The relationship between Vm, K ⁺ , and I _K through Kir channels . (A) The percentage of Kir channels open plotted in purple against membrane potential. As Vm depolarizes from E_K, Kir conductance decreases due to block by Mg²⁺ and polyamines. Plotted in red is a simplified linear driving force for K⁺ to flow through Kir channels. As Vm depolarizes from E_K, there is an increase in driving force for K⁺ to exit muscle. (B) I_K through Kir channels (determined by open probability * K⁺ driving force) plotted versus membrane potential. At E_K (reversal potential), the driving force is 0, so there is no current flow despite open channels. Outward K⁺ current reaches a maximum value at a membrane potential depolarized to E_K, where the open probability is moderately reduced from its maximum value. Further depolarization decreases outward I_K because of decreased conductance. (C) Kir IV relationships used in computer simulations at three different K_ex concentrations. There are hyperpolarized and depolarized shifts in the voltage dependence of outward I_K through Kir channels with decreases and increases in K_ex, respectively. Changes in K_ex also change maximal outward current. See Chang et al. for data on the K⁺ dependence of outward I_K (Chang et al., 2010).