Key Points
- To investigate the role of Na+-Ca2+ exchange and intracellular Na+ activity in regulating contractile tension in canine cardiac Purkinje fibers under varied ionic conditions.
- Simultaneously recorded intracellular Na+ activity, twitch tension, and transmembrane potential in canine Purkinje fibers paced at 1 Hz.
- Assessed fiber responses across normal, low, and high extracellular Na+ or Ca2+ concentrations in the presence and absence of strophanthidin (5 x 10^-7 M).
- Stepwise reductions in extracellular Na+ concentration by 20%, 40%, and 60% lowered the extracellular-to-intracellular Na+ activity ratio while steeply increasing twitch tension.
- Exposure to Na+-free solution depleted intracellular Na+ and markedly increased resting fiber tension.
- The slope of the log twitch tension versus intracellular Na+ activity relationship steepened as intracellular Na+ levels increased, demonstrating that exchange kinetics depend on internal Na+ concentration.
Structured PICO
PPopulationCanine cardiac Purkinje fibers
IInterventionAlterations in extracellular [Na+] or [Ca2+] in the absence and presence of strophanthidin (5 X 10(-7) M)
CComparatorNormal Tyrode solution / baseline conditions
OOutcomeIntracellular Na+ activity (aiNa), twitch tension, and transmembrane potentialsurrogate
In canine cardiac Purkinje fibers, the Na+-Ca2+ exchange mechanism, which regulates contractility, appears to depend on the level of intracellular Na+ activity.