The resting potential in heart muscle

Area of Science:

• Cardiology
• Electrophysiology
• Cell Physiology

Background:

• The cardiac resting potential is largely attributed to potassium diffusion, as proposed by Bernstein.
• Observed deviations from the Nernst equation suggest factors beyond simple diffusion, including ion activity coefficients, measurement errors, ion permeabilities, and electrogenic pumps.

Purpose of the Study:

• To investigate the origin of the cardiac resting potential in ventricular muscle and Purkinje fibers.
• To clarify the roles of ion activity coefficients, ion leakage, and electrogenic pumps in maintaining cardiac membrane potential.

Main Methods:

• Utilized potassium (K+) and sodium (Na+)-selective microelectrodes.
• Exposed cardiac muscle (ventricular and Purkinje fibers) to various ionic solutions.
• Measured intracellular and extracellular ion activities and membrane potential.

Main Results:

• Intracellular K+ activity coefficients were lower than extracellular ones.
• Ventricular muscle membrane potential correlated with the K+ activity gradient (≥10 mM extracellular K+).
• Purkinje fiber resting potential showed correlation with intracellular K+ and inverse correlation with intracellular Na+ activity.
• Significant Na+ and some Ca2+ leakage occurred at normal and low extracellular K+ levels.
• No contribution from electrogenic pumps or extracellular K+ accumulation was detected in the steady resting state.

Conclusions:

• The cardiac resting potential is primarily a K+ diffusion potential, influenced by ion activity coefficients.
• Ion leakage (Na+, Ca2+) plays a role, particularly at lower extracellular K+ concentrations.
• Electrogenic pumps do not significantly contribute to the steady resting potential in cardiac muscle.