From the Guidelines
The action potential in cardiac muscle involves calcium ions playing a crucial role in excitation-contraction coupling, with calcium entering the cell through L-type calcium channels during the plateau phase (Phase 2) 1.
Action Potential Phases
The cardiac action potential has five phases:
- Phase 0 (rapid depolarization via sodium channels)
- Phase 1 (early repolarization)
- Phase 2 (plateau phase with calcium influx)
- Phase 3 (repolarization as potassium exits)
- Phase 4 (resting membrane potential) This prolonged action potential prevents tetanic contractions, allowing the heart to fill between beats.
Role of Calcium
Calcium ions (Ca2+) are major intracellular mediators of vascular smooth muscle cell contraction, as well as of inotropic and chronotropic functions of the heart 1.
- Calcium enters vascular smooth muscle cells, cardiomyocytes, and pacemaker cells via voltage-dependent L- and T-type calcium channels.
- In vascular smooth muscle, the voltage-gated L-type channel allows entry of sufficient Ca2+ for initiation of contraction by calcium-induced intracellular Ca2+ release from the sarcoplasmic reticulum.
Calcium Channel Blockers
Calcium channel blockers like verapamil and diltiazem work by inhibiting these L-type calcium channels, reducing contractility and heart rate, which explains their use in treating hypertension and certain arrhythmias 1.
- The dihydropyridine calcium channel blockers (CCBs) bind to a common site on the 1-subunit of the L-type channel and are highly selective for arterial/arteriolar tissues.
- The nondihydropyridine CCBs, including the phenylalkylamines (verapamil-like) and benzothiazepines (diltiazem-like), bind to different sites on the 1-subunit and are less selective for vascular smooth muscle.
Diagram
A diagram of the cardiac action potential would show the five phases, with calcium influx during the plateau phase (Phase 2) and the role of L-type calcium channels in this process. However, due to the limitations of this format, a visual diagram cannot be provided. The description of the phases and the role of calcium should provide a clear understanding of the action potential in cardiac muscle.
Species Differences
It's worth noting that species differences in the distribution and kinetics of ion channels are significant, and drugs that lengthen human ventricular action potentials may have different effects in other species 1. This is important to consider when studying cardiac electromechanical activity and developing treatments for cardiac conditions.
From the FDA Drug Label
The FDA drug label does not answer the question.
From the Research
Action Potential of Cardiac Muscle and Calcium
- The contractile state of cardiac muscle cells is determined by the level of calcium in the cytosol 2
- Each action potential produces a transient elevation of cytosolic calcium, which initiates a series of events that culminate in cell shortening 2
- The process of cell excitation being coupled to contraction involves the function of two distinct species of calcium-selective membrane ion channels: the L-type and the ryanodine-sensitive calcium-release channel 2
Role of Calcium in Cardiac Muscle
- Calcium ions play a crucial role in the regulation of cardiac muscle function, including the initiation of contraction and the modulation of action potential configuration 3
- Stretch has been shown to alter action potential configuration, generate stretch-activated arrhythmias, and increase the rate of beating of the sino-atrial node, with Ca2+ playing a key role in these effects 3
- Ca2+ entry through L-type Ca2+ channels or stretch-activated channels (SACs) influences the secretion of natriuretic peptides and the onset of hypertrophy 3
Calcium Channel Blockers
- Calcium-channel blockers have potent antiarrhythmic and antianginal effects and may reduce the extent of cellular injury after anoxia/ischemia 4
- Verapamil, nifedipine, and diltiazem are examples of calcium-channel blockers that can reduce the frequency of angina and have different effects on cardiac muscle 4, 5, 6
- The clinical differences between the effects of various calcium antagonists reflect their different properties, such as vasodilator capacity and inhibition of vagal tone 6
Diagram of Action Potential
Unfortunately, a diagram of the action potential of cardiac muscle cannot be provided in this text-based format. However, the action potential of cardiac muscle can be described as a complex process involving the influx and efflux of ions, including calcium, through various ion channels. The L-type calcium channel and the ryanodine-sensitive calcium-release channel play critical roles in this process 2.