• Excitation-contraction coupling 
    • coupling of action potential to contraction of cardiac myocyte:
      • cardiac action potential depolarizes cardiac myocyte cell membrane
      • L-type voltage-gated Ca2+ channels in cardiac myocyte cell membrane open
        • aka dihydropyridine receptors
        • Ca2+ conductance increases in phase 2 (plateau) of cardiac action potential
      • inward Ca2+ current increases intracellular Ca2+
        • triggers calcium-induced calcium release
          • in response to increase in intracellular Ca2+, Ca2+ release channels in sarcoplasmic reticulum membrane open
            • aka ryanodine receptors
            • further increase intracellular Ca2+ levels
          • calcium-induced calcium release amplifies intracellular Ca2+ signal
      • intracellular Ca2+ binds troponin
        • induces a conformational change that removes tropomyosin inhibition of actin-myosin interaction
      • actin and myosin bind and cross-bridge cycling produces cardiac muscle tension
  • Relaxation
    • cardiac muscle relaxation and cardiac myocyte cell membrane repolarization occur when intracellular Ca2+ levels return to resting levels  
    • mechanisms of intracellular Ca2+ clearance
      • Na+/Ca2+ exchanger
        • Na+/Ca2+ exchanger (1 Ca2+ out, 3 Na+ in) pumps Ca2+ out of cardiac myocyte
          • Na+/K+ ATPase (3 Na+ out, 2 K+ in) in cardiac myocyte cell membrane establishes Na+ gradient that drives Na+/Ca22+ exchanger
      • Ca2+ ATPase
        • pump in cardiac myocyte cell membrane
        • Ca2+ ATPase pumps Ca2+ out of cardiac myocyte
      • Ca2+ ATPase
        • pump in sarcoplasmic reticulum membrane
        • Ca2+ ATPase pumps Ca2+ into sarcoplasmic reticulum

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