BIOL 273 - Smooth Muscle (3.5)
What causes calcium release in smooth muscle cells?
Ca2+ entering through the cell membrane causes Ca2+ release from the SR. Ca2+ dependent Ca2+ release from SR and caveolae. Need an initial influx of Ca2+ from the ECF
What does Ca2+ bind to in the cytosol of smooth muscle?
Calmodulin (CaM).
What happens when signal molecules decreases MLCP activity?
Cell becomes more sensitive to Ca2+ and contraction force increases even though [Ca2+] has not changed.
How do these properties of myosin contribute to the characteristics of the smooth muscle as a whole?
Contract more slowly and for longer periods of time tahn skeletal or cardiac muscle.
What happens when signal molecules increase MLCP activity?
Contraction process becomes desensitized to calcium (calcium signal is less effective). Myosin ATPase dephosphorylates and contraction force decreases even though cytosolic Ca2+ concentration has not changed.
Where are single-unit smooth muscles found?
Found on walls of internal organs ex: intestine, blood vessels. AKA visceral smooth muscle because it forms the walls of internal organs.
Where do most smooth muscle neurotransmitters and hormones bind to?
G protein-linked receptors. The second messenger pathways determine the muscle response.
Which second messengers trigger contraction?
IP3
What forms the cytoskeleton of smooth muscle cells?
Intermediate filaments and protein dense bodies in the cytoplasm and along the cell membrane. Cytoskeleton fibers linking dense bodies to the cell membrane help hold actin in place. Protein fibers in the extracellular matrix tie the smooth muscle cells of a tissue together and transfer force from a contracting cell to its neighbors.
What are slow wave and pacemaker potentials caused by?
Ion channels in the cell membrane that spontaneously open and close
MLC Phosphotase
Remove phosphate from myosin. Always active in the cell.
Ca2+ entry from the ECF results in what?
Results in release of SR Ca2+ and Ca2+ from caveolae. Ca2+ induced Ca2+ release.
Pacemaker potentials
Smooth muscle with oscillating membrane potnetials have regluar depolarizations that always reach threshold and fire an AP.
What controls cardiac muscle?
Sympathetic and parasympathethic control as well as hormonal control
Which division of the NS controls smooth muscle?
The autonomic NS, whereas skeletal muscle is controlled by somatic motor division
True or false: Smooth muscle lacks specialized receptor regions
True. Skeletal has motor end plates. Instead, smooth muscle receptors are found all over the cell surface. Neurotransmitter is released from autonomic neuron varicosities close to the surface of the muscle fibers and diffusses across the cell surface until it finds a receptor
Where are pacemaker potentials found?
in some cardiac muscles as well as in smooth muscle.
Is the ATP that is used to activate the myosin through phosphorylation by MLCK used for cross-bridge cycling?
no. Additional ATP is needed for each cross-bridge cycle - MLCK uses the Pi ATP to activate myosin (turn it "on") but additional ATP is needed for contraction. Only need one ATP to turn on Myosin so that it can undergo cross-bridge cycling, but for each crossbridge cycle additional ATP is needed.
true or false: chemical signals may also relax muscle tension without a change in membrane potential
true
true or false: smooth muscles have unstable membrane potentials
true. It can hyperpolarize or depolarize. Can also depolarize without firing APs. Contraction may occur after: a) an AP b) a subthreshold graded potential c) without any change in membrane potential
What are some important smooth muscles?
- bladder sphincter (more waste) open & close organ, one is mooth, other is skeletal. - intestine: move food, nutrients - walls of blood vessels: move blood
What is the latch state in smooth muscle?
- tension is maintained (myosin remains bound to actin) but no ATP is consumed -this process is not yet understood -isometric contraction -significant factor in teh ability of smooth muscle to sustain contraction without fatiguing
What alters smooth muscle sensitivity to Ca2+?
-Chemical signals such as neurotransmitters, hormones, and paracrine molecules alter smooth muscel Ca2+ sensitivity by modulating myosin light chain phosphotase activity -If MLCK and Ca2+/Calmoduling are constant but MLCP activity increase, the MLCK/MLCP ration shifts so that MLCP dominates
How is Ca2+ removed from the cytosol in smooth muscle fibres?
-Pumped back into the Caveolae and SR using ATP and to the extra-cellular env through Ca2+/Na+ anti-port, Ca2+ ATPase. Na+ wants to go into the cell naturally.
