φ = Dapparent /Xmembrane (C2 - C1)
D apparent = Dm*Kmw
Kmw = C membrane/Cwater
*The rate of simple diffusion depends on the ratio of solute centration in the membrane to that in the aqueous phase
Usually, Dm, Kmw and Xm are generally not known for biological membrane. How is simple diffusion calculated?
P = permeability coefficient empirically-determined
Φ = P (C2 - C1)
What determines the net diffusion flux of molecules?
Diffusion molecules see no force, but on average they tend to move to the region of lower concentration
*Not driven by an actual force towards lower concentration
They move due to Brownian motion (thermal agitation)
How is water permeability regulated in the kidney?
Water permeability can be regulated by Antidiuretic hormones (ADH) → allow water movement, but not solute movement in the corical collecting duct ~ 4-fold
What equation describes the membrane’s permeability to water?
*In the cortical collecting duct
*Driven by osmosis (following Na reabsorption)
J(H2O) = P ( π eff2 - π eff1)
P = permeability ~ 10^-4 or 10^-5 cm/sec
π eff = RTσC
R = gas constant
T = temperature (K)
σ = reflection coefficient (0-1, no units) → selectivity of the semi-permeable membrane (1 = actual osmotic pressure, 0 = 100% leaky, no membrane)
C = osmotic concentration (mOsm/l)
What is free diffusion of CO2 across the membrane important for?
(As a dissolved gas)
*Important for acid-base balance
Bicarbonate is reabsorbed by proximal tubule as CO2 dissolved gas (catalyzed by carbonic anhydrase, HCO3- could not diffuse freely)
Found in lumen and in blood as H+ + HCO3-, but diffuses as CO2
In plasma ~ 25 mmol HCO3-
Cells of the proximal tubules secrete H+ (Na+/H+ active transport) into lumen of tubule → combines with the bicarbonate that was filtered → CO2 + H2O (catalyzed by c.a.) → CO2 dissolved gas → diffuses → c.a. remakes H+ + HCO3- in the cells
*Reabsorption of HCO3- driven by excretion of protons
What is the value of D for small molecules in aqueous solution?
D = diffusion coefficient
D = 1x 10^-5 cm2/sec
In actual membrane, D is reduced by the ratio of solute cocentration in the membrane vs in aqueous phase (partition coefficient)
What happens is you expose cells to a solution of NH4+ (ammonium) (ex: ammonium chloride)?
- NH4+ will dissociate → NH3 + H+ (to an equilibrium)
- NH3 will diffuse freely through the cell membrane
- Inside the cell NH3 will assemble with H+ → NH4+ → increase the pH ~7.4
- Regulatory mechanisms will bring it back down a bit (~7.2)
When remove the NH4+ solution: - The NH3 in the cell diffuses back outside the cell to reassemble outside the cell to form NH4+ leaving the H+ ions inside the cell → big decrease in intracellular pH
What happens to the intracellular pH proximal tubule cells when we temporarily put a 5% CO2/ 25mmol HCO3- solution in the lumen
- HCO3 doesn’t diffuse, but CO2 will diffuse into the cells
- Will dissociate into H+ + HCO3- inside the cells bringing the pH down (< 7.0)
- Activate intracellular mechanisms to bring pH back up → secrete protons (Na+/H+ transporter)
- pH goes back up (a bit of overshoot)
When we remove the solution: - Intracellular CO2 diffuse back into the lumen to compensate for the loss of solution, but bicarbonate is left inside the cell → pH goes up (~7.6)
- Some mechanisms, kick in to bring it back down a bit
*Carbonic Anhydrase 4 in the lumen, C.A. 2 inside the cells of the proximal tubule
How do many drugs diffuse through the lipid bilayer?
Through non-ionic diffusion → as the undissociated acid form (HA)
How does clearance of neutral weak acids and neutral weak bases vary depending on pH?
Neutral weak acid: (A- + H+)
At low urine pH → association as HA → diffusion → low clearance/GFR ratio
At high urine pH (~7.6) → not much association with H+ → no-reabsorption → high clearance/GFR ratio (~2.0) and high conjugated base/acid ratio (A-/HA)
Ex: Salicylate
Neutral weak base: (B + H+)
At low urine pH → lots of H+ → association as BH+ → no reabsorption → high excretion → high C/GFR ratio
At high urine pH (~8) → not much H+ in filtrate → no association → reabsorption → C/GFR ~ 0
Ex: Quinine
What are the 3 characteristics of mediated transport?
Both carriers and channels exhibit these characteristics:
- Specificity → only 1 substance, or small group can bind to the transporter and permeate
- Saturation → transport rates reach some maximum (Vmax) when concentration of solute is elevated
- Competition → a second solute may also bind the transporter site although it isn’t necessarily transported
What is the difference between carriers and channels?
Channels ~ holes in the membranes, passive, but still specific
Carriers → involve a conformational change
*Both for large, hydrophilic, charged molecules
What is the importance/formula of the Michaelis-Menten equations?
*Concentration dependence of initial rate of transport
initial flux rate = Vmax/(1 + Km/[A])
Km = concentration of solute A at which the flux rate is 1/2-maximal
C = competitior for the transport site of A → reduces the flux rate for same [A]
What is the difference between Fick’s law diffusion (not mediated) and mediated transport curves ?
*Solute concentration (X-axis) vs Flux rate (Y-axis)
Fick’s law → linear increase, lower slope (lower flux rate than mediated at lower concentration)
Mediated transport → sharp increase at start → plateau when the transporter saturates
How is the turnover number/mechanisms of carriers different than of channels?
*Both are intrinsic membrane proteins
Carriers → lower turnover number (thousands/sec) because conformational change for every molecule that passes through the membrane
ex: Anion exchanger ~ 50,000/sec
Na-Glucose cotransporter ~ 5/sec
Channels → pores and gates
Conformational change in the protein opens the “gate” → allows many ions to pass through per second
Higher turnover number ~ 2-100 millions/sec
What is the structure of AQP1?
N-term-H1-H2-intramembrane loop (NPA)-H3-H4-H5-intramembrane loop (NPA)-H6-C-term
*Both N- and C-term are in the cytoplasm
2 subunits made of the 6 TM segments → 1,2,6 + 3,4,5 with the 2 loops in the middle forming the pore where H2O molecules can pass (NPANPA)
**Found as a tetramer → 1 subunit is glycosylated
Where are aquaporins? What are they inhibited by?
- Abundant in red cells and renal cortex
- Ancient protein family in bacteria and plants
- Initially called CHIP28 → Channel forming Integral membrane Protein of 28 kDa
- Increases water diffusional permeability by 8-fold
- AQP1 is present in apical and basolateral membrane of the proximal tubule
- Very specific to water → not permeable to urea, small ions, H+
- Inhhibited by HgCl2 (reacts at cysteine pore)
