# Shared Flashcard Set

## Details

Blitzkrieg 4
Voltage Clamp Method
25
Biochemistry
10/15/2010

Term
 Explain the current clamp method
Definition
 You stick an intracellular electrode into the cell and then the Vm changes in accordance with Ohm's law.
Term
 Why is the current clamp method not good for measuring Vm changes with drugs/signal transduction?
Definition
 Because if a drug binds a g protein or whatever and this turns off K+ channels, then the cell could rapidly become less negative (depolarized), which could then open voltage gated K+ channels, or inward rectifiers. SO, you can't control Vm or Im (conductance)
Term
 Why is the current clamp method not good for measuring Vm changes with drugs/signal transduction?
Definition
 Because if a drug binds a g protein or whatever and this turns off K+ channels, then the cell could rapidly become less negative (depolarized), which could then open voltage gated K+ channels, or inward rectifiers. SO, you can't control Vm or Im (conductance)
Term
 What is the voltage clamp method?
Definition
 Two microelectrodes inserted into the cell. One of them measures Vm, one of them passes current into the cell. There is a command voltage set by the researcher, so you can measure how the voltage is changing. (ie how much current has to be applied to keep the voltage constant). The current passing into the cell through an electrode is equal and opposite to whatever is passing through the cell membrane at all times.
Term
 Can the voltage clamp method be used in all cells?
Definition
 No, only those that are large!
Term
 What is current? What is the symbol for current?
Definition
 amount of charge moving past per unit time. Im
Term
 What is voltage? What is the symbol for the membrane?
Definition
 Vm. This is the difference between two potentials (like on each side of the membrane)
Term
 What is an I-V plot?
Definition
 This is like when you use a voltage clamp and you are recording the voltage vs the current (Vm vs Im). The slope is the conductance (g).
Term
 The the line on an V-I plot is straight, what does that mean?
Definition
 It means the conductance does not change.
Term
 On a V-I plot, which is what axis?
Definition
 I is on y axis. V is on x axis. G is the slope
Term
 Where would you find the equilibrium potential on a V-I plot?
Definition
 The Vm where I=zero. This is the voltage where current is nonexistant. (obs, because equilibrium is where stuff doesn't move)
Term
 Let's say you are doing voltage clamp with only K+ channels with constant conductance. At equilibrium, I=0 and Vm=-91. Which direction will K+ flow if the Vm becomes -70, -150?
Definition
 If it becomes -70, K+ ions will flow out, because cell became depolarized, and K+ efflux inhibits depolarization. If it becomes -150, K+ will flow in because now the cell is too polarized.
Term
 Which version of Ohm's law is used for I-V plots?
Definition
 I = GV
Term
 What definition can be ascribed to outward and inward currents?
Definition
 Outward currents are positive to the Veq. Inward currents are negative to it."By definition, outwardcurrents have positive values, and inward currents have negative values."
Term
 What would happen to an I-V plot if you took out half of the K+ channels with a drug?
Definition
 The conductance (g) would go in half. However, the Vk would not change. This translates to a shallower slope, but the same x int. (if you doubled the channels, steeper line, but same intercept.
Term
 Why is it that a change in conductance will not change the Vk (or Veq)?
Definition
 Because no matter how many channels you have, the amount of current flowing at equilibrium is still zero.
Term
 Would the Y int change if you change conductance?
Definition
 Yes. It is only the Xint that changes
Term
 What would an inward rectifier's graph look like? (think Gm on y axis and Vm on x axis
Definition
 Well, at polarized points, the K+ influx keeps the channels open by knocking the Mg+ out of the channels. SO, you'd have very low conductance until close to the point where Vm=Vk. At that point, there would be rapid increase in conductance.
Term
 What would an inward rectifier's I-V graph look like in terms of slope?
Definition
 Well, because the conductance changes, the slope would NOT be constant. Somewhere near about -70 or -90 there would be a very rapid increase in slope. At more positive numbers (ie -50) the slope would be pretty low, because conductance is low.
Term
 What would the inward rectifier graph look like in terms of x int?
Definition
 The X int would be -91, because that is the point where there is no flow (no current).
Term
 What would inward rectifier graph look like in terms of Im?
Definition
 At low values of Vm, Im would be low. This is because negative Im (current) means influx of K+, and at hyperpolarized states (very negative Vm) K+ likes to rush in. [Conversely, when Vm is high--depolarized--the Im would be positive, because K+ wants to efflux to inhibit depolarization.
Term
 Tell me about a G vs Vm curve for a voltage gated channel and why
Definition
 Well, you'd have high conductance at high Vm, because this is what would open up a voltage gated K+ channel (the leaflets wouldn't be attracted to the cytosol anymore because its less negative.
Term
 Tell me about a I-V curve for voltage gated channel in reference to the slope
Definition
 Well, at high Vm, you'd have a steeper slope. At low Vm, you'd have a slope that is practically flat.
Term
 Tell me about I-V plot for voltage gated in reference to the current.
Definition
 Current would be positive for high Vm and zero for low Vm. This is because at high Vm, the channels are open. And they are closed the rest of the time.
Term
 How would a voltage gated Na+ channel IV plot look a bit different from K+?
Definition
 Well, there would be a transition from negative to positive current. This is because voltage gated Na+ and Ca+ channels are not unidirectional. Things can flow in either direction.
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