Does a calcium-mediated outward current exist in the atrial myocardium?

Area of Science:

• Cardiology
• Electrophysiology
• Myocardial Physiology

Background:

• Intracellular free calcium (Ca) concentration plays a crucial role in regulating cardiac contractility and action potential dynamics.
• Experimental conditions altering Ca levels are known to influence myocardial function, but the precise mechanisms during repolarization require further elucidation.

Purpose of the Study:

• To investigate the impact of altered intracellular free Ca concentration on the repolarization phase of rabbit atrial myocardium.
• To correlate changes in action potential repolarization with positive inotropic effects under various experimental conditions.

Main Methods:

• Utilized a microelectrode technique to record action potentials in rabbit atrial myocardium.
• Manipulated intracellular free Ca concentration through experimental conditions like increased external Ca, ouabain application, and decreased external K concentration.

Main Results:

• All tested conditions induced a positive inotropic effect, indicating enhanced atrial contraction.
• A rapid initial repolarization phase was observed in the action potentials.
• The positive inotropic effect correlated well with action potential shortening at 25% repolarization.
• A positive correlation was found between the velocity of initial rapid repolarization and the potentiation of atrial contraction after pacing pauses.

Conclusions:

• Experimental conditions that increase intracellular Ca concentration lead to a positive inotropic effect and shortened action potentials in rabbit atrial myocardium.
• These findings suggest the activation of a Ca-sensitive outward current during repolarization, occurring at potentials approximately 40 mV more positive than the resting potential.