Session 4 - Cellular and molecular events

Question 1

What is the main ion which determines RMP?

Answer 1

-K+ (moves out through voltage-insensitive channels)

Question 2

What is the RMP of cardiac myocytes and why does it differ from Ek?

Answer 2

• Approx. -90mV

- The cells are slightly permeable to other ions

Question 3

In brief, summarise how RMP is set up

Answer 3

• K+ moves down its concentration gradient out of the cell through voltage-insensitive channels
• The inside becomes more negative relative to the outside and this sets up an electrical gradient as the extracellular positive K+ are attracted to the -ve intracellular environment
• When the electrical gradient is = and opposite to the chemical gradient there is no net flow of ions as an equilibrium is reached and the RMP becomes constant

Question 4

Does NaKATPase contribute to the RMP?

Answer 4

-No, it establishes the ion gradients but does not set up RMP

Question 5

What happens to the membrane potential of cardiac cells during diastole?

Answer 5

-It remains at a constant level, except within the pacemaker cells

Question 6

Draw the changes in membrane potential of:
-Ventricular cells
-Pacemaker cells
over the cardiac cycle

Answer 6

• Ventricle (starts at -90, steep upstroke to +30, small steep decline then plataeu for approx 200ms then decrease steeply to -90mV) should last aorund 280ms
• SAN (starts at appros -60mV, slow increase for approx 400ms, then upstroke (not vertical) to +30mV then downstroke (not vertical) to below were upstroke startd

Question 7

Describe the membrane permeability changes and the changes in ionic currents underlying the ventricular AP

Answer 7

1) Upstroke-> opening of voltage-gated Na channels (increasing permeability) results in rapid Na influx, moving MP towards Ena
2) Change in MP causes voltage gated K+ channels to open resulting in transient outward K+ with initial small repolarisation. Na channels become inactive
3) Opening of Voltage-gated L-type Ca2+ channels causes Ca2+ influx and a plateau occurs due to balance with K+ efflux
4) Ca2+ channels inactivate (slower than Na) and delayed voltage-gated K+ channels open causing rapid efflux of K+ and membrane repolarisation occurs

Question 8

Describe the membrane permeability changes and the changes in ionic currents underlying the Pacemaker AP

Answer 8

1) Hyperpolarisation Cyclic Nucleotide channels are permeable to Na (and K+ but more Na at this MP) and allow gradual influx establishing the funny current, which slowly depolarises the cell to threshold
2) When the threshold is reached, the upstroke occurs as voltage-gated Ca2+ channels open and Ca influx down its concentration gradient increases MP to approx +30mV
3) Voltage-gated K+ channels then open and Ca2+ channel inactivation occurs resulting in an efflux of K+, which causes hyperpolarisation of the cell

Question 9

Why is it important that K efflux causes hyperpolarisation rather than repolarisation in pacekmaker action potentials?

Answer 9

• In order to generate natrual automaticity
• The HCN channels are voltage-gated channels which open in response to hyperpolarisation
• Hyperpolarisation activation of the HCN channels allows the funny current to be immediately generated again, initiating a cycle

Question 10

At what membrane potential to HCN channels open?

Answer 10

-approx -50mV

Question 11

How does the level of hyperpolarisation in pacemaker cells effect depolarisation?

Answer 11

-The more hyperpolarisation that occurs the more HCN channels open and the quicker the funnycurrent to threshold is established

Question 12

Why is the influx of Ca2+ significant in cardiac myocytes?

Answer 12

-The increase in intracellular calcium is required for actin-myosin interactions to generate a contraction

Question 13

Why is the length of the AP in a cardiac myocyte significant?

Answer 13

-Ensures that the first cell to depolarised is still depolarised when the last cell begins its AP

Question 14

What is responsible for one heart beat of the heart?

Answer 14

-One AP in the pacemaker cells, one systole, one contraction, one heartbeat

Question 15

What areas of the heart can initiate an AP?

Answer 15

• SAN
• AVN
• Purkinje fibres
• Bundle of his

Question 16

Why does the SAN set the rhythm of the heart?

Answer 16

-Fires an AP at the quickest rate when compared with the other areas of the heart

Question 17

What are desmosomes?

Answer 17

• Glycoprotein cadherins which span the membranes of cells and crosses the gap between two adjacent cells
• Mechanically joins cells providing strength during contraction

Question 18

What is the function of gap junctions?

