Ion channels and transporters in the heart

Question 1

2 types of cells in the heart

Answer 1

Cardiac contractile cells and autorhythmic cells

Question 2

Cardiac contractile cells

Answer 2

• Myocytes
• 99% of cardiac cells
• Located in myocardium
• Purpose is mechanical work - contraction
• Contraction = force = pump blood
• Must create cardiac action potential

Question 3

Autorhymic cells

Answer 3

• Pacemaker cells
• 1% of cardiac cells
• Located in conduction fibres
• SAN, AVN, bundle of His, R/L bundle branches and Purkinje fibres
• Don’t contract
• Generates pacemaker potentials (pacemaker activity) which sets pace of heart
• Relays info to contractile cells

Question 4

Cells in the heart

Answer 4

• Myocytes in heart embedded in membrane
• Na+/K+ antiporters = secondary transport
• Driving force to get Na+ into cell by antiporter
• Ca2+ from low to high out of cell
• Digitalis inhibits pumps
• Increased intracellular calcium will increase contraction
• Digitalis = greater contraction

Question 5

Ion channels contracting heart

Answer 5

• Excitable cells generate electrical signals, e.g. neurons, muscle cells touch receptor cells
• Excitable cells maintain different concentration of ions in its cytoplasm than extracellular environment
• Ion channel receptors are multimeric proteins in plasma membrane arranged to form pores extending from one side of the membrane to the other
• Opening of ion channels alters charge distribution across membrane = electrochemical gradient = electrical signal

Question 6

Cardiac mini potentials

Answer 6

• Membrane potential of cardiac cell is -90 mV
• Outside of cell assumed to be 0 mV
• Na+ into cell which increases +ve inside cell - membrane potential more positive
• Potential would repel positive sodium from entering cell
• Calcium has low intracellular concentration but high extra cellular - large conc grad
• When membrane potential is 134mV, no net movement of calcium
• Membrane potential determined by gradients for potassium, sodium and calcium and relative permeability of membrane to these ions
• To determine membrane potentials individual ion equilibrium potentials are multiplied by relative membrane permeabilities and summed
• g’ is relative conductance
• Na+ and Ca2+ diffusing in and K+ diffusing out maintains gradient

Question 7

Cardiac action potentials

Answer 7

• Cardiac myocytes contraction stimulated by APs
• Heart generates own electrical stimulation
• Pacemaker cell myocytes - cardiac conduction system
• Pacemakers lose ability to contract but specialised to transmit APs
• SAN controls HR, cells fire spontaneously to generate APS in atrium
• Myocytes connected by gap junctions which form channels for ions to flow = electrical coupling of neighbouring cells and propagation of signals.
• When signals reach AVN, they follow conduction pathway
• AP is brief reversal in polarity of cell membrane
• When membrane voltage increases, cell is depolarised
• When membrane potential decreases, cell is repolarised
• For AP, must be depolarised to threshold value
• Pacemaker cells have no true AP - starts at -60mV until threshold of -40mV (funny currents)
• Funny channels allow sodium in = depolarisation
• At threshold, Ca2+ in = more depolarisation

Question 8

AP in myocytes

Answer 8

• SR contains lots of calcium
• Stable resting AP at -90mV
• When cell depolarised, more Na+ and Ca2+ inside cell which leak into neighbouring cells
• Ca2+ channels are slow and L-type
• Ca2+ channels close and K+ opens = decrease in membrane potential (early repolarisation)
• Influx of calcium triggers Ca2+ release from SR = calcium-mediated calcium release and then uses sliding filament mechanism
• As Ca2+ close, potassium efflux dominates, resting AP re-established
• Plateau phase means cardiac muscle contracting for longer than skeletal
• Long refractory period because otherwise heart would stop beating

Question 9

What does digoxin do?

Answer 9

Na/K pump inhibited