Submaximal sodium-lack contractures in rapidly perfused frog ventricular strips

Area of Science:

• Cardiovascular Physiology
• Muscle Contraction Mechanisms
• Ion Transport in Cardiac Muscle

Background:

• Understanding the regulation of intracellular calcium concentration ([Ca]i) is crucial for cardiac function.
• Previous research has explored the role of sodium-calcium exchange in cardiac muscle.
• The relationship between extracellular ions and myofilament tension requires further elucidation.

Purpose of the Study:

• To investigate the determinants of intracellular calcium concentration ([Ca]i) during contractures in frog ventricular muscle.
• To test the hypothesis that [Ca]i during contracture plateaus represents a steady-state balance of influx and efflux.
• To examine the role of sodium-dependent and independent mechanisms in calcium handling.

Main Methods:

• Rapid perfusion of frog ventricular muscle strips with sodium-free Ringer solution.
• Measurement of muscle tension and its relationship to extracellular calcium concentration ([Ca]o).
• Assessment of the effects of electrical depolarization, high potassium, low temperature, and dihydroouabain on contracture tension.

Main Results:

• Sodium-free conditions induced rapid contractures, but intracellular calcium was insufficient to saturate myofilaments.
• Tension showed an S-shaped relationship with [Ca]o, indicating calcium-dependent activation.
• Dihydroouabain potentiated peak twitch tension but not plateau tension during contractures, supporting a steady-state model.

Conclusions:

• Intracellular calcium concentration during contracture plateaus is likely a steady-state value.
• This steady-state is maintained by a balance between sodium-dependent calcium influx and sodium-independent calcium transport.
• The potentiating effects of increased intracellular sodium may be dependent on membrane potential.