Final answer:
Myocardial contractility is most closely associated with the concentration of calcium ions (Ca2+), as they play a pivotal role in both maintaining the cardiac action potential's plateau phase and triggering muscle contraction through the troponin-tropomyosin complex.
Step-by-step explanation:
The myocardial contractility is best correlated with the intracellular concentration of calcium ions (Ca2+). Calcium ions play a crucial role in the physiology of cardiac muscle, not only in maintaining the plateau phase of the cardiac action potential but also in initiating muscle contraction. This process starts with calcium entering through slow calcium channels which then combines with the regulatory protein troponin to allow the myosin heads to form cross bridges with actin, providing the power stroke necessary for contraction. The amount of calcium required for muscle contraction is derived partially from influx during the action potential and partially from storage in the sarcoplasmic reticulum.
Additionally, in cardiac autorhythmicity, calcium ions are critically important during spontaneous depolarization of conductive cells, e.g., the SA node. When sodium ions have raised the membrane potential, calcium channels open to allow a more rapid depolarization, leading to the generation of an action potential. After the peak is achieved, calcium channels close, and potassium channels open, resulting in repolarization.