Excitation-contraction coupling in developing mammalian myocardium: evidence from voltage clamp studies

Area of Science:

• Cardiovascular Physiology
• Developmental Biology
• Cellular Electrophysiology

Background:

• Excitation-contraction coupling (ECC) is fundamental to cardiac function.
• Developmental changes in ECC are crucial for understanding heart maturation.
• The role of calcium (Ca2+) handling in developing myocardium requires further elucidation.

Purpose of the Study:

• To investigate maturational changes in excitation-contraction coupling in rabbit ventricular papillary muscles.
• To differentiate the mechanisms of tension development in immature versus mature myocardium.
• To explore the voltage dependence and calcium sensitivity of developing cardiac muscle.

Main Methods:

• Utilized the single sucrose gap voltage clamp technique on rabbit right ventricular papillary muscles.
• Assessed tension responses during controlled depolarizations across different developmental stages.
• Investigated the effects of calcium loading and analyzed the voltage dependence of phasic and tonic tension.

Main Results:

• Immature myocardium showed monotonic tension development, while mature myocardium exhibited peak (phasic) tension followed by relaxation (tonic).
• The ratio of phasic to tonic tension significantly increased with myocardial maturation.
• Phasic tension in mature myocardium showed distinct voltage dependence compared to tonic tension, which was consistent across ages.

Conclusions:

• Immature myocardium relies primarily on trans-sarcolemmal calcium influx for tension.
• Myocardial maturation involves an increasing contribution from sarcoplasmic reticulum calcium handling to tension development.
• A developmental shift in calcium sources underlies maturational changes in cardiac excitation-contraction coupling.