How do inward rectifier potassium channels work in the heart?
According to Boron's medical physiology textbook:


  "Although inward rectifying potassium channels pass current better in
  the inward than the outward direction, the membrane potential (Vm) is
  typically never more negative than Ek (equilibrium potential of potassium across the membrane). Thus,net inward K+ current does
  not occur physiologically. As a result, the activation of GIRK
  channels (G protein coupled inwardly rectifying potassium channel)
  hyperpolarizes cardiac cells by increasing K+ conductance or outward
  K+ current."