Page 19 - Cardiac Electrophysiology | A Modeling and Imaging Approach

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Figure 2.6B, C show V , I and channel state occupancies during the first few milliseconds
Na
m
of the action potential at slow (CL=1000ms) and fast (CL=300ms) rates. During the action potential
upstroke, rapid closed to open transitions generate a large inward current (close to 300μA/μF at
the slow rate) that lasts for a brief time because of fast transitions from the open to the fast
inactivation state. During the rest of the action potential, channels transition to the stable

inactivation states IM and IM . At fast rate, V is depolarized for a longer portion of the time, more
1
m
2
channels transition to IM and IM with less time to recover between action potentials. At the fast
2
1
rate shown (CL=300ms) 20% of channels remain in these unavailable states and the current is
reduced compared to that at slow rate (CL=1000ms) where there is practically no accumulation
of channels in IM and IM . Because of the large margin of safety with which I operates, the direct
2
1
Na
effect of reduced current on the action potential is small. However, indirectly the consequential
reduction of V affects activation of other channels (I , I , I Ca,L ) that are important determinants
Ks
Kr
m
of APD and its rate dependence.
The second depolarizing current during the action potential is I Ca,L . As described earlier, it
provides the trigger for SR Ca release which signals contraction. In terms of the action potential, it
2+
depolarizes and supports its plateau. The α-subunit of the cardiac L-type channel, Ca 1.2, includes
v
four domains each with six membrane spanning segments (similar to Na 1.5), a P-loop between S5
v
and S6 that forms the channel pore, and positive residues on S4 that forms the voltage sensor.
59
A ß-subunit is located on the intracellular face of the channel and interacts with the linker between
domains I and II. The I-II linker and ß-subunit have been linked to I Ca,L inactivation. In addition,

the ß-subunit contains a phosphorylation site for PKA, which makes the channel a substrate for
ß-adrenergic regulation . L-type calcium channels are also regulated by calmodulin (CaM) and the
60
CaMKII regulatory pathway. CaMKII interacts with the C-terminus of the α1-subunit; its activation
causes downstream facilitation of L-type channels by promoting longer and more frequent

channel openings. 61

I Ca,L inactivation is both voltage and calcium dependent. Voltage-dependent inactivation
(VDI) can be studied in isolation by replacing Ca by Ba as charge carrier 62,63
. VDI has been linked
2+
2+
to binding of the I-II linker to a region near the pore. Calcium-dependent inactivation (CDI) is a CaM
mediated process. CaM is activated by binding of Ca to four binding sites; it reduces I Ca,L by
2+
accelerating channel inactivation caused by the I-II linker mediated occlusion of the pore. 61,63

The I Ca,L model of Figure 2.7A represents explicitly activation of each of the protein domains.
Each closed state represents a permutation of the four voltage sensor positions (i.e., in C all four
o
voltage sensors of the four domains are in the resting position; in C one voltage sensor has been
1
activated and three are resting, etc.). Voltage dependent inactivation can occur once the channel

approaches the open conformation (i.e., from C or O, where at least three or all our voltage sensors
3
are activated). These two possible transitions reflect experimentally observed two components of
VDI. The lower tier in Figure 2.7A includes the CDI states which represent accelerated inactivation
facilitated by activated CaM binding to the channel. 64,65,66