Tetrodotoxin-sensitive Ca2+ and Ba2+ currents in human atrial cells

Area of Science:

• Electrophysiology
• Cardiology
• Ion Channel Physiology

Background:

• Human atrial cells possess various ion channels influencing cardiac function.
• The precise nature and properties of calcium currents in these cells are not fully elucidated.
• Distinguishing between different calcium and sodium currents is crucial for understanding cardiac electrophysiology.

Purpose of the Study:

• To characterize a novel tetrodotoxin (TTX)-sensitive fast inward calcium current (ICa,TTX) in human single atrial cells.
• To investigate the biophysical properties and pharmacological sensitivity of ICa,TTX.
• To compare the characteristics of ICa,TTX with known sodium currents (INa).

Main Methods:

• Whole-cell patch-clamp technique applied to human single atrial cells.
• Utilized Na(+)-free solutions and physiological Ca(2+) levels (2 mM).
• Tested ICa,TTX sensitivity to various ions (Ca(2+), Ba(2+)), channel blockers (TTX, Ni(2+), La(3+), Cd(2+)), and activators (veratrine).

Main Results:

• A TTX-sensitive fast inward Ca(2+) current (ICa,TTX) was identified, dependent on extracellular Ca(2+).
• ICa,TTX exhibited unique voltage dependence, prominent at negative potentials, and resembled Na(+) currents in activation/inactivation kinetics.
• The current was insensitive to T-type Ca(2+) channel blockers but suppressed by Na(+) channel inhibitors, suggesting a link to Na(+) channels or modified Na(+) channels.

Conclusions:

• ICa,TTX represents a distinct ionic current in human atrial cells, potentially involving Ca(2+)-conducting channels related to Na(+) channels.
• Alternatively, ICa,TTX might arise from a subpopulation of Na(+) channels exhibiting increased permeability to Ca(2+) and Ba(2+).
• Further research is needed to elucidate the exact molecular identity and physiological role of ICa,TTX in cardiac function.