Biomedical Engineering Reference
In-Depth Information
of extracellular Ca
2
+
into the cardiomyocyte for optimal excitation-contraction
coupling. Delayed repolarization also yields cardiomyocyte refractory to premature
excitation.
Other ionic fluxes arise from Na
+
-Ca
2
+
exchanger, with Na
+
and Ca
2
+
transport
reversal during repolarization, and from Na
+
-K
+
pumps. The transition between
phases 2 and 3 is characterized by an increase in repolarization rate due to the
beginning in activity of inward K
+
rectifiers, expelling K
+
ions.
During phase
3
, delayed rectifier K
+
channels induce rapid repolarization by
K
+
current that counters depolarization induced by Na
+
and Ca
2
+
inflow. These
membrane channels remains closed in the presence of ATP, but as soon as ATP
levels fall, they open for K
+
efflux that leads cardiomyocytes to a hyperpolarized
inactive state.
Throughout phase
4
, the resting membrane potential is regulated by a
background K
+
current. A Na
+
efflux can balance the K
+
influx generated by
the inward K
+
rectifiers to ensure a resting cell potential. This process can be
modulated by voltage-sensitive Cl
−
channels activated by catecholamines via
cAMP messenger. The action potential is shortened by Cl
−
influx and lengthened
by adrenergic effect on Ca
2
+
channels.
6.5.3
Electrophysiological Myocyte Types
Transmural differences in electrochemical properties of the ventricular myocytes
are attributed to changes in both quantity and quality of sarcolemmal (K
+
transient
outward, rapid and slow delayed K
+
rectifiers [
624
], inward K
+
rectifier, Na
+
-
Ca
2
+
exchanger with its reverse and forward modes, and late Na
+
channel) as well
as sarcoplasmic reticulum carriers such as sarcoplasmic reticulum Ca
2
+
AT P a s e .
63
Such variations lead to electromechanical coupling differences, which can also be
due to variations in intracellular Ca
2
+
buffering, as well as in contractile protein
isoform expression.
64
These transmural differences may be required for coordinated contraction and
relaxation of the ventriculomyocyte population across the ventricular wall stimu-
lated at different instants by the propagating electrochemical wave with a delay
on the order of few tens of milliseconds. The ventricular myocardium contains at
least 3 electrophysiologically distinct myocyte types: epicardial, midmyocardial,
and endocardial myocytes.
63
The expression of sarcoplasmic reticulum Ca
2
+
ATPase is greater in epicardial than midmyocar-
dial and endocardial myocytes [
625
]. Consequently, epicardial myocytes exhibit a quicker decay
of intracellular Ca
2
+
content than endocardial myocytes. Transmural expression of Na
+
-Ca
2
+
exchanger does not significantly vary [
625
]. Regional differences in ryanodine channel expression
and/or its binding protein might exist.
64
Myosin isoforms do not have similar ATPase and contractile activities [
626
].
Search WWH ::
Custom Search