Membrane Transporters

Question 1

What is the barrier coefficient and how is it calculated?

Answer 1

The barrier coefficient is a measure of how much a particular solute is reflected by a membrane or tissue (i.e., cell membrane, blood-brain-barrier). A barrier coefficient of 1 is totally impermeable to the solute, a barrier coefficient of 0 offers no resistance to the solute.

Question 2

What is the goal of administering manitol to a DKA patient and what is its advantage over NaCl?

Answer 2

Manitol is given to a DKA patient who is showing signs of cerebral edema. The manitol raises the osmolarity of the plasma, equalizing it to the osmolarity of the interstitial fluid in the brain, and drawing H2O back from the brain. It has a barrier coefficient of 1, unlike NaCl, and does NOT cross the blood brain barrier.

Question 3

What are H/K exchangers and how to they complicate DKA?

Answer 3

H/K exchangers move H+ and K+ ions in opposite directions, either may enter or exit the cell but always in opposition to the other. Under acidotic conditions (high plasma H+, low pH) H+ ions preferentially enter cells through H/K exchangers, and K+ ions are pumped out, resulting in hyperkalemia.

Question 4

What determines membrane potential? How is this demonstrated by cardiac cells?

Answer 4

Relative permeability to ions (Na+, K+). Cardiac cells are highly permeable to K+, so K+ flows out of cells along its electrochemical gradient, pulling the cell’s membrane potential close to K+’s E of -90. Cardiac cells are slightly permeable to Na+, which flows into cells along its electrochemical gradient, pulling the cell’s mmbr potential away from -90 towards Na+’s E of +50(ish).

Question 5

What is the Nernst equation? In conceptual terms, what causes it to change?

Answer 5

E=(60/z)log([X]outside/[X]inside)
Changes in the relative concentrations across a membrane produce changes in E. The stronger the concentration gradient, the greater the value, and the greater effects that changes in the relative concentrations have on E.

Question 6

How does hyperkalemia lead to heart arrhythmias?

Answer 6

Hyperkalemia is an increase in the [K+]outside, which moves the E of K+ closer to zero. Moving E towards zero is “depolarization”, and causes the sensitive voltage gated channels to open at improper times. Heart rhythm is maintained by VG channels, so their improper opening causes arrhythmias/ventricular fibrillation.

Question 7

How does hypokalemia lead to heart arrhythmias?

Answer 7

Hypokalemia is a decrease in the [K+]outside, which makes E even more negative. This results in a greater difference between the Vm of the cell and Ek, causing some K+ channels to close, which makes the cell LESS PERMEABLE to K+. Less permeability always results in Vm moving away from Ek, thus the cell depolarizes triggering VGNa+ channels and causing arrhythmia/Vfib.

Question 8

How is the effect of membrane permeability demonstrated in insulin release by Beta cells?

Answer 8

Glucose enters the beta cell, resulting in ATP creation. The ATP then blocks K+ channels, making the cell less permeable to K+. This decrease in K+ permeability causes the Vm to move away from Ek, depolarizing the cell. Depolarization triggers VGCa channels, Ca binds to SNARE proteins and causes insulin vesicles to bind to the membrane and release insulin into the system.

Question 9

What is [Na+]o?

Answer 9

140mM = [K+]i

Question 10

What is [K+]o?

Answer 10

4mM

Question 11

What is [H2O]?

Answer 11

55.5M (55,500mM)

Question 12

What is [Cl-]o?

Answer 12

115mM

Question 13

What is [HCO3-]?

Answer 13

24mM

Question 14

What is [Ca2+]o?

Answer 14

1mM

Question 15

What is [Ca2+]i?

Answer 15

.0001mM = [H+]i

Question 16

What is [H+]i?

Answer 16

.0001mM

Question 17

What is [H+]o?

Answer 17

.00004mM

Question 18

What is the max urine mosM?

Answer 18

1200mosM

Question 19

What is plasma mosM?

Answer 19

300mosM

Question 20

What distinguishes the apical side of epithelial cells from the basilateral sides?

Answer 20

All of the special transporters in epithelial cells are located on the apical side. The basilateral faces contain the same transporters as all normal cells. Apical membranes also have microglia (many little folds that increase surface area).

Question 21

How do epithelial cells transfer Na+ and Cl- into the ECF/plasma?

Answer 21

The apical faces are highly permeable to Na+, which flows down its electrochemical gradient into the cell, where it is pumped out on the basilateral face to the ECF. Cl- flows into the cell through the apical face following the positive charge created by the influx of Na+. Cl- follows the Na+ pumped out of the cell on the basilateral face.