Repolarising currents.

Question 1

Ikur channels flows through and is encoded by

Answer 1

Ikur: Kv1.5 (KCNA5)

Question 2

where is Ikur present and absent?

Answer 2

the mRNA and the expressed protein is found in both atria and ventricles in many species HOWEVER the current is generally ABSENT in ventricular myocytes suggesting that it also depends on the presence of modulating subunits or scaffolding or trafficking proteins.

Question 3

Kv1.5 structure

Answer 3

alpha subunit consists of typical voltage-sensitive K channel template:
tetramer each with 6 transmembrane segments
P loop betweeen S5 and S6

beta subunits bind to control channel trafficking and appear to influence the activation and inactivation by moving V1/2 to more negative potentials

Question 4

describe the Ikur activation

Answer 4

rapidly activates in the range of 0 to +60mV within 10ms of voltage change, and inactivates over 250ms

outwardly rectifying.

Question 5

pharmacological things re Ikur

Answer 5

Kv1.5 channels appear promising dfrug targets for AF.

Inhibition of Ikur would prolong the atrial APD tehereby increasing the effective refractory period and reducing chances of re-excitation.
It would also lead to a more positive early plateau potential in a range where more ICa,L might be activated. this would increase Ca influx and thus enhance atrial contraction.

Question 6

Ikr channel structure, genes coding it.

Answer 6

Ikr flows through Kv11.1 (hERG), coded by KCNH2

Question 7

generally, problems with Ikr channel

Answer 7

extremely susceptible to drug binding

there are over 450 known mutations of KCNH2, all causing loss of function

Question 8

hERG and LQT syndromes

Answer 8

over 450 mutations of the coding KCNH2 gene.
all cause to a loss of function and abnormal repolarisation of cardiomyocytes.

Epicardial myocytes express different amounts (less than endo) of Kv11.1 than endocardial myocytesm thus there arise a large range of different APDs, meaning that they express heterogeneous repolarisation times and refractory periods. this situation is termed INCREASED DISPERSION OF APD, and is a substrate for reentrant arrhythmias.

Question 9

why is Kv11.1 so susceptible to blockade from a varity of drugs?

Answer 9

compound tend to block the channel in its open state suggesting that the channel gates provide a barrier to drugs preventing their access to the pore and there is likely to be a larger than normal vestibule that accommodates a large range of different agents which can block the pore.

there are several possible structural reasons for this:
1) the channel lacks proline residues in the S6 transmembrane domain. the proline residues in this region is thought to induce a kink in the S6 domain, and this kink allows the lower kalf of the s6 to move and angles it towards the s4-s5 linker.
without this kink the s6 domain is more flexible and may form a larger inner vestibule

2) the presence of two aromatic residues (Y and F) in the same domain (S6) seems important for drug binding.

the drugs appear to block by entering from the cytoplasmic side. larger compunds prevent the channel from closing completely, at which point they can also dissociate from the channel.

all new drugs that are being developed are specifically screened for their ability to bind Kv11.1

Question 10

when is Ikr activated and inactivated? mechanisms?

Answer 10

activated when Em > -40mV (during plateau). V1/2= -30mV

at Em > 0mV it inactivates rapidly, reducing the outward current: behaves like an inward rectifier shutting off at more positive voltages.

this is because the channel inactivation develops faster than activation at positive potentials, so limits the amount of tiem these channels spend in an open state.

as the AP moves into phase3, the Em becomes < 0mV and the channels recover from inactivation opening again.
this leads to further outward current that helps with early phase 3 repolarisation. then the channels inactivate again.

the activity results in the I-V relationship being bell-shaped, much liek the L-type Ca channel behaviour.

the unusually rapid inactivation is due to a voltage dependent C-type inactivation.

Question 11

what is c-type inactivation?

Answer 11

it involves a subtle repositioning around the selectivity filter so that geometry of it changes and decreases the ability to conduct ions. this residue repositioning is relatively slow (compared with N-type inactivation)

Question 12

what is N-type inactivation?

Answer 12

it is known as the ball-and-chain model:
the particle (ball) that is attached to the cytoplasmic face of the channel by a protease-cleavable domain (chain) binds to the site in the conducting pathway and thus blocks the channel pore.

seen in K channels (shaker)

similar is in Nav1.5 but it involves a hinged lid rather than a ball and chain. the hinged lid is made from a loop of peptide linking the s3 and s4 domains.

Question 13

which delayed K channels have a different distribution across the myocardial wall?

Answer 13

Ikr: less in epicardium, more in endocardium

Iks: less in mid-myocardium and less in the apex, more in at the base –> longest APD in the middle of apical myocardium

Question 14

what is the priamry repolarising current?

Answer 14

Ikr

Question 15

wahat is the importance of IKr in increased heart rates?

Answer 15

Peak Ikr is similar in slow nad fast heart rates, so it’s not hte major adaptive component for APD.

Question 16

IKs channel sttructureand gene

Answer 16

Kv7.1, KCNQ1 (for alpha subunit)

KCNE1 is for ebta subunit

Question 17

Iks activation and inactivation patterns

Answer 17

very slow activation on depolarisation., time constant for activation is around 300ms, so it is IKs is never fully activated at normal hearts.

channel inactivation is also slow, has two components with time constants up to 700 ms. –> IKs tends to build at higher rates because there is less time for it to deactivate completely before the next AP, so each depolarisation results in more channels in the active state.
In addition, at faster heart rates, Iks activation is accelerated. this probably due to the channels being in a transition state towards the open state, allowing very rapid opening.

Question 18

the role of Iks

Answer 18

plays a role in determining the later plateau amplitude, and the start of phase 3. can affect AP shape and duration. but it remains controversial.

its activation/inactivation porperties mean that it is more important in APD adaptation to changing heart rates.

Question 19

the role of Ikr

Answer 19

major repoalrising current that brings about the phase 3

Question 20

where is the longest APD in the myocardium and why?

Answer 20

longest APD in the middle of apical myocardium

because there is the least amount of Kv7.1 (IKs)