What effect do calcium channel blockers have on the heart?

Calcium channel blockers are primarily used in the treatment of hypertension and various cardiovascular conditions due to their ability to affect calcium ion movement across cardiac and smooth muscle cell membranes. The correct answer highlights that these agents decrease heart rate and myocardial oxygen demand.

By blocking calcium channels, these medications reduce the influx of calcium ions during depolarization of heart cells. This leads to a decrease in the contractility of the heart (negative inotropic effect), which in turn reduces the overall workload on the heart and lowers myocardial oxygen consumption. Additionally, many calcium channel blockers have a direct effect on the sinoatrial node, leading to a decrease in heart rate (negative chronotropic effect). These combined effects help to alleviate symptoms in patients with conditions like angina and hypertension by allowing the heart to operate more efficiently under reduced stress.

The other options suggest effects contrary to the well-established pharmacological action of calcium channel blockers, such as increasing contractility, promoting arrhythmia, or causing coronary artery constriction, all of which do not align with their therapeutic use and mechanisms of action.