Cellular and molecular events in the CVS

Question 1

What is the resting membrane potential, and how is it set up?

Answer 1

The resting membrane potential is the electrochemical gradient established in excitable cells.
It is set largely due to the movement of K+ ions, due to the permeability of the cell membrane to K+ ions at rest (leak K+ channels open at rest).
A small movement of other ions due to transient opening of ion channels normally closed at rest means that the resting membrane potential is slightly less negative than the equilibrium potential for K+ (in cardiac ventricular myocytes and skeletal muscle RMP= -90mV).

Question 2

What is the role of Na+K+ATPase in the generation of the resting membrane potential?

Answer 2

Na+K+ATPase is not reponsible for the generation of RMP, however it is responsible for the generation of the Na+ and K+ concentration gradients across the cell membrane that leads to the movement of K+ and setup of RMP. Overall Na+K+ATPase contributes -5mV of the RMP. Blocking Na+K+ATPase only depolarises the cell by approx 7mV.

Question 3

How does K+ permeability set the resting membrane potential?

Answer 3

• K+ ions move out of the cell – down their concentration gradient
• Small movement of ions makes the inside –ve with respect to the outside
• As charge builds up an electrical gradient is established

Question 4

Why are cardiac myocytes electrically active?

Answer 4

• Cardiac myocytes are electrically active in order to fire action potentials
• Action potential triggers increase in cytosolic [Ca2+]
• A rise in calcium is required to allow actin and
myosin interaction
– Generates tension (contraction)

Question 5

Which parts of the action potential of the ventricular myocyte is caused by which ion channels?

Answer 5

• Depolarisation- opening of voltage gated sodium channels
• Small repolarisation- transient opening of K+ channels (with a small contribution from reversal of Na+/Ca2+ exchanger and inactivation of Na+ channels involved in depolarisation
• Plateau- opening of voltage gated (L-type) Ca2+ channels (some K+ channels also open)
• Repolarisation- Ca2+ channels inactivate, voltage gated K+ channels open

Question 6

Different composition of channels permeable to which ion will affect the behaviour of cardiac myocytes?

Answer 6

K+

Question 7

Describe an action potential at the sinoatrial node.

Answer 7

-Pacemaker potential leads to If (funny current); a slow depolarisation to threshold mediated by the influx of Na+ ions. Caused by hyperpolarisation-activated-cyclic-nucleotide-gated ion channel (HCN) (activated at -50mV and below)
-Depolarisation mediated by opening of voltage gated Ca2+ channels
Repolarisation mediated by opening of voltage gated K+ channels

Question 8

Does the SA node have a resting membrane potential?

Answer 8

No- none of the pacemaker cells do (same for the AV node)

Question 9

Why does the SA node not have a resting membrane potential?

Answer 9

Natural automaticity is due to unstable membrane potential

Question 10

What is the importance of the SA node?

Answer 10

SA node is fastest to depolarise
– Sets rhythm
– Is the pacemaker
– Other parts of the conducting system (AV node and purkinje fibres) also have automaticity, but are slower. They can take over if SA node is damaged.

Question 11

What is the order of spread of electrical activity in the heart?

Answer 11

SA node-> atria-> AVnode->bundle of His-> ventricular myocytes

Question 12

How are cardiac myocytes joined together?

Answer 12

Mechanically: attached by desmosomes, which give mechanical strength and prevents the myocytes from pulling apart during contraction.
Electrically: connected by gap junctions, containing connexins- different subunits come
together on either side of the membrane to form a pore, that allows any ion to move through. This allows the spread of depolarisation between myocytes.

Question 13

What is a desmosome?

Answer 13

A glycoprotein structure- cadherin- that mechanically rivets cardiac myocytes together.

Question 14

What lies between cardiac myocytes?

Answer 14

Intercalated disks- containing desmosomes and gap junctions.

Question 15

What is the importance of calcium in cardiac myocyte contraction?

Answer 15

• Depolarisation opens L-type Ca2+ channels in T-tubule system (not enough to initiate contraction alone, but crucial for intracellular Ca2+ release
• Localised Ca2+ entry opens Calcium-Induced Calcium Release (CICR) channels in the SR
• Close link between L-type channels and Ca2+ release channels
• 25% enters across sarcolemma, 75% released from SR

Question 16

In the release of intracellular Ca2+, what is different about skeletal muscle compared to cardiac myocytes?

Answer 16

In skeletal muscle, conformational change of T-tubules opens intracellular Ca2+ channels of the SR. This isn’t true of cardiac myocytes.

Question 17

How is intracellular calcium decreased to resting levels during relaxation in diastole?

Answer 17

Most pumped back into SR by Ca2+ATPase (SERCA). Raised [Ca2+]i activates SERCA.
Some exits the cell across the cell membrane.
Pumps and exchangers involved in transporting ions and restoring RMP:
Ca2+ATPase, Na+K+ATPase, Na+Ca2+ exchanger, Na+H+ exchanger

Question 18

How is cardiac muscle contraction regulated?

Answer 18

As with skeletal muscle:
• Ca2+ binds to troponin C
• Conformational change shifts tropomyosin to reveal myosin binding site on actin filament
• Muscle contracts using sliding filament mechanism

Question 19

How is tone of blood vessels controlled?

Answer 19

Tone of blood vessels is controlled by contraction and relaxation of vascular smooth muscle cells
– Located in tunica media
– Present in arteries, arterioles and veins

Question 20

How is the structure of smooth muscle different to that of striated muscle?

Answer 20

Oval shaped cells
Presence of dense bands and dense bodies.
Different arrangement of actin and myosin filaments that does not result in a striated appearance.
NB. smooth muscle cells are joined by gap junctions

Question 21

How is the physiology of smooth muscle different to that of striated muscle?

Answer 21

-The way actin and myosin interact is slightly different to striated muscle.
-There are also different mechanisms of control of Ca2+:
Either depolarisation or GPCR: (G alphaq-> IP3 upreg) IP3 binds to SR, increasing intracellular Ca2+. Can also have activation of GPCRs alone to initiate contraction.

Question 22

Describe excitation contraction coupling in cardiac myocytes.

Answer 22

Increase in intracellular calcium
Calcium ions bind to calmodulin. (1 CaM can bind 4 Ca2+ ions)
Myosin light-chain kinase is activated, enabling phosphorylation of regulatory light chain on myosin head, allowing it to interact with actin. Without this, it cannot contract.
At rest, myosin light-chain phosphatase is active (constitutively), which dephosphorylates the regulatory light-chain kinase on the myosin head. Activation of DAG and PKC by the GPCR during activation during excitation downregulates MLCP, allowing contraction to occur.

Question 23

How can smooth muscle contract for long periods of time?

Answer 23

When the myosin head binds to actin, in smooth muscle to maintain contraction, it can go into latch state, where it remains bound for longer than in striated muscle.

Question 24

Which GPCR mechanism can inhibit contraction in smooth muscle?

Answer 24

Upregulation of cAMP and therefore PKA.

PKA inhibits myosin light chain kinase by phosphorylation, thereby inhibiting contraction.

Question 25

How does relaxation occur in smooth muscle?

Answer 25

Restoration of Ca2+ resting level

Increase in the activity of myosin light chain phosphatase.

Question 26

What is the name of the GPCR that mediates contraction?

Answer 26

Alpha adrenoceptors