Smooth Muscle Contraction Flashcards
what is the organization of contractile proteins in smooth muscle?
has thick myosin and thin actin filaments that are not organized into sarcomeres. the thin filaments are anchored to dense bodies
describe the contraction of smooth muscle.
it is slow in contraction and relaxation but it shortens very much
do smooth muscle cells contain T tubules? what is the organization of the sarcolemma?
there are no t tubules
the sarcolemma contains calveolae- microdomains that have many cell receptors and ion channels
what types of receptors and channels are contained within the calveiolae of smooth muscle cells?
muscarinic ACh receptors and adrenergic receptors
L-type Ca channels, ATP sensitive K channels, and Ca sensitive K channels
what relationships in smooth muscle are similar to those of striated muscle?
temporal relationship of action potential and the concentration of Ca and its relationship to tension
what is a tonic contraction?
a prolonged conraction with slower cross bridge cycling and a longer time spent in the tension generating phase of the cross bridge cycle
what is the result of a longer tension generating phase of the cross bridge cycle?
greater force of contraction with less energy expenditure
what is the reduction of length in a contracting smooth muscle?
up to 1/3 of original length
what is multiunit orgainization? does this lead to spontaneous contraction?
composed of discrete smooth muscle fibers innervated by a single nerve ending.
seldom spontaneous contraction
what is unitary organization of smooth muscle fibers? what kind of cell could this be called?
a sheet of smooth muscle connected by gap junctions -contract as a single unit
syncytium
how is unitary smooth muscle innervated?
little innervation because action potentials spread from cell to cell
which type of smooth muscle responds to stretch?
unitary smooth muscle
what controls the contraction of unitary and multiunit smooth muscle? what are some examples?
unitary- local factors (aiway muscle, eilliary muscle of eye, piloerector muscle)
multiunit- neural factors (small vessels, GI tract, uterus and most arteriolar muscle)
what calcium sources do smooth muscle use for contraction?
extracellular fluid
what are the three pathways of initiation of calcium influx?
L-type Ca channels activated by depolarization
G-protein coupled receptors
Ca induced Ca release from SR
what pathways do G coupled receptors use to cause the influx of calcium into the cell?
they activate phospholipase C->generates inositol triphosphate (IP3)->IP3 induced calcium release from the SR
what is an effective way to reduce contractile activity in both smooth and cardiac muscle?
L-type Ca channel blocking drugs
what are the two mechanisms of calcium reuptake after contraction?
Ca pumps in the sarcolemma and SR
3Na/Ca exchange across the sarcolemma
what is capacitive calcium entry?
the SR is refilled by calcium from outside the cell
how does Ca induce muscle contraction in smooth muscle?
Ca binds to calmodulin on myosin light chain kinase. this phosphorylates the regulatory myosin light chain and activates it
how does smooth muscle relax?
MLCP (phosphatase) dephosphorylates the regulatory light chain and blocks interaction between myosin and actin
what are the two categories of smooth muscle contraction? what do they have in common?
electromechanical and pharmacomechanical
both increase intracellular Ca-> activate MLCK and myosin
what is electromechanical contraction of smooth muscle?
opening of Ca channels in response to stretch or depolarization
what is pharmacomechanical contraction of smooth muscle?
ligand binding to cell surface receptor initiating a metabotropic response. this generates IP3 which opens Ca channels in the SR
can smooth muscle contractile activity be spontaneous? if so in what conditions?
yes- in peristaltic waves of the GI tract and in uterine contractions of labor
what type of smooth muscle does not usually spontaneously contract and what does it respond to?
vascular smooth muscle
contracts in response to stretch, adrenergic neurons, endothelial cells or circulating chemical factors
what is the resting potential of smooth muscle?
