Cardiac Contraction

Question 1

What do cardiomyocytes make up?

Answer 1

Branching myofibres

Question 2

What does each myocyte contain?

Answer 2

Multiple rod-like cross branched strands that run the length of repeating sarcomeres

Question 3

What are t tubules?

Answer 3

Invagination of the muscle cell membrane that penetrates into the centre of cardiac muscle cells

Question 4

What does the cytoplasm between the myofibrils contain?

Answer 4

The single, centrally located nucleus, mitochondria and sarcoplasmic reticulum

Question 5

What are sarcomeres?

Answer 5

Cause muscle contraction when the actin and myosin filaments move relative to each other

Question 6

What is proportional to intracellular calcium?

Answer 6

Force of contraction

Question 7

What are the 5 steps in cardiac muscle contraction?

Answer 7

• sodium channels open allowing sodium to enter and depolarise the cell
• voltage gated calcium channels open
• plateau phase calcium influx and calcium induced calcium release
• calcium channels close and potassium channels open fully allowing potassium to leave and repolarise the cell
• stable sodium/potassium pump when cell is at rest

Question 8

Where would you find ryanodine receptors?

Answer 8

Intracellular calcium channels

Question 9

What does troponin T bind to?

Answer 9

Tropomyosin

Question 10

What does troponin I bind to?

Answer 10

Actin filaments

Question 11

What does troponin C bind to?

Answer 11

Calcium

Question 12

What does binding of calcium to troponin C lead to?

Answer 12

Conformational changes of tropomyosin and exposure of actin binding sites

Question 13

How does the calcium concentration decrease in myocytes?

Answer 13

• action potential repolarises t tubules, and the voltage gated calcium channels and decreased calcium influx
• no calcium influx-> no calcium induced calcium release
• extrusion of calcium from cell by Na/Ca exchanger
• calcium uptake into sarcoplasmic reticulum via SR membrane
• calcium is taken into the mitochondria

Question 14

What is intrinsic stretch a result of?

Answer 14

Increased pressure or volume

Question 15

What is extrinsic control due to calcium conc rise a cause of?

Answer 15

Some pressure or volume but increased contractility

Question 16

What are drugs needed for clinically?

Answer 16

To increase the contractility of the heart

Question 17

What do sympathetic mimetic drugs do?

Answer 17

Increase the voltage gated calcium channel activity

Question 18

What do cardiac glycoside drugs do?

Answer 18

Reduce calcium extrusion

Question 19

What do positive ionotropes do?

Answer 19

Increase the strength of contraction

Question 20

What do beta 1 adrenoreceptors induce?

Answer 20

Increased contractility

Question 21

What does increased pKA lead to?

Answer 21

• increased calcium channel so higher calcium level and greater contraction
• increased potassium channel opening so faster repolarization and shorter action potential leads to a faster heart rate
• increased sarcoplasmic reticulum calcium ATPase, so uptake of calcium into storage by SR allowing fresh relaxation
• overall stronger faster contractions, same diastolic time to allow for filling with blood and coronary perfusion

Question 22

Why are cardiac glycosides not used often now?

Answer 22

Side effects

Question 23

What is digitoxins mechanism of action?

Answer 23

It inhibits the sodium potassium ATPase

• Na build up lowers the concentration gradient
• less calcium epulsion by sodium/potassium exchanger , so it is taken up into stores

Question 24

What drugs are used in acute heart failure?

Answer 24

Dobutamine and dopamine

Question 25

What drug is used in acute heart failure when taking beta blockers (and how does it work)?

Answer 25

Glucagon

Acts as a GPCR, stimulates Gs pathway, and increases cAMP and pKA activity

Question 26

What is amrinone?

Answer 26

Phosphodiesterase inhibitor