How is Ca2+ released from the SR in smooth muscle?
-SR Ca2+ release is mediated both by a ryanodine receptor (RyR) calcium release channel and by an IP3 receptor channel.
How do IP3 channels on the SR open?
-When G protein-coupled receptors activate phospholipase C signal transducation pathways -IP3 is a second messesnger, when it binds to the SR IP3-receptor channel, the channel opens and Ca2_ flows out of the SR into cytosol
What enzyme does Ca2+/CaM activate?
-activates the enzyme myosin light chain kinase (MLCK)
Single-unit arrangement of smooth muscle
-cells coupled by gap junctions. Act in unison as a single group of cells -not necessary to electrically stimulate each individual fibre -rapid cell to cell communication -neuron innervating is autonomic, have varicosities storing neurotransmitter -cells are close to autonomic varicosities get activated and spread the signal to other cells without being directly innervated
Stretch-activated channels
-ex: blood vessels -open when pressur or other force distorts the cell membrane -cell depolarizes, opening neighbouring voltage-gated Ca2+ channels -because contraction originates from a property of muscle fiber itself, it is know as myogenic contraction -some types of smooth muscle adapt if the muscle cells are stretched for an extended period of time. Why the bladder develops tension as it fills, then relaxes as it adjusts to increased volume
cardiac muscle
-features of both smooth adn skeletal muscle -straited, sarcomeres -fibers are shorter than skeletal, may be branched, have a single nucleus -electrically linked, gap junctions in specialized cell junctions known as intercalated disks -some exhibit pacemaker potentials
How do smooth and skeletal muscle compare in size at the cellular level?
-fibres much smaller in smooth muscle than skeletal muscle fibres --> about same diameter as a single myofibril in a skeletal muscle fibre
What happens to the cells of the uterus just prior to labour?
-in the uterus most of the time the smooth muscle is of the multi-unit type -just prior to labour the cells undergo remodelling --> gap junctions are formed between the cells -this turns them into smooth muscles of the single unit type -this may allow a synchronization of electrical signals resulting in more efficient contractions during labour
How is force of contraction increased in multi-unit smooth muscle?
-increasing force of contraction requires recruitment
Autonomic Neurotrasnmitters and hormons influencing smooth msucle activity
-many smooth muscles are under antagonistic control by both sympathethic and parasympathethic division of autonomic NS -some are under tonic control where only one of the 2 autonomic branches controls tehm, the response is graded by inreasing or decreasing amount of neurotransmitter released -chamical signal can have different effects in different tissues, depending on receptor type
Why does smooth muscle have slower and longer periods of contraction?
-myosin ATPase activity is slower -longer actin and myosin filaments allow longer contractions and allow smooth muscle to be stretched yet still be able to contract. In skeletal muscle too much stretching leads to too little overlap and an inability to contract.
Multi-unit arrangement of smooth muscle
-no gap junction between neighbouring cells -stimulated independently to contract -each indiviudal muscle fibre is separately innervated by an axon terminal or varicosity -allows fine control of contraction through selective activation of indv cells - ex: iris and ciliary body of eye, parts of reproductive organs -each cell has receptors and form synapses
How does the SR in smooth muscle compare to the SR in skeletal muscle?
-not much sarcoplasmic reticulum in smooth muscle - SR in smooth muscle varies -arrangement of SR in smooth muscle is less organized -SR in smooth muscle is closely associated with membrane invaginations called caveolae
How does the RyR channels on the SR open in smooth muscle?
-opens in response to Ca2_ entering the cell, process called calcium-induced calcium release (CICR)
Which type of muscle is the slowest to contract and relax?
-smooth muscle 1) Skeletal, 2) cardiac 3) smooth
What additional Ca2+ stores does smooth muscle contain?
-stored in special vesicles called caveolae
Where is smooth muscle found in the body?
-walls of hollow organs & tubes --> not attached to bones of skeleton
how is smooth muscle categorized?
1) By location - smooth mscles can be divided into 6 major groups: vascular, gastrointestinal, urinary, respiratory, reproductive, ocular 2) By contraction pattern - whether it alternates between contraction and relaxation states or whether it is continuously contracted (phasic vs. tonic) 3) By their communication with neighbouring cells: single-unit vs. Multi-unit
What are the main differences in contraction and relaxation of skeletal and smooth muscle?
