Smooth Muscle Flashcards
No _____ are seen in smooth muscle
sarcomeres
What is the actin/myosin ratio in smooth muscle
10:1
Intermediate filaments are cytoskeletal elements which form a structural backbone against which contraction occurs. ____ and ____ are 2 components of this cytoskeleton
desmin; vimentin
Dense bodies contain the protein ____ and are functionally analogous to Z-lines in striated muscle.
actinin
____ _____ allow direct electrical communications between adjacent smooth muscles
gap junctions
Smooth muscle contains no T-tubules and no terminal cistern system. This feature correlates with its small size and unique lack of what?
dependence on a muscle AP to induce contraction
In smooth muscle, the SR is poorly developed. Therefore, it is more dependent on _____ ____ for contraction
extracellular Ca2+
_____ is associated with smooth muscle actin. No ____ is associated with smooth muscle actin.
tropomyosin; troponin
Although _____ and ____ are actin binding proteins whose phosphorylation can modulate smooth muscle contraction by affecting ATPase activity, class thinking is that smooth muscle regulation differs from striated muscle regulation in that it is coupled to ____ levels through myosin-based mechanism
caldesmon; calponin; Ca2+
Myosin has ___ heavy chains and ____ light chains. Which ones serve an essential regulatory function in smooth muscle but probably not in striated muscle
2; 4; light chains
Smooth muscle myosin is thought to be organized in a ____-____ arrangement as opposed to a _____ arrangement seen for striated muscles.
side-polar; bipolar
note: this likely contributes to smooth muscles’s lack of well-defined sarcomeres
There are two types of smooth muscle units: single (visceral) and multiunit. Where is the single unit (visceral) subtype found?
small intestine, colon, uterus, urinary bladder, ureters, lymph vessels, smaller arterioles
Single unit smooth muscle tends to have many ____ _____ between cells. Innervation tends to be relatively ____
gap junctions; sparse
Slow wave potentials are associated with what subtype of smooth muscle?
single (visceral) unit
Single (visceral) unit smooth muscle shows this characteristic which allows slow stretch and lengthening of the muscle without a significant increase in pressure. ex: bladder filling up to a point
plasticity
This type of contraction is seen in single (visceral) unit smooth muscle. hint: think bladder
stretch induced contraction
In regards to multiunit smooth muscle, each smooth muscle cell acts relatively independently of other smooth muscle cells in the organ. In this sense, multiunit smooth muscle is like ____ muscle
skeletal
Where is the multiunit subtype of smooth muscle found?
ciliary muscle, iris, bronchial muscles, tracheal muscles, vas deferens, GI sphincters, and larger blood vessels
Multiunit smooth muscle tends to have less ___ ____ between cells making each cell operated independently. Multiunit smooth muscle tends to have higher _____ ratios than in visceral smooth muscle
gap junctions; innervation
True or false, the membrane potential of multiunit smooth muscle is stable with no spontaneous depolarization
true
Describe the effects of progesterone and estrogen on smooth muscle in pregnancy.
progesterone: reduces the number of gap junctions in myometrial smooth muscle, making it behave more like non-innervated multiunit smooth muscle, and render it relatively quiescent.
estrogen: causes smooth muscle hypertrophy and increase the number of gap junctions. This allows the muscle to behave more as a single unit smooth muscle and participate in parturition
Intracellular calcium levels rise in smooth muscle to trigger contraction as in striated muscle. However, regulation occurs on the ____ filament, not in the ____ filament as in striated muscle.
thick (myosin); thin (actin-troponin-tropomyosin)
In regards to smooth muscle contraction, Ca2+ binds Calmodulin ___:___
4:1
The Ca2+/calmodulin complex binds to which kinase to form an active enzyme.
myosin light chain kinase
The active Ca-calmodulin-MLCK enzyme complex phosphorylates a ___ residue of the myosin light chains associated with each myosin head. This phosphorylation requires ___ ATP and is essential to confer significant ___ activity on smooth muscle myosin
serine; 1; ATPase
Myosin light chain phosphatase is continually removing the phosphates put on myosin light chains by active MLCK. If MLCK is inactive, what is the status of myosin? What about if MLCK activity is high?
MLCK inactive: myosin light chains are relatively dephosphorylated and MLCK has low ATPase activity
MLCK active: myosin is largely phosphorylated and MLCK has high ATPase activity
MLCK activity is in correspondence to low and high ____ __ conditions
intracellular Ca2+
In regards to MLCK activity, when does fact cycling and shortening of smooth muscle occur?
when MLCK/Phosphatase C activity ratio is high
This is a special state that allows smooth muscle to maintain tone with a minimal expenditure of ATP. Shortening is not occurring.
latch state
If myosin chains are dephosphorylated, myosin ATPase activity decreases. What does this mean in regards to the latch state?
it’s more difficult to release myosin heads from actin, which requires ATP hydrolysis - muscle can hold tone
Why is efficiency low in smooth muscle?
because energy is required for both control and cross bridge cycling during shortening
Why is economy high in smooth muscle?
because the ATP use is low to maintain force in the absence of external work - this is the latch state
Low levels of phosphorylated myosin light chains cause _____
relaxation
In regards to smooth muscle relaxation, Ca2+ is removed form the cell by the sarcolemmal ___/___ exchanger and a sarcolemmal ____ ATPase
3 Na+/Ca2+; Ca2+
Phospholamban is a protein that normally inhibits the SR Ca2+ ATPase. Phosphorylation of phospholamban inhibits it. What would the result of this be?
Inhibiting the inhibiter -> promotion of Ca2+ sequestration - relaxation
Resting membrane potential in smooth muscles is relatively positive, perhaps ____ mV
-55
As in skeletal and heart muscle, changes in smooth muscle membrane potential can lead to contraction by opening voltage-sensitive Ca2+ channels. Unlike skeletal and hear muscle, ____-____ potentials as well as muscle ____ ____ can result in contraction
local-graded; action potentials
Describe L-type Ca2+ channels
“L” is for long acting - channels open slowly and close slowly; affected by Ca2+ channel blockers
Describe T-type Ca2+ channels
“T” is for transient - channels open and close quickly. Rapid influx from these channels may be key to Ca2+ induced Ca2+ release from SR, which is dependent on change of Ca2+ concentration, not absolute concentration
Smooth muscle has relatively few of the voltage-gated ____ channels seen in other excitable cell types
Na+
Describe cAMP-dependent relaxation
Protein Kinase A phosphorylates MLCK and prevents the calcium-calmodulin complex from activating it
Describe cGMP-dependent relaxation
decrease in myosin light chain phosphorylation - may be due to increased phosphatase activity or decreased MLCK activity; changes are mediated through phosphorylation by cGMP-dependent protein kinase
Describe PLC-dependent contraction
IP3 is formed and releases intracellular Ca2+ stores. DAG is also made and activates PKC which can phosphorylate an number of proteins that cause smooth muscle contraction
In regards to ATP-sensitive K+ channels, ____ can decrease the intracellular [ATP], open these K+ channels, and hyper polarize the membrane
Ischemia
In regards to ATP-sensitive K+ channels, _______ closes voltage gated, L-type Ca2+ channels, reduces Ca2+ influx, and relaxes the smooth muscle
hyperpolarization
In regards to G-protein coupled K+ channels, agonsists (ACh) bind their receptors, causing receptor interaction with a G-protein of the ____ family
G alpha i
The G alpha i subunit that is released directly, binds K+ channels and opens them. The cell _____, ___-___ Ca2+ channels are ____, and relaxation occurs
hyperpolarizes; L-type; closed
