Smooth Muscle Contraction

Question 1

what is the organization of contractile proteins in smooth muscle?

Answer 1

has thick myosin and thin actin filaments that are not organized into sarcomeres. the thin filaments are anchored to dense bodies

Question 2

describe the contraction of smooth muscle.

Answer 2

it is slow in contraction and relaxation but it shortens very much

Question 3

do smooth muscle cells contain T tubules? what is the organization of the sarcolemma?

Answer 3

there are no t tubules

the sarcolemma contains calveolae- microdomains that have many cell receptors and ion channels

Question 4

what types of receptors and channels are contained within the calveiolae of smooth muscle cells?

Answer 4

muscarinic ACh receptors and adrenergic receptors

L-type Ca channels, ATP sensitive K channels, and Ca sensitive K channels

Question 5

what relationships in smooth muscle are similar to those of striated muscle?

Answer 5

temporal relationship of action potential and the concentration of Ca and its relationship to tension

Question 6

what is a tonic contraction?

Answer 6

a prolonged conraction with slower cross bridge cycling and a longer time spent in the tension generating phase of the cross bridge cycle

Question 7

what is the result of a longer tension generating phase of the cross bridge cycle?

Answer 7

greater force of contraction with less energy expenditure

Question 8

what is the reduction of length in a contracting smooth muscle?

Answer 8

up to 1/3 of original length

Question 9

what is multiunit orgainization? does this lead to spontaneous contraction?

Answer 9

composed of discrete smooth muscle fibers innervated by a single nerve ending.
seldom spontaneous contraction

Question 10

what is unitary organization of smooth muscle fibers? what kind of cell could this be called?

Answer 10

a sheet of smooth muscle connected by gap junctions -contract as a single unit
syncytium

Question 11

how is unitary smooth muscle innervated?

Answer 11

little innervation because action potentials spread from cell to cell

Question 12

which type of smooth muscle responds to stretch?

Answer 12

unitary smooth muscle

Question 13

what controls the contraction of unitary and multiunit smooth muscle? what are some examples?

Answer 13

unitary- local factors (aiway muscle, eilliary muscle of eye, piloerector muscle)
multiunit- neural factors (small vessels, GI tract, uterus and most arteriolar muscle)

Question 14

what calcium sources do smooth muscle use for contraction?

Answer 14

extracellular fluid

Question 15

what are the three pathways of initiation of calcium influx?

Answer 15

L-type Ca channels activated by depolarization
G-protein coupled receptors
Ca induced Ca release from SR

Question 16

what pathways do G coupled receptors use to cause the influx of calcium into the cell?

Answer 16

they activate phospholipase C->generates inositol triphosphate (IP3)->IP3 induced calcium release from the SR

Question 17

what is an effective way to reduce contractile activity in both smooth and cardiac muscle?

Answer 17

L-type Ca channel blocking drugs

Question 18

what are the two mechanisms of calcium reuptake after contraction?

Answer 18

Ca pumps in the sarcolemma and SR

3Na/Ca exchange across the sarcolemma

Question 19

what is capacitive calcium entry?

Answer 19

the SR is refilled by calcium from outside the cell

Question 20

how does Ca induce muscle contraction in smooth muscle?

Answer 20

Ca binds to calmodulin on myosin light chain kinase. this phosphorylates the regulatory myosin light chain and activates it

Question 21

how does smooth muscle relax?

Answer 21

MLCP (phosphatase) dephosphorylates the regulatory light chain and blocks interaction between myosin and actin

Question 22

what are the two categories of smooth muscle contraction? what do they have in common?

Answer 22

electromechanical and pharmacomechanical

both increase intracellular Ca-> activate MLCK and myosin

Question 23

what is electromechanical contraction of smooth muscle?

Answer 23

opening of Ca channels in response to stretch or depolarization

Question 24

what is pharmacomechanical contraction of smooth muscle?

Answer 24

ligand binding to cell surface receptor initiating a metabotropic response. this generates IP3 which opens Ca channels in the SR

Question 25

can smooth muscle contractile activity be spontaneous? if so in what conditions?

Answer 25

yes- in peristaltic waves of the GI tract and in uterine contractions of labor

Question 26

what type of smooth muscle does not usually spontaneously contract and what does it respond to?

Answer 26

vascular smooth muscle | contracts in response to stretch, adrenergic neurons, endothelial cells or circulating chemical factors

Question 27

what is the resting potential of smooth muscle?

Answer 27

-50- -60 mV

Question 28

how do smooth muscle action potentials compare to contraction time? what initiates superimposed spike potentials in some visceral smooth muscle?

Answer 28

spike action potentials- much shorter relative to contraction time slow waves- pacemaker waves

Question 29

in smooth muscle cells, what causes depolarization and repolarization of the action potential? what ion is not included in the action potential current?

Answer 29

depolarization- Ca repolarization- K Na not involved

Question 30

what is a basal electric rhythm? what do they dictate?

Answer 30

waves of rhythmic depolarization of intestinal smooth muscle cells (pacemaker potentials) determine frequency, propagation velocity and direction of contractions

Question 31

what must be added to the BER to initiate action potentials?

