“MEMPOT”© – Computer Simulation of Membrane Potential Measurements INTRODUCTION
“MEMPOT”© is an interactive graphical simulation program that is designed to show how cell membrane potentials are measured with microelectrodes in the laboratory situation. The simulation shows how the membrane potential depends on the external potassium, [K+]o and external sodium, [Na+]o concentrations and the relative permeability of the membrane to these ions. The program simulates membrane potential measurements in any excitable cell permeable to K+ and Na+ ions (e.g., nerve or muscle).
The Mempot shortcut is located in the Physiology folder on the desktop. EXERCISE 1 – RESTING POTENTIAL MEASUREMENTS
From the menu, open the “Resting Potential Experiment”. This exercise illustrates the effect of varying the external potassium concentration, [K+]o on the resting membrane potential.
Choose “Start Experiment”. You should obtain measurements of the resting membrane potential in at least 6 different potassium concentrations from 0.5 to150 mmol/l. (0.5, 1.0, 5.0, 10.0, 20.0, 40.0, 100.0 and 150.0).
To do this, enter the value (eg 0.5) into the box and click “ok”. Use the arrow keys to sample each of the 5 cells. Put the electrode into the cell, but be careful not to rupture the cell. The membrane potential will appear on the left of the screen (Vm). Sample each of the 5 cells to get an average resting potential (Vav) and an estimate of error (SEM) at each [K]o. Enter this data into Table 1, along with the internal and external cation concentrations.
Once a reasonable range of [K+]o has been investigated, click “Finish Experiment” and return to the “Menu”
Open “Graph Resting Membrane Potential Results”. This part of the prac requires you to re-enter the data into the program (“Enter/Edit Data”), which will then generate a graph of resting membrane potential against [K+]o.
Plot a curve of the relationship between resting membrane potential and [K+]o. The program will plot the predicted curve based on the Goldman-Hodgkin-Katz equation for each permeability ratio entered. The objective is to obtain the curve that best fits the data. You should then obtain an estimate of the relative Na+ to K+ permeability by trial-and-error entry of values for PNa/PK. This tells you how permeable the cell is to Na+ to K+ when the cell is at rest.
Best value of PNa/PK: 0.0110
Average Sum of squares = 30.830
Standard Error of Estimate, Sy= 1.82463
Once you have found the curve based on the GHK equation (red), you should also have the program plot the ideal curve (based on the Nernst equation, blue dotted) that would be predicted if Na+ were impermeant.
The blue line represents the Nernst Equation while the red represents the GHK equation
The Nernst Potential for an ion EX (volts) is calculated from
RT [X] o
--- ln (------)
zF [X] i
where [X] is the concentration of the ion of interest. This may be potassium, sodium, choline etc.
Substituting appropriate values for R,T, z & F, the Nernst Potential for an ion EX (millivolts) is calculated from
EX (mV) = (approx)
60 log10 (------)
Compare this to the GHK equation (in your notes). How many ions does the GHK equation take into account?
The Goldman-Hodgkin-Kaz (GHK) equation only takes into account three ions (potassium, sodium and chloride ions), while the Nernst equation only considers the concentration of one ion of interest. That is, the Nernst equation calculation tells us what potential would exist...
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