Activation in frog atrial trabeculae: dependence on temperature and length

Area of Science:

• Cardiology
• Physiology
• Biophysics

Background:

• Cardiac muscle contractility is crucial for heart function.
• Understanding the interplay of ionic concentrations, temperature, and muscle length is key to cardiac physiology.

Purpose of the Study:

• To investigate the effects of external sodium concentration ([Na+]o) and temperature on isolated frog atrial trabeculae contractility.
• To explore the length-dependent activation of cardiac muscle under varying conditions.

Main Methods:

• Isolated frog atrial trabeculae were subjected to sodium withdrawal to induce steady-state contractures.
• Isometric tension was measured at different external sodium concentrations (0.25–45 mM) and temperatures (4°C and 20°C).
• Rapid temperature jumps were applied during contractures and twitches to analyze transient mechanical responses.

Main Results:

• Peak contracture tension was dependent on relative muscle length and increased with stretch at warmer temperatures.
• Temperature jumps during cold contractures induced tension transients, with the initial increase proportional to preceding tension.
• Peak twitch tension decreased with increasing temperature, and temperature jumps during twitches showed temperature-dependent relaxation rates.

Conclusions:

• Cardiac myocyte activation (cross-bridge attachment) is significantly less than maximal at warmer temperatures during twitch responses.
• Results provide evidence for length-dependent activation in atrial trabeculae, influencing steady-state tension development.
• The study highlights the complex interaction between sodium ions, temperature, and muscle length in regulating cardiac contractility.