Biomedical Engineering Reference
In-Depth Information
This study raises questions concerning the level of contribution of HCN channel
inhibition in the bradycardic actions of clonidine observed in mouse. However, the
IC
50
for HCN2 and HCN4 seems very high to explain a major contribution of
I
f
inhibition in the therapeutic effects of clonidine in humans.
6.2 Zatebradine (UL-FS 49) and Cilobradine (DK-AH 269)
Zatebradine (UL-FS 49) and Cilobradine (DK-AH 269) (see chemical structures in
Fig.
3
) are bradycardic agents derived from structural modification of the calcium-
antagonist verapamil. Both zatebradine and cilobradine cause a use-dependent
block of
I
f
in cardiac Purkinje fibers, isolated SAN cells and the
I
h
current in
nerve cells. More specifically, these compounds induce a concentration- and volt-
age-dependent inhibition of
I
f
, which slows diastolic depolarisation and decreases
the spontaneous firing rate [
58
-
63
].
Cilobradine and zatebradine have a similar IC
50
for the different HCN subtypes.
However, use-dependent block kinetics depend on the isoform of the channel under
consideration. Indeed, a study using the heterologous expression of HCN in
HEK293 cells revealed that 5
M cilobradine induces a tenfold faster use-depen-
dent block for HCN3 and HCN4 than for HCN2 and HCN1 [
63
]. However, it is
important to delve further into this apparent HCN isoform selectivity since the
kinetics of block depend on protocol parameters such as the frequency and duration
of hyperpolarizing pulses.
Interestingly,
I
f
recorded in murine SAN cells presents a similar use-dependent
block compared to HCN4 [
63
]. Although cilobradine and zatebradine have similar
use-dependent blocking properties, these drugs present a marked difference in their
potency. Indeed, cilobradine blocks
I
f
and heterologously expressed HCN channels
more effectively and faster than zatebradine [
62
,
63
]. This is essentially a conse-
quence of a slower dissociation rate associated presumably with a higher associa-
tion rate.
To date, the binding site of zatebradine and cilobradine has not been fully
elucidated. An investigation of the blocking mechanism of
I
f
by zatebradine in
rabbit SAN cells indicates that this molecule blocks the channel by entering the
open channel pore from the intracellular side for a distance of 39% of the membrane
thickness [
64
]. Furthermore, a recent study using an alanine scanning mutagenesis
approach revealed that mutations A425G or I432A in the S6 segment of HCN2
attenuated the block by cilobradine [
65
]. This block was even less effective in the
double mutant I432A/A425G. These results indicate that cilobradine probably
interacts with these specific residues of the S6 segment.
Besides its inhibitory action on
I
f
and
I
h
, zatebradine has been shown to block
potassium currents [
58
,
66
]. As a consequence, blocking the repolarizing current
I
k
in the myocardium would prolong the action potential in this tissue [
67
-
69
].
Furthermore, patients treated with zatebradine developed symptoms of visual
disorders, which could be explained by the inhibitory effect of zatebradine on
m
