The cell membrane is permeable to a number of ions, the most important of which are Na+, K+, Ca++ and Cl-. These ions pass across the membrane through specific ion channels that can open (become activated) and close (become inactivated). Therefore, these channels are said to be gated channels. Their opening and closing can occur in response to
- voltage changes (voltage gated channels)
- activation of receptors (receptor gated channels)
- specific ions and chemical ligands
Cardiac cells can have several different channels for a given ion. For example, there are many different types of potassium channels that play an important role in resting membrane potential and in action potentials. It is the opening and closing of ion channels that alters specific ion conductances in a manner that determines resting potentials and generates action potentials. For example, when an action potential is elicited in a cardiomyocyte, sodium channels transiently open and potassium channels close, which leads to depolarization. Shortly thereafter (within a few milliseconds), the sodium channels close and calcium channels open to maintain a depolarized state. This is followed by inactivation of the calcium channels and a reopening of the potassium channels, which leads to membrane repolarization.
The following table summarizes some of the important ion channels that are found in cardiac and vascular smooth muscle cells.
|Fast Na+||Phase 0 depolarization of non-pacemaker cardiac action potentials|
|Slow Na+||"Funny" pacemaker current (If) in cardiac nodal tissue|
|Inward rectifier (Iir or IK1)||Maintains phase 4 negative potential in cardiac cells and contributes to late repolarization|
|Transient outward (Ito)||Contributes to phase 1 of non-pacemaker cardiac action potentials|
|Delayed rectifier (IKr)||Phase 3 repolarization of cardiac action potentials|
|ATP-sensitive (IK, ATP)||KATP channels; inhibited by ATP; therefore, open when ATP decreases during hypoxia; in vascular smooth muscle, adenosine removes the ATP inhibition and opens these channels, producing hyperpolariation and vasodilation.|
|Acetylcholine-activated (IK, ACh)||Activated by acetylcholine; Gi-protein coupled|
|Calcium-activated (IK, Ca or BKCa)||Open in response to Ca++ influx in vascular smooth muscle|
|L-type (ICa-L)||Slow inward, long-lasting current; phase 2 non-pacemaker cardiac action potentials and late phase 4 and phase 0 of SA and AV nodal cells; important in vascular smooth muscle contraction|
|T-type (ICa-T)||Transient current that contributes to early phase 4 pacemaker currents in SA and AV nodal cells|