1) Ca2+ comes from the ECF as well as teh SR 2) An AP is not required for Ca2+ release 3) There is no troponin so Ca2+ initiates contraction through a cascade that includes phosphorylation of myosin light chains 4) An additional step in smooth muscle relaxation is dephosphorylation of myoisn light chains by myosin phosphotase
How does relaxation occur in smooth muscle?
1) Ca2+ is removed from the cytosol 2) Decrease in Ca2_ levels in the cytosol causes Ca2_ to unbind from calmodulin which inactivates MLCK and can no longer phosphorylate myosin heads. Myosin light chains are dephosphorylated
How do signals that increase cAMP production cause muscle relazation?
1) Free cytosolic Ca2+ concentrations decrease when IP3 channels are inhibited and SR Ca2+ ATPase is activated 2) K+ leaking out of the cell hyperpolarizes it and decreases the likelihood of voltage-activated Ca2+ entry 3) Myosin phosphotase activity increases
How do pathways that increase IP3 cause contraction?
1) IP3 opens IP3 channels on teh SR to release Ca2+ 2) DAG (product of the phospholipase C signal pathway) indirectly inhibits myosin phosphotase activity. Increases MLCK/MLCP ratio
Why is it difficult to study smooth msucle?
1) Many types through the animal kingdom, several variations 2) Muscle fibre arrangement is not linear, but in many directions (difficult to study sections) 3) Electrical properties make direct stimulation difficult and smooth muscle is also hormone controlled 4) Conflicting stimuli alter response ex: can contract or relax. The same stimulus can have different effects in different regions.
What are 2 ways smooth muscle can be arragned?
1) Single unit 2) Multi-unit
tonic smooth muscles
1) Some continuously contracted because they are always maintaining some level of muscle tone. Ex: esophageal and urinary sphincters close off opening of hollow organ. Relax to allow materail to enter or leave. 1) Some have their contraction varied as needed. Ex: in blood vessesls which maintain intermediate level of contraction that are under tonic control by the NS.
How are smooth and skeletal muscle different on the whole muscle level?
1) contraction of smooth muscle changes muscle shape, not just length 2) smooth muscle develops tension (force) slowly. Takes longer to develop but lasts longer 3) smooth muscle can maintain contraction longer without fatiguing --> important because some are contracted most of the time ex: internal bladder pshincter 4) Smooth muscles must operate over a range of lenghts 5) Within an organ, the layers of smooth muscle may run in several directions 6) Smooth muscle uses less energy to generate and maintain a given amount of force. Can develop force rapidly but have the ability to slow down their myosin ATPase so that crossbridges cycle slowly as they maintain force. Fewer mitochondria and relies more on glycolysis for ATP production 7) Have small, spindle-shaped cells with single nucleus
phasic smooth muscles
1) muscles that undergo periodic contraction and relaxation cycles. ex: lower esophagus which contracts only when food passes through it. 2) Some phasic smooth muscles cycle rhythmically through contraction alternating with relaxation. ex: intestine
What are the different types of membrane potentials in smooth muscle?
1) slow wave potentials: fire APs when they reach threshold 2) pacemaker potentials: always depolarize to threshold 3) Pharmacomechanical coupling: occurs when chemcial signals change muscle tension through signal transduction pathways with little or no change in membrane potential
What 2 principles that apply to skeletal muscle alos apply to smooth muscle?
2) force is created by actin-myosin crossbridge interaction between sliding filaments 2) contraction is initiated by an increase in free cytosolic Ca2+ concentrations
Ligand-gated Ca2+ channels
AKA receptor-operated calcium channels (ROCC) -Open in response to ligand binding and allow enough Ca2+ into cell to induce calcium release from SR
What happens when myosin is not phosphorylated?
ATPase activity is blocked
What happens when myosin is phosphorylated?
ATPase is active. The phosphorylated myosin can now interact with actin and go through crossbridge cycling and allow contraction to occur in the smooth muscle cell.
How are actin and myosin arrangmed in smooth muscle?
Actin & myosin are arranged in long bundles diagnoally around periphery of cell
How do myosin and actin differ between smooth and skeletal muscle?
