Ion transporters and channels in the heart

Question 1

What are ion channels?

Answer 1

passage for ions to move across membranes

down electrochemical gradient

Question 2

what are transporters?

Answer 2

pumps powered by ATP

move ions against concentration gradient

Question 3

what are the most important ions in the heart?

Answer 3

sodium
potassium
calcium

Question 4

location of sodiim

Answer 4

mainly found extracellularly

Question 5

location of potassium

Answer 5

mainly found intracellularly

Question 6

location of calcium

Answer 6

found in lower concentrations intracellularly and extracellularly
less in cytosol
mostly in ER

Question 7

location of calcium

Answer 7

found in lower concentrations intracellularly and extracellularly
less in cytosol
mostly in ER

Question 8

how is a cardiac myocyte action potential different to a neuron action potential?

Answer 8

prolonged depolarisation due to calcium

Question 9

how is a cardiac myocyte action potential generated

Answer 9

calcium ion inflow through T-type channels contributes to depolarisation
plateau phase occurs because L-type calcium channels let calcium into cell and counteract loss of potassium ions from the cell
L-type calcium ion channels close and repolarisation occurs
potassium ions flow out through many different types of channels

Question 10

why are L-type calcium ion channels important for cardiac function?

Answer 10

lengthen depolarisation so lengthens refractory period prevents the cell from contracting again so contractions are coordinated to ensure efficiency

Question 11

describe the process of cardiac muscle contraction

Answer 11

1. action potential arrives
2. depolarisation of membrane in T tubule
3. calcium enters the cell via L-type calcium channels
4. calcium that entered the cell activates/ stimulates opening of ryanodine receptors so calcium can leave the sarcoplasmic reticulum and move into cytosol in larger quantities
5. calcium binds to troponin C and activates contraction
6. after contraction excess calcium is transported back into the sarcoplasmic reticulum by the SERCA pump - ATPase and requires ATP
7. maintains low calcium levels in cytosol so contraction only occurs when required
8. transporters in cell membrane transport calcium ions out of the cell
9. sodium/ potassium pump re-establishes electrochemical gradient

Question 12

what transports are found in the cell membrane of cardiac myocytes to transport calcium?

Answer 12

sodium/ calcium exchanges

1 calcium out and 3 sodium in

Question 13

sodium/ potassium pump

Answer 13

2 potassiums in and 3 sodiums out

Question 14

where is there a pacemaker cell action potential?

Answer 14

SAN
AVN
Bundle of His

Question 15

shape of pacemaker cell action potential?

Answer 15

same as a neuronal action potential

Question 16

how is a pacemaker cell action potential generated?

Answer 16

leaky potassium ion channels maintain resting potential
slow movement of sodium ions into cell via funny ion channels which open on repolarisation/ hyperpolarisation - when membrane potential drops to -60mV
funny sodium ion channels cause sodium to depolarise cell until reaching threshold then calcium ions take over
opening of T-type calcium channels allows rapid influx of calcium causing depolarisation
triggers action potential
T-type calcium channels close rapidly on depolarisation
L-type calcium channels open and then close rapidly
potassium outflow allows repolarisation

Question 17

what is special about pacemaker cell action potentials?

Answer 17

no resting membrane potential/ resting phase

Question 18

what do pacemaker cells do?

Answer 18

generate action potentials and then transmit them to muscle tissue to initiate action potentials in cardiac myocytes

Question 19

fast Na+ channels

Answer 19

mostly in cardiac myocytes
voltage gated
cause rapid depolarisation
targetted by anaesthetics

Question 20

slow Na+ channels

Answer 20

hyperpolarising -activated cyclic nucleotide gated channels
in auto-rhythmic/ pacemaker cells
voltage gated in response to hyperpolarisation
cause spontaneous depolarisation

Question 21

Na+/ Ca2+ exchanger

Answer 21

mostly in cardiac myocytes
antiporter
pumps 3 Ca2+ out for every 1 Na+ in using secondary active transport
maintains calcium electrochemical gradient across cell membrane

Question 22

L-type channels

Answer 22

transport calcium ions
long-opening
in auto-rhythmic cells and cardiac myocytes and smooth muscle cells
voltage gated
allow calcium inflow from extracellular space
responsible for plateau of cardiac action potential and phase 0 of pacemaker potential - depolarisation above threshold
calcium ion influx results in calcium induced calcium release from ryanodine receptors

Question 23

L-type channels

Answer 23

transport calcium ions
long-opening 
in auto-rhythmic cardiac myocytes
voltage gated
allow calcium inflow from extracellular space

Question 24

T-type channels

Answer 24

transient channels for calcium ions
in autorhythmic cells
voltage gated
involved in phase 4 of pacemaker action potential - depolarisation

Question 25

ryanodine receptors

Answer 25

ligand gated for calcium ions
found in cardiac myocytes
calcium induced calcium release from intracellular calcium stores

Question 26

plasma membrane Ca2+ ATPase

Answer 26

found in cardiac myocytes
ATPase
maintain resting intracellular (low) calcium ion concentraction by pumping Ca2+ ions into extracellular space

Question 27

sarcoplasmic or endoplasmic reticulum Ca2+ ATPase

Answer 27

SERCA
for calcium ions
in cardiac myocytes 
ATPase
pump calcium back into intracellular calcium store

Question 28

what are the different types of potassium channels?

Answer 28

inward rectifier 
delayed rectifier ultrarapid
delayed rectifier fast
delayed rectifier slow
transient outward
muscarinic acetylcholine gated

Question 29

where are potassium ion channels found?

Answer 29

in auto-rhythmic cells

cardiac myocytes and smooth muscle cells

Question 30

what type of channels are potassium channels?

Answer 30

voltage gated or ligand gated

Question 31

what is the function of potassium channels?

Answer 31

repolarisation and stabilisation of resting membrane potential - increase K+ permeability across sarcolemma during resting phase

Question 32

potassium channel blockers

Answer 32

antidysrhythmics

Question 33

drugs targeting slow Na+ channels?

Answer 33

ivabridine

Question 34

what drugs target L-type calcium channels?

Answer 34

non-dihydropyridines - verapamil or diltiazem

Question 35

what drug targets RYR2

Answer 35

class I antidysrhythmics 
Flecainide