Lecture 19: Membrane Potential and Electrophysiology of the Heart

Question 1

Two critical qualities of plasma membrane

Answer 1

1. Capacity to maintain ion gradient between icf and ecf
2. Capacity to respond to a wave of depolarization by briefly opening and closing specific channels which influence the contractile performance of the heart

Question 2

Resting membrane potential voltage

Answer 2

-90 mV

Question 3

The Na/K pump is inhibited by

Answer 3

Cardiac glycosides

Ex. digoxin

Question 4

Cardiac glycosides increase…

used to treat…

Answer 4

the contractibility of the heart and are widely used to treat heart failure

Question 5

Cardiac glycosides are commonly called

Answer 5

digitalis glycosides because most come from digitalis (foxgrove) plant

Question 6

Cardiac glycosides influence Na/K flow in cardiac muscle, increasing the contraction of

Answer 6

the atrial and ventricular myocardium (positive inotropic action)

Question 7

5 steps of cardiac glycoside mechanism

Answer 7

1. Inhibition of the Na/K pump cell membrane
2. This cause an increase in intracellular Na+ concentration, altering the Na+ gradient across the cell membrane
3. Inward directed Na+ down the concentration decreases leading to a decrease in the Na-Ca exchange activity
4. As less Ca is pumped out of the cell by Na/Ca exchanger, the intracellular Ca concentration increases
5. Since tension is directly proportional to the intracellular Ca concentration, cardiac glycosides produce an increase in tension by increasing the intracellular Ca concentration - a positive inotropic effect

Question 8

Ca ATPase

Answer 8

• Assists with Ca extrusion from the cell

- Seems to be involved in maintaining diastolic cytostolic calcium concentration

Question 9

Na-Ca exchanger

Answer 9

• Na flows inward along its electrochemical gradient
• Ca flows outward against an electrochemical gradient
• Major mechanism of Ca extrusion through the plasma membrane into the extracellular fluid

Question 10

What allows the myocardium to relax during repolarization

Answer 10

Ca pump of SR and NA/Ca exchanger lowers the cytoplasmic Ca concentration

Question 11

In heart muscle, it seems that adrenergic stimulation acts via their receptors on the same channel depolarization acts on because

Answer 11

no amount of adrenergic stimulation will open the Ca channels in the absence of an accompanying voltage stimulus

Question 12

Voltage gated channels in the heart

Answer 12

• Entry of extracellular Ca activates Ca sensitive release channels (ryanodine channels) in the SR through which stored Ca flows into the cytoplasm
• A rapid increase in intracellular Ca occurs and tension starts to develop (calcium induced calcium release)

Question 13

Tetrodotoxin

Answer 13

Inhibits Na channels, resulting in a decrease in excitability of the heart

Question 14

Vermapamil

Answer 14

Calcium channel blocker

Question 15

Antiarrhymythic drugs

Answer 15

Ca channel blacokers that decrease Ca entry into the cell resulting in a decreased rate of contraction

Question 16

Four examples of adrenergic agonists

Answer 16

• Dopamin
• Dobutamine
• Amrinone
• Milrinone

Question 17

Dopamin and dobutamine

Answer 17

• Positive inotropic agents

- Used to treat heart failure

Question 18

Amrinone and milrinone

Answer 18

• Inhibitors of phosphodiesterase which hydrolyses cAMP

- Used to treat heart failure

Question 19

Autorhythmicity

Answer 19

Heart beats rhythmically due to it’s ability to create action potentials

Question 20

Two types of specialized cardiac cells

Answer 20

1. Contractile cells

2. Autorhythmic cells

Question 21

Contractile cells

Answer 21

99% of cardiac muscle cells that do the mechanical work of pumping. They don’t initiate their own action potential

Question 22

Autorhythmic cells

Answer 22

• Remainder of cardiac cells. Small and do not contract

- Initiate and conduct action potentials responsible for contraction of the working cells

Question 23

Autorhythmic cells action potential

Answer 23

• Have no resting membrane potential

- Display pacemaker activity

Question 24

Ions involved in autorhythmic action potential

Answer 24

1. Decrease in K efflux
2. Ib: Constant Na influx via Na leak channels
3. If: Pacemaker current. Na influx via voltage-dependent Na channels
4. Ca: Brings the membrane potential to threshold

Question 25

What is the basis for the pacemaker potential

Answer 25

Ifunny

Question 26

Ifunny is called the

Answer 26

pacemaker current

Question 27

Ifunny is activated by

Answer 27

increasing negative voltages

Question 28

Ifunny is carried by

Answer 28

Na via voltage-dependent Na channels

Question 29

Ifunny channels are opened when

Answer 29

membrane potential becomes more negative than -50mV

Question 30

Sympathetic stimulation does what to Ifunny

Answer 30

Increases number of open channels leading to an increase in heart rate

Question 31

Parasympathetic stimulation does what to Ifunny

Answer 31

decreases the number of these open channels leading to decrease heart rate

Question 32

Phase 0 of Pacemaker potential

Answer 32

Depolarization: inward Ca carried by L-Ca channels

Question 33

Phase 3 of pacemaker potential

Answer 33

repolarization: K efflux

Question 34

Phase 4 of pacemaker potential

Answer 34

Spontaneous depolarization. Accounts for automaticity of SA nodal cells

Question 35

What sets the heart rate?

Answer 35

Rate of depolarization