Things that change contractility

Question 1

Summarise the effect of sympathetic stimulation on contraction (4)

Answer 1

• more and faster Ca2+ release
• faster cross bridge cycling
• more and faster contraction
• faster relaxation

Question 2

Factors affecting heart rate are called what?

Answer 2

Chronotropic factors

Question 3

Factors affecting contractility are called what?

Answer 3

inotropic factors

Question 4

Factors affecting contraction and relaxation are called what?

Answer 4

lusitropic factors

Question 5

As well as affecting heart rate via the cells in the SA node, how does sympathetic stimulation affect contraction via the muscle cells?

Answer 5

When NE binds to the β1-adrenoreceptor on the cardiac muscle cells, there is a rise in cAMP which activates PKA

Question 6

What is the effect of there being a rise in cAMP in the cardiac muscle cells when noradrenaline binds to the β1-adrenoreceptor on contractility and contraction duration?

Answer 6

PKA phosphorylates

• voltage gated L-type Ca2+ channels which increases Ca2+ concentration
• RyR which increases the SR Ca2+ release
• TnI which limits the interaction of Ca2+ with TnC which DECREASES Ca2+ sensitivity
• PLB which releases the SERCA inhibition to increase SR Ca2+ uptake

Question 7

How does sympathetic stimulation affect the Frank-Sterling mechanism and the pressure volume loop?

Answer 7

It shifts the Frank-Starling curve up because there is a higher SV for the same EDV
On the pressure volume loop, it means there is an increase in contractility which means that the SV increases, the ESV decreases

Question 8

Due to an increase in heart rate by noradrenaline/norepinephrine, the duration of the twitch has to __________

Answer 8

shorten

Question 9

Why is it important that noradrenaline leads to a decrease in Ca2+ sensitivity of the myofilaments?

Answer 9

to allow the heart to contract more quickly and relax more quickly to maintain appropriate ventricular filling during diastole

Question 10

What 5 things come in response to sympathetic stimulation?

Answer 10

• increased heart rate
• elevated contractility
• reduction in duration of diastole and systole
• increase in conduction velocity
• better synchronisation of atrial and ventricular contractions

Question 11

Which hormones lead to an increase in CONTRACTILITY (not contraction duration)? How do they do this?

Answer 11

Short term:
- catecholamines such as adrenaline and dopamine which act the same way as the sympathetic stimulation
- glycosides such as digoxins/digitalis
Long term:
- Angiotensin II
- endothelin
- thyroid hormone

Question 12

Ηοw do glycosides affect contractility?

Answer 12

They inhibit the Na+/K+ pump and so there is an accumulation of Na+. This inhibits the Ca2+ efflux through the Na+/Ca2+ exchanger

Question 13

What are three hormones that decrease contractility?

Answer 13

Acetylcholine
Calcium blockers
β blockers

Question 14

What receptor does acetylcholine bind to cause a decrease in contraction?

Answer 14

M2 muscarinic receptors

Question 15

What is the effect of acetylcholine binding to the cardiac cells leading to a decrease in contractility?

Answer 15

• there is a decrease in cAMP which means that there is less PKA activity which means that there is:
• no activation of L-type Ca2+ channels which leads to a decrease in Ca2+ entering the cell
• no activation of the RyR which decreases the Ca2+ release from the SR
• there is INCREASED Ca2+ sensitivity because TnI does not limit the interaction with TnC
• activates PLB which increases SERCA inhibition to decrease in SR Ca2+ uptake

Question 16

What is the effect of beta-blockers leading to a decrease in contractility?

Answer 16

β-blockers block the β adrenoreceptor which reduces the cAMP levels and reduces the PKA activity. This leads to a eduction in the activity of the voltage gated T-type calcium channels, RyR and SERCA. It also increases the Ca2+ sensitivity of TnC and reduces the need for ATP so there is less stress on the heart