1.4 Modulation of the Cardiac Action Potential and Contraction

Question 1

What are the 10 steps of EC coupling in the heart?

Answer 1

1. AP enters from adjacent cell
2. Voltage gated Ca channels open and Ca enters the cell
3. Ca induces Ca release from the SR through ryanodine receptors
4. local release causes Ca spark
5. Summed Ca sparks cause Ca signal
6. Ca binds to troponin C to intiate contraction
7. Relaxation when Ca unbinds
8. Ca pumped back into SR via SERCA
9. Ca exchange with Na by NCX antiporter
10. Na gradient maintained by the Na/K ATPase

Question 2

how is Ca cleared

Answer 2

1. reuptake into SR via SERCA

2. Ca pumped out via Na Ca exchanger and some via Ca-ATPase

Question 3

what inhibits SERCA?

Answer 3

phospholamban

Question 4

Once Ca is in the SR what happens?

Answer 4

It binds to calsequestrin

Question 5

what is chronotropism and what cells does it affect?

Answer 5

relates to HR - ICS

Question 6

what is bathmotropism and what cells does it affect?

Answer 6

in regard to AP threshold - ICS

Question 7

what is dromotropism and what cells does it affect?

Answer 7

in regard to AVN conduction delay - ICS

Question 8

What is Inotropism and what cells does it affect?

Answer 8

Contractile force - monocytes

Question 9

What is Lusitropism and what cells does it affect?

Answer 9

rate of relaxation - monocytes

Question 10

What is the main adrenoceptor in the heart?

Answer 10

Beta 1

Question 11

what is the parasympathetic and sympathetic innervation to the heart?

Answer 11

Vagal (PS): ACh - nodes only (inhibitory action)

Symp: diffuse innervation (excitation) via NA

Question 12

which autonomic input is usually of greater effect in modulating HR?

Answer 12

Vagal output is larger and typically dominates over sympathetic

Question 13

What receptors on the SA node do vagal and symp signals work?

Answer 13

Vagal: muscarinic (M2)
Symp: B1

Question 14

what are examples of Ca channel blockers and where do they work best?

Answer 14

Verapamil: cardiac tissue
Nifedipine: vessels

Question 15

Effect of Ca channel blockers on AV node and what kind of tropism is it

Answer 15

reduces amplitude and shortens AP: currents decrease for depolarisation of surrounding cells (negative dromotrope)

Question 16

Effect of Ca channel blockers on SA node and what kind of tropism is it

Answer 16

Amplitude decreases but prolongs Polarised membrane potential decay: heart rate decreases - negative chonotrope

Question 17

Effect of Ca channel blockers on monocytes and what kind of tropism is it?

Answer 17

Amplitude decrease and plateau phases decreases (CaL blocked) - decrease HR
Negative inotrope

Question 18

What happens to the AP during hyperkalaemia

Answer 18

RMP increases 
Inactivation of Na channels 
lose sodium currents in monocytes that are needed ot make APs 
Aps start to resemble those in ICS 
loss of phase 0 and 1

Question 19

What happens to the AP in hypoxia?

Answer 19

Drop in Na/KATPase 
- shorter AP, less Ca influx, decrease contractility, sympathetic activation, increase HR 
Store overload (mechanisms causing overload)