A) The action potential lasts hundreds of milliseconds, as long as the cardiac cycle
B) The SR empties its Ca²⁺ with every heart beat
C) The SA node fires too frequently for the cardiomyocytes to follow
D) The absolute refractory period is longer than the action potential
E) The bundle of His has no reserve capacity

A) Stretch of the cardiac muscle with Increasing frequency of stimulation
B) Increased in the sensit ivity of the myofilaments to Ca₂+
C) Phosphorylation of TnC
D) Increased [Ca₂+] in the mitochondria
E) Increased [Ca₂+] transients due to loading of the SR

A) There is no cardiac action potential without extracellular Ca²⁺
B) There is no calcium-induced-calcium release without extracellular Ca²⁺
C) Ca²⁺ cannot enter over the NCX to activate the myofilaments
D) Ca²⁺ is pumped out of the cell by the PMCA
E) TnC is insensitive to Ca²⁺ when the heart is relaxed