-50- -60 mV
how do smooth muscle action potentials compare to contraction time? what initiates superimposed spike potentials in some visceral smooth muscle?
spike action potentials- much shorter relative to contraction time
slow waves- pacemaker waves
in smooth muscle cells, what causes depolarization and repolarization of the action potential? what ion is not included in the action potential current?
depolarization- Ca
repolarization- K
Na not involved
what is a basal electric rhythm? what do they dictate?
waves of rhythmic depolarization of intestinal smooth muscle cells (pacemaker potentials)
determine frequency, propagation velocity and direction of contractions
what must be added to the BER to initiate action potentials?
must be a release of stimulatory neurotransmitters from nerves to increase the depolarization above threshold
where does the BER rhythm change speed?
it changes speed in different intestinal segments depending, mostly, on size (slower in the stomach and faster in duodenum)
what extra step is included in the smooth muscle crossbridge cycle?
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
why is the latch bridge state important?
the cell may need to maintain tension when intracellular Ca decreases
why is the latch state maintained under dephosphorylation of the RLC? what happens if the myosin is not attached at this time?
because dissociation of myosin when it is dephosphorylated is very slow
if the myosin is not already attached it will remain inactive
what is endothelin and where is it produced?
endothelin is a peptide produced by vascular endothelium
what activates endothelin from its precursor? what 6 things is this stimulated by?
ET-1 is cleaved by endothelin converting enzyme
stimulated by angiotensin II, antidiuretic hormone, thrombin, cytokines, ROS and shearing forces
what two effects does endothelin have on metabolic processes?
activates NO production
activates ETa and ETb which phosphorylates Gq-> creates IP3
what effect does NO have on the smooth muscle cell?
increases cGMP decreasing contraction and increasing relaxation
what effect does IP3 have on the smooth muscle cell?
causes calcium release by the SR and therefore smooth muscle contraction
what is the general effect of endothelin on surrounding vasculature?
transient vasodilation and hypotension followed by prolonged vasoconstriction and hypertension
how does endothelin related to heart pathology?
it is released by failing myocardium where it can contribute to calcium overolad and hypertrophy
how does epinephrine stimulate vasodilation in some vasculature and vasoconstriction in others?
the vessels that contract contain alpha 1 receptors and those that dilate have beta 2 receptors
how does epinephrine cause vasoconstriction?
a1 receptors activate Gq which makes IP3
IP3 causes the release of Ca and contraction
how does epinephrine cause vasodilation?
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)
Epinephrine stimulates vasoconstriction and dilation to which areas?
vasoconstriction- skin and gut arterioles
vasodilation- skeletal and heart arterioles and bronchiloar smooth muscle
how is acute control of local blood flow accomplished?
by rapid changes in local vasodilation of arterioles and precapillary sphincters
what causes the increase of blood flow through the tissues?
when availability of oxygen to the tissues decreases
what is hyperemia?
when the rate of blood flow through the tissue increases
what is the oxygen lack theory?
that low oxygen causes smooth muscle relaxation of the sphincter supplying that area
what is the vasodilator theory?
that substances are released by active muscle to cause the relaxation of the sphincter (adenosine)
what is an important local vasodilator?
adenosine
what kind of receptors receive adenosine?
A2 adenosine receptor: g coupled receptors
how does adenosine cause relaxation of sphincters?
Gs is activated by adenosine to stimulate adenylyl cyclase to produce cAMP->active PKA-> inhibit MLCK
what is another theory of relaxation methods of adenosine?
A1 adenosine receptor: couples to ATP sensitive K+ channels-> hyperpolarization and decrease of Ca influx, relaxing the muscle
what is stress relaxation?
a property of smooth muscle
when there is a sudden increase in volume inside the vessel, pressure will increase and then decrease slowly isovolumetrically
what is receptive relaxation and what does it apply to?
the ability of smooth muscle to relax as the volume increases. important in the GI tract
what reflexes are triggered by stomach stretching?
vago-vasal and intrinsic reflexes
what is the pathway of NO production in smooth muscle relaxation?
ACh binds to the muscarinic receptor and stimulates Gq. this increases IP3 and the production of nitric oxide which relaxes the smooth muscle
what is the effect of vasoactive intestinal polypeptide on smooth muscle?
it binds to receptor on the smooth muscle cell and causes delayed relaxation through an increase in cAMP
what receptor mediates NO relaxation?
gyanylate cyclase->produces cGMP
