Ion transporters and channels in the heart Flashcards
What are ion channels?
passage for ions to move across membranes
down electrochemical gradient
what are transporters?
pumps powered by ATP
move ions against concentration gradient
what are the most important ions in the heart?
sodium
potassium
calcium
location of sodiim
mainly found extracellularly
location of potassium
mainly found intracellularly
location of calcium
found in lower concentrations intracellularly and extracellularly
less in cytosol
mostly in ER
location of calcium
found in lower concentrations intracellularly and extracellularly
less in cytosol
mostly in ER
how is a cardiac myocyte action potential different to a neuron action potential?
prolonged depolarisation due to calcium
how is a cardiac myocyte action potential generated
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
why are L-type calcium ion channels important for cardiac function?
lengthen depolarisation so lengthens refractory period prevents the cell from contracting again so contractions are coordinated to ensure efficiency
describe the process of cardiac muscle contraction
- action potential arrives
- depolarisation of membrane in T tubule
- calcium enters the cell via L-type calcium channels
- 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
- calcium binds to troponin C and activates contraction
- after contraction excess calcium is transported back into the sarcoplasmic reticulum by the SERCA pump - ATPase and requires ATP
- maintains low calcium levels in cytosol so contraction only occurs when required
- transporters in cell membrane transport calcium ions out of the cell
- sodium/ potassium pump re-establishes electrochemical gradient
what transports are found in the cell membrane of cardiac myocytes to transport calcium?
sodium/ calcium exchanges
1 calcium out and 3 sodium in
sodium/ potassium pump
2 potassiums in and 3 sodiums out
where is there a pacemaker cell action potential?
SAN
AVN
Bundle of His
shape of pacemaker cell action potential?
same as a neuronal action potential
how is a pacemaker cell action potential generated?
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
what is special about pacemaker cell action potentials?
no resting membrane potential/ resting phase
what do pacemaker cells do?
generate action potentials and then transmit them to muscle tissue to initiate action potentials in cardiac myocytes
fast Na+ channels
mostly in cardiac myocytes
voltage gated
cause rapid depolarisation
targetted by anaesthetics
slow Na+ channels
hyperpolarising -activated cyclic nucleotide gated channels
in auto-rhythmic/ pacemaker cells
voltage gated in response to hyperpolarisation
cause spontaneous depolarisation
Na+/ Ca2+ exchanger
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
L-type channels
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
L-type channels
transport calcium ions long-opening in auto-rhythmic cardiac myocytes voltage gated allow calcium inflow from extracellular space
T-type channels
transient channels for calcium ions
in autorhythmic cells
voltage gated
involved in phase 4 of pacemaker action potential - depolarisation
ryanodine receptors
ligand gated for calcium ions
found in cardiac myocytes
calcium induced calcium release from intracellular calcium stores
plasma membrane Ca2+ ATPase
found in cardiac myocytes
ATPase
maintain resting intracellular (low) calcium ion concentraction by pumping Ca2+ ions into extracellular space
sarcoplasmic or endoplasmic reticulum Ca2+ ATPase
SERCA for calcium ions in cardiac myocytes ATPase pump calcium back into intracellular calcium store
what are the different types of potassium channels?
inward rectifier delayed rectifier ultrarapid delayed rectifier fast delayed rectifier slow transient outward muscarinic acetylcholine gated
where are potassium ion channels found?
in auto-rhythmic cells
cardiac myocytes and smooth muscle cells
what type of channels are potassium channels?
voltage gated or ligand gated
what is the function of potassium channels?
repolarisation and stabilisation of resting membrane potential - increase K+ permeability across sarcolemma during resting phase
potassium channel blockers
antidysrhythmics
drugs targeting slow Na+ channels?
ivabridine
what drugs target L-type calcium channels?
non-dihydropyridines - verapamil or diltiazem
what drug targets RYR2
class I antidysrhythmics Flecainide
