Biomedical Engineering Reference
In-Depth Information
6.5.3.1
Epicardial Myocytes
The epicardial action potential exhibits a prominent early notch (spike-and-dome
morphology). The action potential notch is due to a transient outward K
+
cur-
rent [
627
] that decays in amplitude from the epicardial to endocardial surfaces (the
notch is smaller in midmyocardial myocytes and absent in endocardial myocytes).
The transient outward K
+
current is also larger in right than left ventricular
epicardial myocytes. Higher transient outward K
+
currents lead to faster rate of
repolarization, hence causes larger L-type Ca
2
+
current through Ca
V
1 channel and
greater increase in [Ca
2
+
]
i
than slower currents [
628
].
Epicardial and, to a lesser extent, midmyocardial myocytes indeed exhibit faster
sarcomere shortening kinetics than endocardial myocytes. The number of inward
K
+
rectifiers and characteristics of delayed K
+
rectifiers differ between endocardial
and epicardial cells. Because the amount of sarcoplasmic reticulum Ca
2
+
AT P a s e i s
greater in epicardial myocytes, Ca
2
+
reuptake is quicker in ventricular epicardium
than endocardium.
There is no transmural difference in rapid-activating delayed rectifier K
+
currents
and inward rectifier K
+
currents, but apicobasal differences can exist.
6.5.3.2
Midmyocardial Myocytes
Midmyocardial, or M myocytes, have been observed in the human heart [
629
]. They
localize between the deep subendocardium and midmyocardium in the anterior wall,
between the subepicardium and midmyocardium in the lateral wall, throughout the
wall of the right ventricle outflow tracts, as well as in papillary muscles, trabeculae,
and interventricular septum [
630
].
Midmyocardial myocytes that can extend to the deep subendocardium exhibit
longer action potential duration due to smaller slow components of the delayed K
+
rectifier density and/or unitary conductance [
624
] as well as a larger density in Na
+
-
Ca
2
+
exchangers [
631
] and higher late Na
+
current [
632
]. In the midmyocardium,
higher density of sodium channels, larger channel conductance, and greater proba-
bility of reopening yield stronger late Na
+
currents.
Prolonged action potential augments myocyte contraction by increased
intracellular Ca
2
+
concentration. Midmyocardial myocytes have a greater rate of
depolarization (higher conduction velocity than in other cardiomyocyte types, but
lower than in nodal cells). Midmyocardial myocytes are more sensitive to frequency
changes.
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