Membrane calcium current in ventricular myocardial fibres

Area of Science:

• Cardiovascular Physiology
• Cellular Electrophysiology

Background:

• The rapid sodium current (I(Na)) is well-characterized in cardiac cells.
• A slower inward current component has been observed but its ionic basis and role remain less clear.

Purpose of the Study:

• To investigate the ionic mechanism of the slow inward current in dog ventricular preparations.
• To determine the role of this current in cardiac action potential generation.

Main Methods:

• Utilized voltage clamp techniques to isolate and measure ionic currents.
• Manipulated extracellular ion concentrations ([Ca](o), [Na](o)) and applied pharmacological agents (tetrodotoxin, [Mg](o)).

Main Results:

• Identified a slow inward current, distinct from I(Na), that is highly sensitive to extracellular calcium ([Ca](o)).
• Demonstrated that tetrodotoxin and magnesium ions do not affect this current, supporting a calcium ion (Ca2+) as the charge carrier.
• Characterized the voltage-dependence and inactivation kinetics of the calcium current.

Conclusions:

• The slow inward current in cardiac tissue is primarily carried by calcium ions.
• This calcium current plays a significant role in shaping the cardiac action potential plateau.