Role of sarcomere mechanics and Ca²⁺ overload in Ca²⁺ waves and arrhythmias in rat cardiac muscle

Ca²⁺ release from the sarcoplasmic reticulum (SR) depends on the sarcoplasmic reticulum (SR) Ca²⁺ load and the cytosolic Ca ²⁺ level. Arrhythmogenic Ca²⁺ waves underlying triggered propagated contractions arise from Ca²⁺ overloaded regions near damaged areas in the cardiac muscle. Ca²⁺ waves can also be induced in undamaged muscle, in regions with nonuniform excitation-contraction (EC) coupling by the cycle of stretch and release in the border zone between the damaged and intact regions. We hypothesize that rapid shortening of sarcomeres in the border zone during relaxation causes Ca²⁺ release from troponin C (TnC) on thin filaments and initiates Ca²⁺ waves. Elimination of this shortening will inhibit the initiation of Ca²⁺ waves, while SR Ca²⁺ overload will enhance the waves. Force, sarcomere length (SL), and [Ca²⁺]_i were measured and muscle length was controlled. A small jet of Hepes solution with an extracellular [Ca²⁺] 10 mM (HC), or HC containing BDM, was used to weaken a 300 μm long muscle segment. Trains of electrical stimuli were used to induce Ca²⁺ waves. The effects of small exponential stretches on triggered propagatory contraction (TPC) amplitude and propagation velocity of Ca²⁺ waves (V_prop) were studied. Sarcomere shortening was uniform prior to activation. HC induced spontaneous diastolic sarcomere contractions in the jet region and attenuated twitch sarcomere shortening; HC+ butanedione monoxime (BDM) caused stretch only in the jet region. Stimulus trains induced Ca²⁺ waves, which started inside the HC jet region during twitch relaxation. Ca²⁺ waves started in the border zone of the BDM jet. The initial local [Ca²⁺]_i rise of the waves by HC was twice that by BDM. The waves propagated at a V_prop of 2.0 ± 0.2 mm/sec. Arrhythmias occurred frequently in trabeculae following exposure to the HC jet. Stretch early during relaxation, which reduced sarcomere shortening in the weakened regions, substantially decreased force of the TPC (F_TPC) and delayed Ca²⁺ waves, and reduced V _prop commensurate with the reduction F_TPC. These results are consistent with the hypothesis that Ca²⁺ release from the myofilaments initiates arrhythmogenic propagating Ca²⁺ release. Prevention of sarcomere shortening, by itself, did not inhibit Ca²⁺ wave generation. SR Ca²⁺ overload potentiated initiation and propagation of Ca²⁺ waves.