Answer 31

must be a release of stimulatory neurotransmitters from nerves to increase the depolarization above threshold

Question 32

where does the BER rhythm change speed?

Answer 32

it changes speed in different intestinal segments depending, mostly, on size (slower in the stomach and faster in duodenum)

Question 33

what extra step is included in the smooth muscle crossbridge cycle?

Answer 33

the latch bridge state occurs when the Ca concentration in the sarcoplasm fall and the myosin regulatory light chain is dephosphorylated while the head is still bound to actin

Question 34

why is the latch bridge state important?

Answer 34

the cell may need to maintain tension when intracellular Ca decreases

Question 35

why is the latch state maintained under dephosphorylation of the RLC? what happens if the myosin is not attached at this time?

Answer 35

because dissociation of myosin when it is dephosphorylated is very slow if the myosin is not already attached it will remain inactive

Question 36

what is endothelin and where is it produced?

Answer 36

endothelin is a peptide produced by vascular endothelium

Question 37

what activates endothelin from its precursor? what 6 things is this stimulated by?

Answer 37

ET-1 is cleaved by endothelin converting enzyme | stimulated by angiotensin II, antidiuretic hormone, thrombin, cytokines, ROS and shearing forces

Question 38

what two effects does endothelin have on metabolic processes?

Answer 38

activates NO production | activates ETa and ETb which phosphorylates Gq-> creates IP3

Question 39

what effect does NO have on the smooth muscle cell?

Answer 39

increases cGMP decreasing contraction and increasing relaxation

Question 40

what effect does IP3 have on the smooth muscle cell?

Answer 40

causes calcium release by the SR and therefore smooth muscle contraction

Question 41

what is the general effect of endothelin on surrounding vasculature?

Answer 41

transient vasodilation and hypotension followed by prolonged vasoconstriction and hypertension

Question 42

how does endothelin related to heart pathology?

Answer 42

it is released by failing myocardium where it can contribute to calcium overolad and hypertrophy

Question 43

how does epinephrine stimulate vasodilation in some vasculature and vasoconstriction in others?

Answer 43

the vessels that contract contain alpha 1 receptors and those that dilate have beta 2 receptors

Question 44

how does epinephrine cause vasoconstriction?

Answer 44

a1 receptors activate Gq which makes IP3 | IP3 causes the release of Ca and contraction

Question 45

how does epinephrine cause vasodilation?

Answer 45

b2 receptors activate Gs which stimulates the production of cAMP. This activates protein kinase A which phosphorylates MLCK and inactivates it (prevents myosin phosphorylation and contraction)

Question 46

Epinephrine stimulates vasoconstriction and dilation to which areas?

Answer 46

vasoconstriction- skin and gut arterioles | vasodilation- skeletal and heart arterioles and bronchiloar smooth muscle

Question 47

how is acute control of local blood flow accomplished?

Answer 47

by rapid changes in local vasodilation of arterioles and precapillary sphincters

Question 48

what causes the increase of blood flow through the tissues?

Answer 48

when availability of oxygen to the tissues decreases

Question 49

what is hyperemia?

Answer 49

when the rate of blood flow through the tissue increases

Question 50

what is the oxygen lack theory?

Answer 50

that low oxygen causes smooth muscle relaxation of the sphincter supplying that area

Question 51

what is the vasodilator theory?

Answer 51

that substances are released by active muscle to cause the relaxation of the sphincter (adenosine)

Question 52

what is an important local vasodilator?

Answer 52

adenosine

Question 53

what kind of receptors receive adenosine?

Answer 53

A2 adenosine receptor: g coupled receptors

Question 54

how does adenosine cause relaxation of sphincters?

Answer 54

Gs is activated by adenosine to stimulate adenylyl cyclase to produce cAMP->active PKA-> inhibit MLCK

Question 55

what is another theory of relaxation methods of adenosine?

Answer 55

A1 adenosine receptor: couples to ATP sensitive K+ channels-> hyperpolarization and decrease of Ca influx, relaxing the muscle

Question 56

what is stress relaxation?

Answer 56

a property of smooth muscle when there is a sudden increase in volume inside the vessel, pressure will increase and then decrease slowly isovolumetrically

Question 57

what is receptive relaxation and what does it apply to?

Answer 57

the ability of smooth muscle to relax as the volume increases. important in the GI tract

Question 58

what reflexes are triggered by stomach stretching?

Answer 58

vago-vasal and intrinsic reflexes

Question 59

what is the pathway of NO production in smooth muscle relaxation?

Answer 59

ACh binds to the muscarinic receptor and stimulates Gq. this increases IP3 and the production of nitric oxide which relaxes the smooth muscle

Question 60

what is the effect of vasoactive intestinal polypeptide on smooth muscle?

Answer 60

it binds to receptor on the smooth muscle cell and causes delayed relaxation through an increase in cAMP

Question 61

what receptor mediates NO relaxation?

Answer 61

gyanylate cyclase->produces cGMP