Cell Bio E1 Ch 8

Aquaporin → ↑ water permeability →

↑ rate of water transport

hypothesis

If aqp13a encodes an aquaporin, then Xenopus oocytes expressing aqp13a will have an increased rate of water uptake when placed in hypotonic solution compared with oocytes that do not express aqp13a.

Hypotonic solution creates

driving force

Aquaporin = _______ rate of water movement

increases but does not determine the direction of water movement

What is electrophysiology used for?

measure resting membrane potential or measure current at various membrane potentials

Whole-Cell Patch Clamp

You have a:

recording micropipette/electrode recording electrical activity.

↓

approach plasma membrane

↓

form a tight seal

↓

brief strong suction breaks the membrane underneath the pipette

↓

pipette becomes continuous with the cytoplasm

↓

whole-cell configuration

Now you're able to measure electrical properties across the whole cell membrane.

What can whole-cell patch clamp measure?

Whole-cell membrane potential or whole-cell ionic current

What does voltage clamp do?

Change membrane potential and record currentI-V curve

X-intercept = Veq for the ion

equilibrium potential.

Why does current switch directions at Veq?

At one side of Veq, the electrochemical driving force pushes K⁺ one direction and at exactly Veq there is no net electrochemical driving force, so i = 0 and the other side the driving force reverse so current can reverse.

Voltage-INSENSITIVE K⁺ Channel

essentially available/open regardless of Vm, whose current changes with electrochemical driving force.

Voltage-SENSITIVE K⁺ Channel

Where I-V curve is flat → voltage-sensitive channels are closed. x-value where the I-V curve increases linearly → threshold potential for that channel.

Voltage-Sensitive Na⁺ Channel

orange U-shaped-looking curve.

At negative voltages before threshold:

Na⁺ channel is closed

↓

GNa ≈ 0

↓

current ≈ 0.

Then threshold is reached:

voltage-sensitive Na⁺ channels open. Na conductance increases and inward Na+ current plotted so downward below zero than driving force gets smaller and Na current decreases at I = 0 so curve returns to x-axis

Below x-axis

inward cation current

Above x-axis

outward cation current

How would acute hyponatremia affect the I-V curve for Na+?

↓ extracellular Na⁺

then

Na⁺ gradient becomes smaller

then

Na⁺ Veq becomes less positive.

Remember: x-intercept = Veq.

Therefore, the Na⁺ I-V curve's x-intercept shifts LEFT to a less-positive voltage.

Draw the I-V curve for a ligand-gated Na⁺ channel when ligand is always present.

channel is ligand-gated so channel stays available/open, behaves like a voltage-insensitive channel with respect to Vm. linear I-V curve (starting at negative then postive)

How about when absent?

No ligand means the channel is closed so I = 0 at every voltage.

shown as a flat line along I = 0.

Cell-attached patch clamp

measures SINGLE CHANNEL events, shown as little jumps that represent individual channels switching between closed and open.

Channel Inhibitor

fewer opening events, inhibitor reduces the frequency/probability of channels being open so there would be less peaks