Biomedical Engineering Reference
In-Depth Information
Table 5.11.
Main ion channels determine the typical shape of action potential in the cardiomy-
ocyte. Fast depolarization (phase 0) is induced by an inward sodium flux. Next, a partial early
repolarization (notch; phase 1) is caused by outward potassium flux through rapidly activating
and inactivating K
+
channels. The plateau phase (phase 2) results from a balance between
inward depolarizing (mainly Ca
2
+
influx) and outward repolarizing (mainly K
+
efflux) ion fluxes.
Potassium ion motions strongly contribute to phases 3 (rapid final repolarization) and 4 (resting
potential). In addition to transient outward current (
i
K
,
to
) through rapidly inactivating K
V
4.2 and
K
V
4.3 (
i
K
,
to
(
r
)
: fast component) and K
V
1.4 (
i
K
,
to
(
s
)
: slow component), a rapidly activating, slowly
inactivating delayed rectifier current (
i
K
,
slow1
) and another component (
i
K
,
slow2
) through K
V
2.1
with slower ina
ctivation kinetics than K
V
1.5 explain repolarization in ventricular m
yocytes [
467
].
Phase
Event
Involved ion channels (currents)
0
Fast depolarization
Na
V
1.5
1
Early repolarization
K
V
1.4/4.2/4.3 (
i
K
,
to
)
2
Plateau
K
V
1.5/2.1 (
i
K
,
ur
and
i
K
,
slow2
);
Ca
V
1.2
3
Final repolarization
K
V
11.1/7.1 (
i
K
,
r
and
i
K
,
s
)
4
Rest
K
IR
2.1/3.1 (
i
K1
)
α
q
/
11
subclass are
involved in the development of pressure overload-induced maladaptive cardiac
hypertrophy. This effect is counteracted by RGS4 GTPase-activating protein.
Activated G
α
heart behavior via G
s
-mediated signaling. Members of the G
α
β
activation, and subsequent
processing of phosphatidylinositol (4,5)-bisphosphate into diacylglycerol, an ac-
tivator of PKC, and inositol (1,4,5)-trisphosphate, a ligand of IP
3
receptors.
In cardiomyocytes, the plasmalemmal PIP
2
modulates activity of ion carriers, such
as K
IR
6.2 channel (
i
K
(
AT P
)
q
proteins trigger phospholipase-C
current) and Na
+
-Ca
2
+
-K
+
exchanger.
α
q
/
11
subclass protein concentrations influences dynamics of
Ca
2
+
transients (appearance, amplitude, and recovery), hence Ca
2
+
handling, as
well as electrophysiological properties of cardiomyocytes [
465
]. In the absence of
G
Modulation of G
α
q
and in the presence of G
α
11
, recovery is accelerated; in the absence of both
G
α
q
and G
α
11
, recovery is decelerated.
5.10.4
Myocardial Potassium Channels
Approximately 60 K
+
-selective channel subunits exist in the myocardium that carry
outward currents in the physiological range of potentials to repolarize excitable
cardiac cells and set the resting potential that governs excitability [
466
](Table
5.11
).
Depolarization-activated K
+
currents determine the amplitude and duration of
action potentials in cardiomyocytes.
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