Cadmium block of isometric contractions of isolated bullfrog atrial cells

Area of Science:

• Cardiovascular Physiology
• Cellular Electrophysiology
• Pharmacology

Background:

• Understanding the mechanisms of calcium handling is crucial for regulating cardiac contractility.
• Field-stimulated bullfrog atrial cells provide a model to study ion channel function.
• Distinct components of cellular force generation are influenced by various ion channels and exchangers.

Purpose of the Study:

• To investigate the effects of cadmium, verapamil, and quinacrine on the force of contraction in isolated bullfrog atrial cells.
• To differentiate between various calcium entry and exit pathways involved in cardiac muscle contraction.
• To elucidate the roles of L-type calcium channels and Na-Ca exchange in regulating twitch force.

Main Methods:

• Isolated, single bullfrog atrial cells were field-stimulated.
• Rapid application and removal of agents (cadmium, verapamil, quinacrine) were used to minimize confounding intracellular concentration changes.
• Two components of twitch force, distinguished by their sensitivity to cadmium, were analyzed.

Main Results:

• Two components of twitch force were identified: one sensitive to micromolar cadmium and another to millimolar cadmium, contributing equally to activation.
• The cadmium-sensitive force component (≤100 µM Cd2+) showed characteristics of L-type calcium channels, including sensitivity to verapamil and potentiation by catecholamines.
• The cadmium-insensitive force component was abolished by removing extracellular calcium and potentiated by quinacrine, suggesting it is not Na-Ca exchange.

Conclusions:

• Activating calcium enters atrial cells through both an inactivating, cadmium-sensitive L-type channel and a non-inactivating, cadmium-insensitive mechanism.
• Calcium exits the cell primarily via Na-Ca exchange.
• These findings clarify the distinct roles of different calcium transport pathways in cardiac contractility.