Sodium-calcium exchange in heart: membrane currents and changes in [Ca2+]i

Area of Science:

• Cardiology
• Cell Physiology
• Biophysics

Background:

• The sodium-calcium exchanger (Na-Ca) plays a crucial role in regulating intracellular calcium ion concentration ([Ca2+]i) in mammalian heart cells.
• Understanding the electrogenic properties of the Na-Ca exchanger is vital for comprehending cardiac electrophysiology and calcium handling.

Purpose of the Study:

• To investigate and characterize the electrogenic, voltage-dependent sodium-calcium (Na-Ca) exchanger in mammalian cardiac myocytes.
• To elucidate the relationship between membrane potential, intracellular calcium, and membrane current mediated by the Na-Ca exchanger.

Main Methods:

• Utilized voltage clamp techniques on guinea pig ventricular myocytes.
• Employed fura-2, a fluorescent calcium indicator, to measure real-time changes in intracellular calcium ion concentration ([Ca2+]i).
• Applied various organic channel blockers and impermeant ions to isolate and identify Na-Ca exchange currents and their effects.

Main Results:

• Observed slow increases in [Ca2+]i upon depolarization, dependent on internal sodium, external calcium, and membrane voltage.
• Detected net calcium efflux and a decline in inward current upon repolarization, linked to Na-Ca exchange.
• Established a linear relationship between [Ca2+]i and membrane current, which was modulated by hyperpolarization.

Conclusions:

• Confirmed the operation of an electrogenic, voltage-dependent sodium-calcium exchanger in mammalian heart cells.
• Demonstrated that the Na-Ca exchanger significantly influences intracellular calcium homeostasis and membrane currents during changes in membrane potential.
• Provided quantitative insights into the Na-Ca exchanger's contribution to cardiac calcium dynamics.