Slow inward tail currents in rabbit cardiac cells

Area of Science:

• Cardiac Electrophysiology
• Ion Transport Mechanisms
• Cellular Physiology

Background:

• Slow inward tail currents play a role in cardiac action potential repolarization.
• The precise mechanisms and modulators of these currents are not fully understood.
• Investigating these currents is essential for comprehending cardiac function and dysfunction.

Purpose of the Study:

• To elucidate the ionic basis of slow inward tail currents in rabbit ventricular and atrial myocytes.
• To test the hypothesis that these currents are mediated by the sodium-calcium exchanger.
• To investigate the influence of intracellular calcium and sarcoplasmic reticulum calcium uptake on tail current dynamics.

Main Methods:

• Whole-cell voltage-clamp technique with suction microelectrodes.
• Enzymatic dispersion of rabbit ventricular and atrial myocytes.
• Application of various stimulation protocols, Tyrode solutions, and pharmacological agents.

Main Results:

• Slow inward tail currents were consistently observed and characterized in rabbit myocytes.
• Current amplitude and decay kinetics were dependent on the duration of preceding depolarizations.
• Evidence supports the involvement of the sodium-calcium exchanger, modulated by intracellular calcium levels.
• Inhibition of the Na(+)-K+ pump or reduced extracellular K+ increased tail current amplitude.
• Barium ions (Ba2+) inhibited the currents, while strontium ions (Sr2+) did not, consistent with Na(+)-Ca2+ exchanger activity.

Conclusions:

• The slow inward tail current in rabbit myocytes is primarily generated by the electrogenic sodium-calcium exchanger.
• Intracellular calcium release from the sarcoplasmic reticulum modulates the sodium-calcium exchanger activity.
• Sarcoplasmic reticulum calcium uptake significantly influences the time course of the slow inward tail current.
• These findings enhance the understanding of cardiac calcium homeostasis and electrophysiological mechanisms.