Final answer:
Diltiazem, a calcium channel blocker, slows heart rate and increases the action potential duration in the heart by inhibiting calcium ion influx, leading to a longer plateau phase and an extended refractory period. These effects reduce myocardial excitability and the risk of abnormal heart rhythms.
Step-by-step explanation:
The drug diltiazem is a type of calcium channel blocker that impacts the heart by affecting the calcium ion influx, which has significant roles in heart rate (HR) and myocardial contractility. Specifically, diltiazem binds to calcium channels in the heart and inhibits or slows the inward movement of calcium ions, resulting in a decreased heart rate. This effect also influences the cardiac action potential duration by elongating the plateau phase, a feature that is critical to ensuring that cardiac muscle cells fully contract before another electrical event occurs.
Calcium ions are essential for cardiac muscle physiology due to their role during the plateau phase of the action potential and their contribution to the regulatory protein troponin in the troponin-tropomyosin complex, necessary for myocardial contraction. Diltiazem effectively reduces the excitability of the heart muscle and the likelihood of abnormal electrical activity such as cardiac dysrhythmia. Furthermore, by extending the refractory period of the cardiac action potential, diltiazem helps in preventing the heart from experiencing excessively rapid beats that could lead to cardiac dysfunction or arrest.