Smooth Muscle Contraction Flashcards

1
Q

A smooth muscle has thick and thin filaments but lacks what structure?

A

A sarcomere. The thin filaments are anchored to a cytoskeletal specialization called a dense body.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What characterizes smooth muscle contractions?

A

They are have slow contraction velocity and slow relaxation time, but large contraction (1/3 initial size)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

How is the action potential of smooth muscle propagated?

A

It is Ca dependent instead of Na dependent. The inward depolarizing current is carried out by Ca ions.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What are caveoli? What do they “replace”?

A

Caveoli are micro domains that are enriched for cell receptors and ion channels. They function similar to the T-tubules in other muscle types. T-tubules are not found in smooth muscle. The caveoli are thus in close proximity to SR or mitochondria.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is unitary smooth muscle?

A

It refers to smooth muscle in which millions of cells organized into sheets or bundles contract in a coordinated fashion as a single unit. The smooth muscle is electrically coupled via gap junctions and can be spontaneously active (ie. peristalsis)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is multiunit smooth muscle?

A

It is composed of discrete smooth muscle fibers each of which is innervated by a single nerve ending as in skeletal muscle.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Which muscle types utilize two sources of Ca, one from the SR and one from the extracellular fluid?

A

Cardiac and smooth muscle

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Decreasing the influx of Ca with L-type Ca channel blocking drugs is an effective means of reducing contractile activity in what muscle types?

A

Smooth and cardiac muscle.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

In smooth muscle there are multiple ways for Ca to enter a cell, what are they?

A

Activation of a G protein (Gq), activates phospholipase C leads to formation of IP3 and the release of Ca from SR. There is also some Ca induced Ca release in smooth muscle. Some smooth muscle opens Ca channels in response to stretch.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

There is no Troponin C in smooth muscle so how is contraction triggered?

A

Calcium activation of Myosin Light Chain Kinase (MLCK) results in contraction of smooth muscle. Ca binds to a calmodulin moiety on MLCK resulting in phosphorylation of the Regulatory Light Chain (RLC). The conformation change permits myosin to interact with actin.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

How does smooth muscle relax?

A

MLCP, a phosphatase in the sarcoplasm, dephosphorylates the regulatory light chain of myosin inhibiting the interaction of myosin and actin causing the muscle to relax. Reduction of Ca also results in relaxation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What are the two categories of smooth muscle contraction?

A

Electrochemical (involving an AP or stretch) and pharmacomechanical (ligand binding to cell surface receptor). They both have the same downstream effect and operate via increased cytosolic Ca and MLCK

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Describe the membrane potential of smooth muscle and the role of specific ions in APs

A

The resting potential is -50 - -60mV. Depolarization is caused mainly by an inward current of Ca followed by depolarization by an outward current of potassium. Most smooth muscle APs have no Na current.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is the basal electric rhythm?

A

It refers to waves of rhythmic depolarization of intestinal smooth muscle cells which originate at a specific point and then are propagated along the length of the GI tract. There is no tension produced with the depolarizations unless stimulatory neurotransmitters are added.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

In smooth muscle the cross bridge cycle includes what additional step?

A

The “latch-bridge” state. Despite drops in Ca and MLC dephospho rylation the muscle maintains tension. This occurs when MLCP dephosphorylation of the myosin light chain happens while the myosin head IS bound to actin. If it is NOT bound then it doesn’t occur.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

How does endothelin produce dual acting effects?

A

ETa and ETb receptors both linked to Gq proteins are responsible for the different effects seen during the administration of endothelin. Systemic administration causes endothelial ETb activation and vasodilation followed by prolonged vasoconstriction (smooth muscle Eta and ETb activation

17
Q

Where are alpha 1 receptors found? What happens when they bind their agonist?

A

They are found in all arterioles and binding results in vasoconstriction. The alpha 1 receptor is Gq protein. The Gq protein creates IP3 which releases Ca and ultimately causes MLCK activation. Skin and gut arterioles have alpha 1.

18
Q

Where are beta 2 receptors found? What happens when they bind their agonist?

A

They are found predominantly in arterioles in skeletal muscle and heart muscle, as well as, smooth muscle in the bronchioles. The result is vasodilation. b-2 receptors act via Gs to stimulate adenylate cyclase which produces cAMP and activates PKA which pohsporylates and inactivates MLCK.

19
Q

How is acute control of blood flow achieved?

A

By rapid changes in local vasodilation or vasoconstriction of the arterioles. It occurs within second to minutes to provide very rapid maintenance of appropriate local tissue blood flow. (compensate for Oxygen loss)

20
Q

What effect does adenosine have on smooth muscle?

A

It causes relaxation in skeletal arteriolar SM and heart arteriolar SM. It does so by binding A2 receptor which is Gs coupled leading to cAMP and PKA activity. A1 receptors can couple ATP-sensitive K channels causing SM hyper polarization and decrease Ca influx.

21
Q

What is stress relaxation in smooth muscle?

A

There is a decline in pressure (stress) over time at a instant volume (strain). Relaxation of smooth muscle relates to the content of elastin.

22
Q

What releases NO?

A

Autonomic neurons AND endothelial cells that line blood vessels.

23
Q

What is the mechanism of action for NO? Describe the first and second phase.

A

NO activates guanylyl cyclase and raises cGMP which contributes to the first phase of relaxation. cGMP activates CGMP dependent protein kinase I cGK1 which though several steps elevates MLCP activity. In the second phase VIP binds to receptors on the smooth muscle cell and causes a delayed relaxation through an increase in cAMP or a decrease in Ca.

24
Q

What is responsible for rapid phase of relaxation? What about delayed relaxation?

A

Acetylcholine and NO for rapid. VIP for delayed.