Determining K+ channel activation curves from K+ channel currents

Potassium ion channels are generally believed to have current-voltage (IV) relations which are linearly related to driving force (V - E-K), where V is membrane potential and E-K is the potassium ion equilibrium potential. Consequently, activation curves for K+ channels have often been measured by normalizing voltage-clamp families of macroscopic K+ currents with (V - E-K), where V is the potential of each successive step in the voltage clamp sequence. However, the IV relation for many types of K+ channels actually has a non-linear dependence upon driving force which is well described by the Goldman-Hodgkin-Katz relation. When the GHK dependence on (V - E-K) is used in the normalization procedure, a very different voltage dependence of the activation curve is obtained which may more accurately reflect this feature of channel gating. Novel insights into the voltage dependence of the rapidly inactivating I-A channels Kv1.4 and Kv4.2 have been obtained when this procedure was applied to recently published results.