Membrane Potential II

Question 1

Electrical Neutrality

Answer 1

Cations external = anions external

Cations internal = anions internal

Question 2

Donnan Rule

Answer 2

[Kout][Clout] = [Kin][Clin]

Question 3

Rules for determining ICF and ECF composition

Answer 3

1. ) Osmotic Balance
2. ) Nernst equation
3. ) Donnan Rule
4. ) Electrical neutrality (total cation external = total anion external, same for internal. Remember proteins may carry a charge but they are not considered anions/part of charge balance equation).

Question 4

How did we show cells are actually permeable to Na?

Answer 4

We bathed RBC’s in radioactive Na.. they became radioactive. When then bathed in regular solution they lost radioactivity.

Question 5

Na

Answer 5

Concentration is higher outside than in, also inside in electrically neg so it leaks in… and is PUMPED OUT.
This takes work from ATP.
otherwise Na comes in, water follows, cell bursts.

Question 6

Na/K pump

Answer 6

Exchange pump.
Pump Na out and K in (K conc is higher inside cell). These ions must be present simultaneously for this pump to work.
Stoichometry: ratio is 3Na/2K
Pump is electrogenic

Question 7

Saturable pump

Answer 7

Has a rate of maximum activity.

Characteristic of carrier mediated transport (not ion channels)

Question 8

Structure of Na/K pump

Answer 8

One large alpha subunit, small beta. 
Double gated (one inside one outside). Gates NEVER open at same time, pump changes ion binding affinity depending which gate is open (ATP gives energy for this).

Question 9

Na/K pump cycle

Answer 9

1. ) Both gates closed, 2K ions inside
2. ) ATP binds, inner gate opens, affinity changes from K to Na. So K leaves and Na enters.
3. ) ATP split, pump is phosphorylated. Inner gate closes.
4. ) Spontaneously, outer gate opens and affinity changes from Na to K (so K can be pumped in)
5. ) Pump loses phosphate group and outer gate closes, completing cycle. (go back to step 1).

Question 10

How do channels affect Vm?

Answer 10

The number of channels. Continuous struggle b/w ions to get the Vm of the cell close to their E value.
Relative permeability determines Vm.. so if you have more K channels you have Vm closer to Ek.

Question 11

Nerve and muscle cells Vm

Answer 11

Closer to -70 to -90 mV (closer to Ek).

Question 12

Ion concentration and Vm

Answer 12

In normal resting cells not important b/c all cells have similar ICF and ECF…
exception is Potassium. Small change in ECF potassium concentration has huge effect on Ek and Vm.

Question 13

Driving force

Answer 13

The difference b/w E and Vm (if they are equal, driving force is 0).

Question 14

What determines membrane potential?

Answer 14

Relative conductance

Question 15

Relative conductance equation

Answer 15

Vm = (Ek + Gr*Ena) / (Gr + 1)

Question 16

Goldman Equation

Answer 16

Vm= 60log [ (Pk[Kout] + Pna[Naout] / (Pk)[Kin] + Pna*[Nain])
For all ions that cross the membrane. Na and K are most important.

Question 17

Depolarization

Answer 17

Inward movement of positive charge. Inside of cell becomes less negative.

Question 18

Changes in [k]

Answer 18

These have huge effect because permeability of K is so large, so it can cause big depolarizations etc.

Question 19

Hyperpolarization

Answer 19

Inside of cell gets more negative

Question 20

What if Pna increases?

Answer 20

This means that Na ions can move into the cell much more easily, which depolarizes the membrane (remember Na wants to move in b/c higher conc. outside).

Question 21

ECF charge

Answer 21

IT IS ALWAYS NEUTRAL. If you take away positive you must also take away negative.

Question 22

Hyperkalemia causes

Answer 22

Most K is in cells. Lose K from cells… crush injuries, burns, traumas, immunological attack.
Status of kidney (normal job is to excrete excess K).

Question 23

Diagnose hyperkalemia?

Answer 23

With EKG to see cardiac arrhythmias, measure plasma K conc.

Question 24

Treat Hyperkalemia

Answer 24

CBIGK. Give calcium to help heart, then reduce K conc in plasma by alkalinizing blood or giving sugar to juice up ATP supply. Or give enema that is ion exchanger.

Calcium, Bicarb, insulin + glucose, Kayexalate

Or dialysis.