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Cardiovascular System > Cardiac Heart Muscle 2 > Flashcards

Cardiac Heart Muscle 2 Flashcards

1
Q

How does cardiac muscle differ from skeletal muscle

A

Each cardiac myocyte contributes to each heart contraction.

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2
Q

How does cardiac muscle force still vary, and why?

A

To maintain CO to meet the body’s metabolic demand.

  • The heart MUST pump out all the blood which comes to it (CO=VR)
  • Since all muscle fibres are activated we cannot use recruitment of new fibres. Hence the heart MUST modulate the level/rate of activation of the fibres and/or the contractility of actin/myosin
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3
Q

Resting CO vs exercise CO

A

Resting CO: ~5L/min = 70mL (SV) x 70BPM (HR)

Exercise CO: can increase to 20L/min

  • HR can increase 3-fold (70 x 3). But as 210BPM x 70mL only reaches 14.7L so not all is HR!
  • the remainder must be in the SV (70x ?)
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4
Q

Consequence of exercise CO (increased CO)

A

there is LESS TIME for filling and ejection (most time is lost in DIASTOLE)

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5
Q

How is force modulated?

A

The heart can…

1) Increase rate of automaticity
2) increase dimensions of ventricle (stretch)
3) use neurotransmitters to alter rate and calcium handling (direct and rate effects)
4) use inotropic drugs

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6
Q

What does contraction generate and what does it depend on?

A

Contraction generates both ‘isometric force’ (ventricular pressure) and ‘isotonic force’ (rapid shortening during ejection phase)

Contraction force depends on [calcium]i and [calcium]total in a highly non-linear way.

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7
Q

What are the two main ways to change the strength of contraction

A

1) Altering calcium transient (amplitude (amount released by SR) and duration (slower, force for longer))
2) Altering myofilament calcium sensitivity

Drugs affect one or both of these things!

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8
Q

Myofilament Calcium Sensitivity

A

Many factors affect

  • inc pH (inc)
  • inc sarcomere length (FS response) (inc)
  • inc catecholamines eg PKA (decreas, lusitropy)
  • decr intracellular ATP (inc)
  • inc caffeine (inc)

Increases sensitivity means more force is generated for the same release of calcium

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9
Q

How is the pH of myofilaments regulated at 7.4?

A

By H+ equivalent membrane transporters, tightly control pH

More acidic: NHE(Na+/H+ exchanger)*** and NBC “acid extruders”

less acidic: CHE and CBE “acid-loaders”

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10
Q

The basis of HF is

A

reduction in myocyte contractility

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11
Q

HF would best be treated by a drug that inc or decr myofilament calcium sensitivity

A

Neither! Although increasing sensitivity would lead to a greater contraction, this would also mean a decrease in the time to relax, this compromises CO.
And vice versa for decreasing. A no win situation.

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12
Q

Force/length relationshhip of myofilaments in regards to stretch and force of contraction.

A

Peak(SL 2-2.2micrometres): Maximum optimisation of force generated at this resting length

2.2: overstretched, there is insufficient overlap of the myofilaments, so force produced is reduced.

SL = sarcomere length

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13
Q

Frank-Starlings law of the heart: EDV and stretch

A

That an increase in LV end diastolic volume (VR) will increase the stretch of the muscle fibres therefore increasing the preload. Therefore the heart is able to generate a greater force of contraction (via a stretch-induced increase in cardiac contractility) and eject the extra volume of blood INCREASING SV

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14
Q

Biphasic response to stretch

A

Stimulate muscle via an electrical stimulus to contract.

Rapid Response: As you increase muscle length you increase the force of contraction . (FS law of the heart)

Slow Force Response: In the same experiment but only a step increase in muscle length. You will again see the rapid response BUT then a SLOW INCREASE IN FORCE due to the increasing calcium transient

Therefore the 2 things that can increase force at the cellular level.

  • calcium sensitivity of the myofilaments (FS law/stretch)
  • increase in the calcium transient
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15
Q

Also, when HR is increased will increase the force of contraction. This is due to many events….

