Force responses to rapid length changes in single intact cells from frog heart

Area of Science:

• Cardiovascular physiology
• Muscle biophysics

Background:

• Understanding the mechanical properties of cardiac muscle is crucial for comprehending heart function.
• Previous studies have characterized force transients in skeletal muscle, but less is known about atrial cells.

Purpose of the Study:

• To investigate force transients and recovery mechanisms in frog atrial cells following controlled length perturbations.
• To compare the mechanical properties and cross-bridge dynamics of frog atrial cells with those of frog skeletal muscle fibers.

Main Methods:

• Isolated frog atrial myocytes were subjected to rapid length steps (0.5 ms) during isometric twitches at varying temperatures (6-15°C).
• Force recovery was measured using high-frequency force transducers (2-4 kHz).
• Force-extension relationships (T1 and T2 curves) were analyzed to characterize instantaneous force and early recovery phases.

Main Results:

• A rapid force recovery phase was observed in atrial cells, occurring before slower recovery processes.
• This quick recovery phase accelerated with larger shortening steps, reaching rates comparable to skeletal muscle ( >1000 s⁻¹).
• The force-extension analysis indicated that force becomes negligible with ~1.4% shortening, and recovery is substantial with small stretches/releases (up to 0.3% L0).

Conclusions:

• Frog atrial cells display mechanical properties and cross-bridge transition rates similar to fast skeletal muscle fibers.
• These findings suggest conserved mechanisms governing contractility across different muscle types in the frog.