Answer 18

-Subunits of connexons which align on two cell membranes to form a pore, allowing cells to be electrically coupled to one another via permitting the movement of any ions or molecules and thus allows spread of depolarisation

Question 19

What structures are inside an intercalated disc?

Answer 19

• Desmosomes

- Gap Junctions

Question 20

Where in the cardiac cell do Ca channels open in response to depolarisation?

Answer 20

-Depolarisation spreads along the sarcolemma and down the T-tubules, and opens the Ca L-type channels in the T-tubules

Question 21

Describe how intracellular Ca is increasedin a cardiac myocyte for a contraction to occur

Answer 21

• Depolarisation opens L-type Ca2+ channels in t-tubules
• Influx of Ca
• Localised Ca entry causes calcium-induced calcium release channels to open in the sarcoplasmic reticulum
• Ca influx from the SR produces a high rise in intracellular calcium, sufficient to initiate contraction via the sliding filament model

Question 22

What proportion of intracellular calcium needed for contraction comes from extracellularly and what proportion from the SR?

Answer 22

• 25% from L-type channels

- 75% from SR

Question 23

What is the force of contraction of each myocyte proportional to?

Answer 23

-The Ca concentrationat a given degree of stretch

Question 24

After a contraction has occured, how is the intracellular calcium reduced?

Answer 24

• Re-uptake into the intracellular stores as it is pumped into the SR by SERCA
• Expulsion of Ca2+ by sarcolemmal Ca ATPase and
• Mainly decreased by NCX (high capacity low affinity)

Question 25

What is necessary in order for relaxation of cardiac myocytes to occur?

Answer 25

-Reduction in intracellular calcium

Question 26

How does quicker hyperpolarisation of pacemaker cells increase HR?

Answer 26

• Rapid hyperpolarisation will occur which leads to an increased HCN channel activation
• This would cause the funny current to reach the threshold for depolarisation quicker, thus depolarisation would occur quicker -> increased HR as the intervals between AP would be reduced

Question 27

What effect does slow repolarisation have on the depolarisation of pacemaker cells and thus the HR?

Answer 27

-Repolarisation is slower and therefore less HCN channels open and depolarisation to threshold is slower and thus HR is slower

Question 28

What controls the tone of blood vessels?

Answer 28

-Contraction and relaxation of vascular smooth muscle located within the tunica media

Question 29

What is the result of increased contraction of smooth muscle cells in blood vessels?

Answer 29

-Increased tone, narrowed lumen

Question 30

How are the actin filaments arranged in smooth muscle?

Answer 30

-They are attaches to dense bodys and bands within the cells which anchor them into place

Question 31

How does smooth muscle contraction occur? (from high intracellular calcium)

Answer 31

• Calcium binds to calmodulin forming a 4Ca-CaM complex
• This Ca-CaM complex binds to and activates myosin-light-chain-kinase
• MLCK-CaM enables MLCK to phosphorylate a regulatory light chain on myosin head-> this results in binding to actin and contraction to occur

Question 32

In what two ways can intracellular calcium be raised in smooth muscle in order to initiate contraction?

Answer 32

• Depolaritsation via Ca channels
• Non-depolarisaiton method -> Ligand binds to GPCR located in smooth muscle cell membrane. Gaq dissociates and activates PLC which cleaves PIP2 into IP3 and DAG.
• > IP3 binds to SR IP3 receptor which opens calcium channels allowing influx into the cell

Question 33

Why is smooth muscle contraction longer and maintained compared with skeletal and cardiac muscle?

Answer 33

-The myoson head enters the latched state and stays bound to actin for longer

Question 34

Why is sustained contraction of smooth muscle cells important?

Answer 34

-In order to maintain vascular tone

Question 35

How is vascular tone decreased?

Answer 35

1) Reduced release of Ca from SR or reduced Ca entry into the cell -> reduced MLCK activation by reduced ca-cam -> reduced contraction
2) Inhibition of MLCK by increased cellular concentrations of cAMP-> activates PKA -> phosphorylates MLCK and inactivates it -> reduced contraction
3) Myosin-light chain phosphatase activation which dephosphorylates the regulatory light chain on the myosin head -> reduced contraction

Question 36

Discuss how MLCP causes dephosphorylation of myosin head, and thus decreased tone

Answer 36

• MLCP normally inhibited by DAG, decreased ligand binding reduces Gaq-mediated production of DAG and thus MCLP not inhibited
• MLCP dephosphorylates the myosin heads -> decreased contraction -> decreased tone