Cardiac transient outward potassium current


The cardiac transient outward potassium current is one of the ion currents across the cell membrane of heart muscle cells. It is the main contributing current during the repolarizing phase 1 of the cardiac action potential. It is a result of the movement of positively charged potassium ions from the intracellular to the extracellular space. Ito1 is complemented with Ito2 resulting from Cl− ions to form the transient outward current Ito.

Mechanism

Ito1 is rapidly activated and deactivated. It is activated after the fast increase of the membrane potential following the phase 0 of the cardiac action potential. Once activated, ions from inside the cells flow to the extracellular space. This outward flow of positively charged ions constitutes the Ito1 and causes the transmembrane voltage to decrease.This decrease of the transmembrane potential is known as repolarization. Ito1 is then quickly deactivated, stopping the repolarization and ending the phase 1 of the action potential.
Ito1 is Ca2+-independent and has been clearly demonstrated in myocytes from different cardiac regions and species. There are two kinetic variants of cardiac Ito1: fast Ito1, called Ito1,f, and slow Ito, called Ito1,s. The channel responsible for Ito1,f is formed by assembly of Kv4.2 subunits, Kv4.3 subunits or a combination of the two, while the channel responsible for Ito1,s is composed of Kv1.4 subunits. In addition, several regulatory subunits and pathways modulating the level and biophysical properties of cardiac Ito have been identified.
Ito1 affects the opening of Ca2+ channels during Phase 2 of the Action Potential. As a result, changes in Ito1 modulate changes in the action potential duration.

Role in disease