Calcium entry blockers, or calcium channel blockers - CCBs for short - are vasodilators, or medications that promote dilation of blood vessels. These medications are mainly used to treat hypertension, or high blood pressure, and angina pectoris, which is a pain caused by reduced blood flow to the heart muscle. Now, by definition, blood pressure is the force that blood exerts on the walls of blood vessels and it’s basically what keeps blood flowing and perfusing tissues to deliver oxygen and nutrients. Hypertension happens when this pressure is higher than it should be. In most cases, the cause is unknown.
But basically, we can do a number of things to help lower the blood pressure. First, we can decrease the heart rate or the myocardial contractility, so the heart pumps less blood into the blood vessels. In other words, diminish the amount of blood that exerts force upon the same area. Or, we can vasodilate the peripheral blood vessels, which increases the area for the same amount of liquid that exerts force. Angina, on the other hand, is a type of chest pain caused by insufficient oxygen supply to the myocardium to meet its demand. Generally, the underlying cause is the presence of atheromatous plaques in the coronary arteries which decreases the blood flow to the heart. So, to help diminish the symptoms, it’s important to decrease the oxygen demand of the heart, again by decreasing heart rate or myocardial contractility; and increasing the oxygen supply by vasodilating the coronary arteries.
Now, let’s look at how calcium channels affect heart function. First off, the heart rate depends on the rate that the pacemaker cells in the sinus and atrioventricular node generate action potentials. These action potentials start automatically when sodium channels slowly let in a stream of sodium ions, which causes the membrane potential of the pacemaker cells to become more positive. When this reaches the threshold membrane potential, it’s the cue for voltage-gated calcium channels to open, allowing a large influx of calcium ions, which depolarizes it further. Then, these calcium channels close and potassium channels open to let potassium out of the cell, so the membrane potential goes back down, or repolarizes. Now, each cycle of depolarization and repolarization represents a single heartbeat, so how fast this process repeats in one minute determines the heart rate.
Okay, so now let’s look at the cardiac muscle and vascular smooth muscle contraction, which also depends on calcium. Voltage-gated calcium channels in the membrane of the muscle cell open when they receive an action potential and this allows calcium ions to flow into the cell from the extracellular space. The extracellular calcium causes the release of intracellular calcium ions stored in the sarcoplasmic reticulum. All these calcium ions then bind to troponin regulatory proteins, which change shape and release the thin filaments in the muscle fiber. This allows the thin filament to bind to the thick filament, eventually leading to muscle contraction. In the heart, this means greater myocardial contractility. In blood vessels, this means vasoconstriction.
Okay, so calcium channel blockers, as the name suggests, block voltage-gated calcium channels. We can divide calcium channel blockers into dihydropyridines and non-dihydropyridines. Dihydropyridines include medications that end with the suffix “-dipine,” like nifedipine, nicardipine, amlodipine, and nimodipine. These medications act mainly on the smooth muscles of the blood vessels. Non-dihydropyridines include verapamil and diltiazem, which both have a greater effect on the heart compared to dihydropyridines, but are less effective for vasodilation.
So, let’s start with the dihydropyridines, which are mainly used to treat hypertension. They preferentially exert their effects on arterial smooth muscle, and nifedipine is the prototype of this class. Besides treating hypertension, dihydropyridines are also commonly used to treat other disorders. Since they can also dilate coronary arteries, they are effective for preventing angina. They can treat Raynaud Phenomenon, a disease caused by vasoconstriction of the arteries in the tips of fingers, causing them to turn white, then blue, and finally red. Lastly, they are used to prevent cerebral vasospasms after a subarachnoid hemorrhage.