Evidence that reverse Na-Ca exchange can trigger SR calcium release

Area of Science:

• Cardiology
• Cell Physiology
• Biophysics

Background:

• The Na-Ca exchange mechanism is implicated in regulating intracellular calcium and cardiac contractility.
• The precise role of Na-Ca exchange as a trigger for sarcoplasmic reticulum (SR) calcium release and subsequent contractions requires further elucidation.

Purpose of the Study:

• To investigate whether Na-Ca exchange can function as a primary trigger for SR Ca release and cardiac contractions.
• To determine the influence of intracellular sodium concentration on the voltage-dependence of triggered contractions.

Main Methods:

• Electrophysiological recordings of cellular contractions under varying voltage clamp conditions.
• Manipulation of intracellular and extracellular sodium concentrations to assess Na-Ca exchange activity.
• Use of pharmacological agents (ryanodine, thapsigargin, nifedipine, XIP) to differentiate Ca current and Na-Ca exchange pathways.

Main Results:

• Contractions can be triggered even when Ca current is significantly blocked, suggesting an alternative pathway.
• The relationship between voltage and contraction strength is dependent on intracellular Na concentration, unlike Ca current-voltage relationships.
• Experiments in the absence of a Na gradient demonstrate that SR Ca content does not influence the shape of the shortening-voltage relationship, implicating reverse Na-Ca exchange.

Conclusions:

• The Na-Ca exchange, particularly reverse exchange, contributes to triggering SR Ca release and cardiac contractions.
• The observed voltage-dependent asymmetry in contractions is primarily due to the contribution of reverse Na-Ca exchange, not SR Ca content.
• Reverse Na-Ca exchange may serve as a significant inotropic mechanism, potentially enhancing contractility during increased heart rates due to elevated intracellular Na.