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Apparently in cardiomyocytes, there is an inward rectifying potassium channel that operates during phase 4 of the cardiomyocyte action potential. I have heard that despite this potassium channel being referred to as an inward rectifying channel, that in vitro the channel in fact still transports potassium from the inside to the outside of the cell. It appears that wikipedia seems to suggest otherwise (?).

Can someone please explain what the inward rectifier potassium channel does and its role in phase 4 of the myocyte action potential?

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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."

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