From the Guidelines
Calcium plays a crucial role in the action potential of cardiac muscle, and its influx through L-type calcium channels is essential for the plateau phase and contraction of cardiac muscle. The action potential of cardiac muscle is unique compared to skeletal muscle, with a plateau phase (Phase 2) that prolongs contraction, which is essential for effective pumping of blood. During depolarization, voltage-gated calcium channels (L-type) open, allowing calcium ions to enter the cell 1. This calcium influx triggers calcium-induced calcium release from the sarcoplasmic reticulum through ryanodine receptors, significantly increasing intracellular calcium concentration. This calcium binds to troponin C, enabling actin-myosin interaction and muscle contraction. The prolonged plateau phase in cardiac muscle is maintained by this calcium influx balancing potassium efflux, keeping the membrane depolarized longer than in skeletal muscle.
Some key points to consider about calcium and cardiac muscle action potential include:
- The role of L-type calcium channels in allowing calcium ions to enter the cell and trigger contraction
- The importance of calcium-induced calcium release from the sarcoplasmic reticulum in increasing intracellular calcium concentration
- The binding of calcium to troponin C, enabling actin-myosin interaction and muscle contraction
- The balance between calcium influx and potassium efflux in maintaining the plateau phase of the action potential
Calcium channel blockers like verapamil and diltiazem work by inhibiting these L-type calcium channels, reducing contractility and heart rate, which is why they're used to treat conditions like hypertension and arrhythmias 1. The unique calcium-dependent action potential of cardiac muscle allows for coordinated contractions and prevents tetanic contractions that would be fatal for heart function. Overall, the management of calcium in cardiac muscle is critical for maintaining proper heart function and preventing morbidity and mortality.
From the Research
Action Potential of Cardiac Muscle and Calcium
- The action potential of cardiac muscle is influenced by calcium, which plays a crucial role in the contraction of cardiac muscle cells 2
- Calcium-channel blocking agents, such as verapamil, nifedipine, and diltiazem, affect the movement of calcium through channels in cardiac muscle, causing changes in the slow inward current of the cardiac action potential, myocardial contractility, and metabolism 3
- The L-type, dihydropyridine-sensitive calcium channel and the ryanodine-sensitive calcium-release channel are two distinct species of calcium-selective membrane ion channels involved in cardiac muscle excitation-contraction coupling 2
Role of Calcium in Cardiac Muscle Excitation-Contraction Coupling
- The contractile state of cardiac muscle cells is determined by the level of calcium in the cytosol, with each action potential producing a transient elevation of cytosolic calcium 2
- The interaction of calcium ions with the contractile elements of the cell initiates a series of events that culminate in cell shortening 2
- Calcium antagonists, such as verapamil, nifedipine, and diltiazem, have a basically different mode of action from beta-adrenoceptor blockade, although both ultimately act on the free cytoplasmic calcium ion concentration 4
Clinical Use of Calcium-Channel Blocking Agents
- Calcium-channel blocking agents are used to treat various cardiovascular diseases, including angina, hypertension, and supraventricular tachycardias 3, 4
- Verapamil is useful intravenously for treating supraventricular rhythm disturbances, while nifedipine has proven useful for hypertension, coronary-artery spasm, and exertional angina 3
- Diltiazem has been shown effective in the treatment of coronary-artery spasm, and limited studies indicate it may be useful in treating exertional angina, hypertension, and possibly arrhythmias 3