Physiology: Cardiac Myocytes

Question 1

What effect does Digoxin have on the heart, and how?

Answer 1

• Positive inotropic effect
  
  • Inhibits the Sodium-potassium pump, which indirectly inhibits the sodium Calcium-Sodium exchanger.

Question 2

• What does hyperkalemia do the resting membrane potential of cardiac myocytes, what effect does this have on action potentials?

Answer 2

• Hyperkalemia reduces the Nernst K+ equilibrium potential and, therefore, the resting membrane potential.
• This, in turn, diminishes the action potential, because the low resting potential prevents many Na+ channels from resetting from the inactivated state to the closed- but-activatable state.

Question 3

Label 1-3

Answer 3

Question 4

• What effects do the following agents have on the contractility of the heart, and through what mechanisms?
  
  1. Propranolol
  2. Verapamil
  3. Caffeine (+ milrinone)

Answer 4

1. Propanolol
   
   • Negative inotropic agent
   • A non-selective beta blocker that does not allow NE to bind to the Gs subunit of Adenyl cyclase.
     
     • This stops the conversion of ATP to cAMP.
2. Verapamil
   
   • Negative inotropic agent
     
     • Blocks the influx of Ca2+ through the L-type Calcium channels, thus decreasing calcium within the cells and decreasing contraction force.
3. Caffeine (+ milrinone)
   
   • Positive inotropic agent
   • Inhibits the conversion of cAMP to AMP by phosphodiesterase III.

Question 5

• Describe how the state of the voltage-gated sodium channels changes along this action potential
  
  • (Open vs closed, active vs inactive)

Answer 5

Question 6

This is the cardiac action potential of a ventricle.

1. What causes the initial upstroke, seen at point 0?
2. What causes the sharp stop in depolarization, seen at point 1?
3. What causes the plateau phase, seen at point 2?
4. What causes the steady repolarization, seen at 3?

Answer 6

Question 7

1. What kind of channel?
2. What kind of channel?
3. What complex?

Answer 7

Question 8

Concerning myocardial cells,

• Give the order from least to greatest for intracellular ion concentration for
  
  1. K +
  2. Na +
  3. Ca 2+
  4. Cl -
• Give the order from least to greatest extracellular ion concentration for
  
  1. K +
  2. Na +
  3. Ca 2+
  4. Cl -
• Give the order from least to greatest for Nernst equilibrium potential for
  
  1. K +
  2. Na +
  3. Ca 2+
  4. Cl -

Answer 8

• Give the order from least to greatest for intracellular ion concentration
  
  • Ca 2+ , Na + , Cl - , K +
• Give the order from least to greatest for extracellular ion concentration
  
  • K +, Ca 2+, Cl -, Na +
• Give the order from least to greatest for ion Nernst equilibrium potential
  
  • K + , Cl -​, Na +​, Ca 2+​

Question 9

Where would the following refractory periods go in this graph?

• Effective refractory period (ERP)
• Absolute refractory period (ARP)
• Relative refractory period (RRP)
• Supranormal refractory period (SRP)

Answer 9

• The effective refractory period (ERP) includes the absolute refractory period (ARP) and the first half of the relative refractory period (RRP).
• The RRP begins when the ARP ends and includes the last portion of the ERP.
• The supranormal period (SNP) begins when the RRP ends.

Question 10

What do cardiac glycosides do to the contraction of the heart?

Answer 10

• They have a positive inotropic effect
  
  • Positively inotropic agents increase the strength of muscular contraction.

Question 11

How does adrenaline as well as pufferfish toxin affect the action potentials of Purkinje fibers?

Answer 11