Voltage dependence of Na-Ca exchanger conformational currents

Area of Science:

• Biophysics
• Molecular Physiology
• Cardiovascular Research

Background:

• The sodium-calcium exchanger (Na-Ca) plays a critical role in regulating intracellular calcium in cardiac myocytes.
• Understanding the conformational dynamics of Na-Ca is essential for elucidating its transport mechanism.
• A transient current (Icont) has been proposed to represent conformational changes of Na-Ca after calcium binding.

Purpose of the Study:

• To investigate the properties of the transient current (Icont) associated with Na-Ca exchanger conformational changes.
• To characterize the voltage dependence of Icont and its relationship to Ca2+ binding.
• To explore the effects of the Na-Ca exchange inhibitor 3',4'-dichlorobenzamil (DCB) on Icont.

Main Methods:

• Utilized flash photolysis of caged Ca2+ to induce rapid intracellular Ca2+ concentration jumps in isolated cardiac myocytes.
• Employed the whole-cell patch-clamp technique to measure membrane currents, specifically Icont.
• Developed an experimental protocol to compensate for caged compound photoconsumption, revealing subtle voltage dependence.

Main Results:

• A shallow voltage dependence of Icont was resolved, previously unobserved.
• Analysis suggests Na-Ca exchanger molecules exist in states with the Ca2+ binding site facing either inside or outside the cell.
• Cytosolic dialysis with 3',4'-dichlorobenzamil (DCB) increased Icont magnitude and altered voltage dependence, indicating interference with Na+ binding or translocation.

Conclusions:

• The transient current (Icont) likely arises from charge movement during Na-Ca exchanger conformational changes post-Ca2+ binding.
• The voltage dependence of Icont provides insights into the distribution of exchanger conformational states.
• DCB's effect suggests it interferes with Na+ interaction or transport steps within the Na-Ca exchanger cycle.