Current Voltage Relationships

Current is the movement of charge, and in cells the flow of ions through ion channels can be measured. By expressing cloned ion channels in heterologous systems or by silencing other channels with chemical blockers, the current from a single ion channel can be isolated. In these settings, it is often useful to apply different electrical potentials to the membrane and measure the magnitude and direction of the current as a function of voltage. If current through the ion channel is linearly related to the membrane potential as in Fig. 3a, then the ion channel is said to be ohmic because it behaves like a resistor and follows Ohm's law:

where V is voltage, I is current, and R is the resistance. For the purposes of studying ion channels, it is useful to modify Ohm's law into the form

Here, V is replaced with (V-Ex) because I = 0 at the equilibrium potential, and R is replaced with its inverse, the conductance G. The conductance is the ease with which ions will flow through an ion channel, which is more intuitive since ion channels with a high conductance will conduct larger currents. (V-Ex) constitutes the driving force, which together with the conductance determines the amount of current that flows through the ion channel. From Fig. 3a or Eq. (4), we can see that G represents the slope of the I-V curve. Ion channels with a higher conductance will have steeper I-V curves.

I-V curves show the sensitivity of an ion channel to voltage. One should bear in mind that all ion channels are not ohmic. Ion channels that conduct more ions in one direction are said to rectify. For example, inward rectifier potassium channels preferentially conduct more K+ into the cell than out of the cell (Fig. 3b). Other ion channels display outwardly rectifying currents.

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