Lecture 19: Membrane Potential and Electrophysiology of the Heart Flashcards

1
Q

Two critical qualities of plasma membrane

A
  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
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2
Q

Resting membrane potential voltage

A

-90 mV

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3
Q

The Na/K pump is inhibited by

A

Cardiac glycosides

Ex. digoxin

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4
Q

Cardiac glycosides increase…

used to treat…

A

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

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5
Q

Cardiac glycosides are commonly called

A

digitalis glycosides because most come from digitalis (foxgrove) plant

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6
Q

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

A

the atrial and ventricular myocardium (positive inotropic action)

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7
Q

5 steps of cardiac glycoside mechanism

A
  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
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8
Q

Ca ATPase

A
  • Assists with Ca extrusion from the cell

- Seems to be involved in maintaining diastolic cytostolic calcium concentration

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9
Q

Na-Ca exchanger

A
  • 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
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10
Q

What allows the myocardium to relax during repolarization

A

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

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11
Q

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

A

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

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12
Q

Voltage gated channels in the heart

A
  • 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)
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13
Q

Tetrodotoxin

A

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

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14
Q

Vermapamil

A

Calcium channel blocker

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15
Q

Antiarrhymythic drugs

A

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

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16
Q

Four examples of adrenergic agonists

A
  • Dopamin
  • Dobutamine
  • Amrinone
  • Milrinone
17
Q

Dopamin and dobutamine

A
  • Positive inotropic agents

- Used to treat heart failure

18
Q

Amrinone and milrinone

A
  • Inhibitors of phosphodiesterase which hydrolyses cAMP

- Used to treat heart failure

19
Q

Autorhythmicity

A

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

20
Q

Two types of specialized cardiac cells

A
  1. Contractile cells

2. Autorhythmic cells

21
Q

Contractile cells

A

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

22
Q

Autorhythmic cells

A
  • Remainder of cardiac cells. Small and do not contract

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

23
Q

Autorhythmic cells action potential

A
  • Have no resting membrane potential

- Display pacemaker activity

24
Q

Ions involved in autorhythmic action potential

A
  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
25
What is the basis for the pacemaker potential
Ifunny
26
Ifunny is called the
pacemaker current
27
Ifunny is activated by
increasing negative voltages
28
Ifunny is carried by
Na via voltage-dependent Na channels
29
Ifunny channels are opened when
membrane potential becomes more negative than -50mV
30
Sympathetic stimulation does what to Ifunny
Increases number of open channels leading to an increase in heart rate
31
Parasympathetic stimulation does what to Ifunny
decreases the number of these open channels leading to decrease heart rate
32
Phase 0 of Pacemaker potential
Depolarization: inward Ca carried by L-Ca channels
33
Phase 3 of pacemaker potential
repolarization: K efflux
34
Phase 4 of pacemaker potential
Spontaneous depolarization. Accounts for automaticity of SA nodal cells
35
What sets the heart rate?
Rate of depolarization