Activation of classical Kir (K+ inwardly rectifying) channels (KCNJ2, 4, 12 and 14) results in K+ influx which contributes to the maintenance of the membrane potential (Phase 4 of the action potential). The current created by this flow of K+ is called the inward rectifying current (IK1). A channel that is inwardly-rectifying is one that passes current more easily into the cell than out of the cell. At membrane potentials negative to potassium's reversal potential, KCNJs support the flow of K+ ions into the cell, pushing the membrane potential back to the resting potential. Two factors regulate K+ permeability - cell permeability to K+ is increased at more negative membrane potentials and increasing extracellular K+ concentrations.
When the membrane potential is positive to the channel's resting potential (such as in Phase 3 of the action potential), these channels pass very little charge out of the cell. This may be due to the channel's pores being blocked by internal Mg2+ and endogenous polyamines such as spermine (Shin & Lu 2005).
Inwardly rectifying (Kir) channels contribute to potassium leak, stabilizing cells near the equilibrium reversal potential of potassium (EK). Kir channels pass small outward currents because of pore blockade by internal magnesium and polyamines; at potentials negative to EK, large inward currents are passed upon relief from blockade.