Potassium channels

Types of K+ current

1. Inwardly rectifying (i_K1)
2. Transient outward (i_to)
3. Delayed rectifier (i_K)

Inwardly rectifying K+ current (i_K1)

• Responsible for outward K+ current at RMP
• Outward K+ current decreased when Vm positive
  * i.e. Phase 1 and 2
• Outward K+ current increases as Vm become more negative in Phase 3
  --> Repolarisation accelerated

Transient outward K+ current (i_to)

• Starts in phase 1
• Due to outward pressure of both electrical and concentration gradients
• Help determine the duration of plateau

e.g.

• In atrial myocytes, i_to is more pronouced
  --> K+ current exceeds Ca2+ current early
  --> Plateau less pronouced in atrial cells

Delayed rectifier K+ current (i_K)

• Activated near the end of phase 0
• Activation really slow (few hundred ms)
• Very little contribution in phase 2
• Significant contribution in phase 3

Two types: more rapid activation (i_Kr) and slower activation (i_Ks)
--> Distinction may contribute to difference in duration and AP shape between endocardium and epicardium
* [BL8:p19]