Lecture 12: Vascular Smooth Muscle Function Flashcards

1
Q

describe the cell shape and features of a vascular smooth muscle cells

A
  • spindle shaped
  • overlapping actin and myosin contractile unit
  • thin filament longer
  • poorly developed SR (rely primarily on diffusion of Ca2+ ion from ECF to initiate contraction)
  • no striations, appear wrinkled
  • actin filament anchored to dense body instead of Z band in cytoplasm
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2
Q

how is vascular smooth muscle contraction triggered?

A

contraction triggered by a rise in intracellular calcium

most of the intracellular calcium arises from influx of extracellular calcium

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3
Q

what are the 2 broad groups of excitation-contraction coupling? (how we get an increase in intracellular calcium)

A
  1. electromechanical coupling - depol of the vsm cell = opening of L type voltage gated calcium channels
  2. pharmacomechanical coupling - binding of signalling molecule to a receptor leads to increase in intracellular calcium via g protein or receptor operated calcium channel
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4
Q

describe the vascular smooth muscle contraction

A
  • increase in intracellular calcium caused by electromechanical or pharmacomechanical
  • calcium and calmodulin form a complex
  • calcium and calmodulin complex with myosin to form myosin light chain kinase
  • myosin LCK phosphorylised
  • myosin forms a cross bridge with actin
  • contraction of SMC
  • no troponin in SMC must form complex with calmodium
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5
Q

what is different about vascular smooth muscle?

A
  • contracts slowly, maintains tone
  • low energy requirements
  • poorly developed SR
  • cell to cell conduction with gap junctions
  • no troponin, no VO Na channels
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6
Q

what is vascular tone

A

the state of contractile tension in vessel walls
basal level not fully relaxed
vasal tone produced by vascular contraction and maintained throughout animals life - maintaining MAP

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7
Q

what are the factors controlling vascular tone?

A
  • intrinsic mechanisms; entirely within the vasculature myogenic response, vasoactive, endothelial secretions responsible for auto regulation and active/ reactive hyperaemia
  • extrinsic mechanisms: outside the vasculature: neural control, hormonal control
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8
Q

what is the myogenic response

A

intrinsic mechanism
VSM contracts in response to stretch and relaxes with reduction in tension
important because - contributes to basal tone, stabilises tissue blood flow if arterial pressure changes

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9
Q

where is the myogenic response particularly prominent?

A

cerebral and renal arteries - where we need constant oxygen

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10
Q

how is the myogenic response initiated and sustained?

A

mediated by depol and Ca2+, depol initiated by stretch-activated channels and sustained by TENSION sensitive channels

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11
Q

what re vasoactive metabolites

A

metabolic products released into interstitial fluid causing alterations in local blood flow

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12
Q

what is the function of vasoactive metabolites?

A

to match blood flow to metabolic demands of the tissue

eg/ increase metabolic work - increase oxygen demand - increase release of vasodilators and brings in more blood

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13
Q

what is metabolic hyperaemia

A

increase blood flow in response to an increase in metabolic rate (O2 demand)

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14
Q

what is reactive hyperaemia

A

temporary increase in blood flow following a period of reduced blood flow.
leads to rapid restoration of oxygen supply and wash out of accumulated waste products, restoring potentially hypoxic tissue

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15
Q

how is the duration of the reactive hyperaemia variable?

A

peak flow and duration of hyperaemia is proportional to duration of occlusion

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16
Q

what is a major function of endothelial cells

A

regulation of vascular smooth muscle tone

17
Q

what is endothelial induced flow dependent relaxation

A

with increased flow and sheer stress of endothelial cells, the EC’s produce NO

18
Q

what is NO signalling

A

sheer stress stimulates NO synthase to produce NO via L-arginine
NO diffuses to SMC and activates gunnel cyclase
GC converses GTP to cGMP
cGMP activates kinases that promote relaxation

19
Q

what is autoregulation

A

the maintenance of constant blood flow even in the face of vascular resistance
preserves tissue flow and protects capillaries from excessive hydrostatic pressure
achieved by: myogenic response and endothelial cell flow induced relaxation

20
Q

where does the myogenic response predominate?

A

in resistance vessels

21
Q

where does the metabolic vasodilators predominate?

A

in most metabolically active tissues e.g. capillaries

22
Q

what are the two major nerves involved with extrinsic neural control

A

sympathetic nerves - predom vasoconstrictor

parasymp nerves - predom vasodilator

23
Q

what do the sympathetic fibres release and how does it act?

A

symp fibres release NA
NA activates a-adrenoreceptors causing vasoconstriction
symp activity = increase TPR
decrease blood flow –> displacement of blood from periphery to central

24
Q

describe the action of NA on smooth muscle contraction

A

NA –> a-adrenoreceptor (g protein coupled) –> activates phospholipase C –> cleaves memb phospholipid –> IP3
IP3 opens channels in the SR
= DIRECT action on smooth muscle cell

25
Q

what do the parasympathetic fibres release and how does it act?

A

parasymp fibre release ACh
ACh activates M3 receptor on EC –> synth NO –> dilation
= INDIRECT action on SMC

26
Q

describe how hormonal control contributes to basal tone

A

adrenaline
from adrenal medulla
a1 and a1 = predom most tissues cause contrstriction
b1 = heart dilation
b2 = vessel dialtion
b2 innervated ONLY by circulating levels of adrenalin, not by symp fibres.

27
Q

e.g. of how extrinsic control can override intrinsic control

A

HAEMORRHAGE

  1. blood vessel not ligated properly = blood pooling, tissue start becoming hypoxic, decreased CBF
  2. blood pooling increases metabolites
  3. intrinsic system wants to dilate (would make problem worse)
  4. extrinsic overrides and begins sympathetic stim
  5. increase TPR, decrease local blood flow and vasoconstriction = displacement of blood
  6. partial restoration fo cardiac filling