A

HR when increased will increase the force of contraction. This is due to many events
1) With each excit/contract coupling event, you get an increase of calcium and sodium.
Sometimes at a higher HR frequency the Na+ is not all removed by the Na+/K+ pump as quickly as it could, so at higher HR’s you tend to get an increase in intracellular sodium. Then by Na+/Ca2+ exchange that high sodium inside will cause calcium to be brought into the cell

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16
Q

Force Frequency relationship in healthy individual (why do you go from 1Hz to 2Hz?)

A

Normal: FF positive (1Hz = 50BPM)

Force of contraction steadily increases until about 3Hz.

As frequency increases the number of calcium transients per minute increases (due to retained Ca2+), which tends to load the SR with calcium, and the gain of ex/contract coupling increases (amount of calcium released per calcium stimulus)

17
Q

Force Frequency relationship in an unhealthy individual

A

At rest: someone with heart failure isn’t very different to a normal heart in terms of force and calcium load. BUT if you increase the HR they are less able to increase the force of contraction, SR calcium load stays constant and it drops off rapidly. Calcium transient remains the same.

18
Q

Effect of increasing rate/frequency

A

As each time the AP is activated/ heart contracts calcium comes into the cell.

Increasing rate leads to:

  • less time for calcium extrusion
  • decrease in the average membrane potential (-80mV) which decreases overall calcium efflux via NCX
  • increased numbers of APs lead to increased intracellular Na+ AND Ca2+ via effect on NCX

Overall effect is to “load” the SR and for the amplitude of the calcium transient to increase.

19
Q

Whats the importance of intracellular [Na+] ??

A

Na+ gradients exist within myocytes, with higher concentrations found in the sub-sarcolemmal space, as well as local “hot” and “cold” spots depending on the transporters present
Hotspot: NCX and NKA present
The NCX which are also located in the hotspots will function differently depending on the levels of intracellular Na+

Can get high levels of intracellular Na+ just after AP

20
Q

Modulation of force by Neurotransmitters

A

The heart is innervated by sympathetic and parasympathetic nerves

PS (vagal) decreases SA node discharge rate & hence force

Sympathetic nerves:

  • increase SA node discharge rate
  • increase calcium influx via calcium channels
  • increase SR pump rate (SERCA, lusitropic effect) quick SR loading
  • decrease sensitivity of troponin for calcium (seems counteractive, but enables relaxation to occur quickly)
21
Q

B-adrenergic stimulation

A

-Sympathetic nerve endings and B-ARs (1,2 and 3) are widely distributed throughout the heart.

  • B-agonists act via adenylyl cyclase, increasing cAMP. This activates cAMP-dependent protein kinase (PKA) which phosphorylates key proteins:
  • Tnl
  • SL calcium channels
  • phospholambam
  • SR calcium release channels

Activation of B1-ARs can give rise to all of the major cardiac adrenergic effects: inotropy, lusitropy and chronotropy

22
Q

Modulation of force by drugs

A

Best known: ‘cardiotonic steroids’ (eg digoxin) that increase [Na]i by inhibiting Na pump, hence reducing Ca extrusion by NCX

Sympathomimetics: acting via B-1 receptors, act like adrenaline (limited by de-sensitization, lose of #receptors)

Bipyridines: act via inhibiting phosphodiesterase that usually breaks down cAMP(increasing cAMP. Clinical trials indicate limited use

23
Q

Cardiac failure results in an increase in ___?

What does this mean for current therapies

A

ventricular dimensions which decrease efficiency.

Targeted at reversing the increase in the hearts dimmensions by decreasing filling pressure

  • NO donors to relax vasculature
  • diuretics to decrease blood voume
  • ACE inhibitors to depress angiotension
24
Q

In a myofilament Ca sensitivity graph, the sigmoidal curve indicates

A

The co-operative binding of Calcium to the troponin-C binding sites

25
Q

In isolated myocytes you measuring isotonic force, “shortening”. As pH decreases you get

A

Diminished shortening.

26
Q

Although HF patients have variable Calcium sensitivity, they ALWAYS have….

A

Decreased maximum activity/force that can be generated

Cardiovascular System (34 decks)

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