The molecular basis of the steep force-calcium relation in heart muscle

Area of Science:

• Cardiovascular Physiology
• Muscle Contraction Biophysics
• Molecular Biology

Background:

• Heart muscle contraction is regulated by calcium (Ca2+) binding to troponin on thin filaments, enabling myosin interaction and force generation.
• The relationship between force and Ca2+ concentration follows the Hill equation, characterized by EC50 and the Hill coefficient (nH), which indicates cooperativity.

Purpose of the Study:

• To investigate the molecular basis of cooperativity in cardiac muscle contraction, specifically evaluating the hypothesis that force-generating myosin cross-bridges are responsible.
• To determine if the cooperative mechanism is an intrinsic property of the thin filaments or dependent on cross-bridge interactions.

Main Methods:

• Analysis of steady-state force-Ca2+ relationships in demembranated ventricular trabeculae.
• Examination of the effects of interventions altering maximum force and EC50 on the Hill coefficient (nH).
• Assessment of nH values in the presence and absence of myosin inhibitors to abolish force generation.

Main Results:

• The Hill coefficient (nH) is typically around 3, suggesting cooperativity, despite troponin having only one Ca2+ binding site per molecule.
• Interventions modifying maximum force and EC50 generally did not significantly alter nH.
• Complete inhibition of force generation by myosin inhibitors did not affect nH values for Ca2+ binding or troponin structural changes.

Conclusions:

• The cooperative mechanism responsible for the high Hill coefficient (nH) in cardiac muscle contraction is not driven by force-generating myosin cross-bridges.
• Evidence strongly supports that cooperativity is an intrinsic property of the thin filaments themselves.
• This finding has implications for understanding the precise molecular regulation of cardiac contractility under physiological conditions.