Activation heat in rabbit cardiac muscle

Area of Science:

• Physiology
• Biophysics
• Cardiovascular Research

Background:

• Understanding myocardial activation heat is crucial for comprehending cardiac energy metabolism.
• Previous methods for estimating activation heat have limitations.
• The role of calcium (Ca2+) in excitation-contraction coupling and energy production requires precise quantification.

Purpose of the Study:

• To accurately estimate the activation heat in rabbit ventricular papillary muscles using multiple myothermic techniques.
• To compare the validity of different methods, including gradual pre-shortening and quick-release maneuvers.
• To investigate the influence of extracellular calcium concentration and muscle length on activation heat.

Main Methods:

• Myothermic measurements of activation heat in isolated rabbit right ventricular papillary muscles.
• Employing gradual pre-shortening to a non-force-generating length (lmin).
• Utilizing quick-release techniques during the latency period and from lmax to lmin to establish heat-stress relationships and measure activation heat.

Main Results:

• Gradual pre-shortening yielded lower activation heat estimates (1.59–2.06 mJ g-1).
• The latency release method provided a heat-stress relation estimating activation heat at 3.27 mJ g-1, closely matching direct measurement (3.46 mJ g-1).
• Activation heat magnitude correlated with extracellular Ca2+ concentration and peak stress, and was reduced by prolonged shortening or hyperosmotic solutions.

Conclusions:

• The quick-release method, particularly latency release, is a reliable technique for estimating myocardial activation heat.
• Activation heat constitutes approximately 30% of the total active energy flux per contraction, comparable to skeletal muscle.
• Estimated Ca2+ release per beat suggests submaximal myofibrillar ATPase activity under experimental conditions.