I want to calculate voltage for half-maximal activation and the slope factor from a series of membrane potential (voltage) and amplitudes currents for a given ionic channel.
Hi Guillermo,
Which type of channel are you working with? If you are working with voltage-gated calcium or sodium channels you can plot the current I as a function of the command voltage V and fit the curve (so-called I/V curve) with the following modified Boltzmann equation:
I(V)=[Gmax×(V−Vrev)] / [1+exp ((V1/2− V) / k)]
with I(V) being the peak current density at the command potential V, Gmax the maximum conductance, Vrev the reversal potential, V1/2 the half-activation potential, and k the slope factor.
Hope this helps. Best, Norbert
One of the best ways to do this in whole cell mode is to plot the amplitude of the tail current for your channel against test voltage. The cell is usually held at a negative voltage and then jumped to a "test" potential to activate the channel, then the voltage is then jumped back to a third voltage were the the channels close . This third voltage is nowhere near the reversal potential for the current and the current trace shows a relaxation current the amplitude of which should be plotted against the second "test" voltage.The curve of tail current amplitude vs test voltage is a sigmoid curve which can be fitted to a Boltzmann function in software packages such as Prism graph.
This procedure works, but you may have to change reversal potential by changing Ringers, and can be more complicated if you channel shows rapid inactivation.
Thanks Norbert, I am working with voltage gated-channels so I will try to apply the procedure that you explain.
Also thanks to Marcus for the protocol on measuring on whole cell and the software packages to do the fitting
Marcus Christopher Allen How do you do a Boltzmann function curve fitted with graph pad prism ? I tried using the Boltzmann fitting already installed in the graph pad prism, but the slope factor and the values its selecting looks not true enough.
I am working with voltage gated potassium channels. Could you advise ?
What you could do is put some simulated data into Graph pad Prism and see if it gives you sensible answers. It should give you values similar to the V50 and the slope you used to generate the data.
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