Actin and myosin filaments are longer and overlap more in smooth muscles. They aren't arranged in sarcomeres. Smooth muscles have less myosin. Myosin are surrounded by actin filaments and arranged so that each myosin molecules is in the center of a bundle of 12-1 actin molecules. Contractil units are arranged so that they run parallel to the long axis of the cell.
Why must smooth muscle fibers act as integrating centers?
Because several different signals might reach a muscle fiber simultaneously. Received contradicting messages from 2 sources, must integrate the signals and execute an appropriate response.
Why are smooth muscle contractions called graded contrations?
Because the force varies according to the strength of the ca2+ signal
What would happen if your bladder were lined with muscle organized in sarcomeres?
Bladder would not be able to contract. Optimal degree of overlap. Fills up with urine, stretches teh bladder. If arranged in sarcomeres, when streteched it cannot contract.
How can contraction in smooth muscle be initiated?
By electrical or chemical signals or both.
What controls contraction in smooth muscle?
Cytosolic Ca2+ levels control contraction.
How does Ca2+ enter from the ECF into the ICF?
Different types of receptors 1) through voltage gated channels --> opens when cell depolarizes. 2) through stretch activated channels (mechanical) --> open when membrane stretched 3) Through chemically gated channels --> open in response to hormones
How is it possible for smooth muscle to contract without any change in membrane potential? i.e. pharmacomechanical coupling
Due to signal transduction through neurotransmitters, hormones, or paracrine signals. May be either excitatroy or inhibitory, modulate contraction by second messenger action at the level of myosin as well as by influencing Ca2+ signals. SIgnal transduction may cuase muscle relaxation as well as contraction.
What did scientist originally believe nitric oxide was?
Endothelium-derived relaxing factor, EDRF.
How is the myosin in smooth muscle regulated? How does this differ from skeletal muscle?
In smooth muscle myosin is regulated via phosphorylation of myosin. Thus MLCK regulates the interaction between actin and myosin. In skeletal muscle, myosin: actin interaction is regulated via troponin/tropomyosin
What is the signal to initiate contraction in smooth muscle?
Increase in cytosolic Ca2+, influx from the ECF.
Is there more myosin per unit actin smooth muscle or skeletal muscle?
Less myosin per unit actin in smooth muscle than skeletal muscle. Have more actin in smooth. actin to myosin ratio is 10-15: 1, in striated muscle it's 2-4:1.
what happens when MLCK is activated by Ca2+/CaM?
MLCK activates myosin by phosphorylating the light chain of the myosin molecule in the head using energy and Pi from ATP. This ATP is used to activate the myosin through phosphorylation.
Is Myosin ATPase activity faster in mooth or skeletal muscle?
Myosin ATPase is faster in skeletal, and much slower in smooth muscle. Due to different isoforms of myosin which affects hyrolysis of ATP.
Where are myosin heads located in smooth muscle?
Myosin heads are located along all parts of myosin molecule, not just at the ends.
What is an important paracrine molecule that affects smooth muscle contraction?
Nitric oxide. Synthesized by endothelial lining of blood vessels and relaxes adjacent smooth muscle that regulates the diameter of blood vessels.
What is the effect of not having T-tubules?
No direct coupling of the AP of Ca2+ release from the SR through DHP receptor-ryanodine receptor coupling as in skeletal muscle.
Are there T-tubules in the sarcolemma of smooth muscle?
No,
Does smooth muscle compose a large % of body weight?
No, but much more improtant in how the body functions. Move material through the body, 40% of body weight is skeletal muscle.
Has smooth muscle been studied for a long time?
No, only recently. We know more about skeletal muscle.
Do smooth muscle have troponin?
No. However it is associated with tropomyosin.
Does myosin automatically relax after being dephosphorylated?
No. This allows the smooth muscle to enter the latch state.
Are actin and myosin arranged into sarcomeres in smooth muscle?
No. Thus, no banding pattern (no striations).
How is force of contraction increased in single-unit/visceral smooth muscle?
Since all fibers contract every time, no reserve units are left to be recruited to increase contraction force. Instead the amount of Ca2+ that enters the cell determines force of contraction.
electromechanical coupling
Smooth muscle contraction caused by electrical signaling
What is the major difference between contraction of smooth muscle and cardiac muscle?
The role of phosphorylation in regulating the smooth muscle contraction process
How doe the cell monitor their SR ca2+ stores?
When SR Ca2+ stores decrease, a protein sensor (STIM1) on the SR membrane interacts with store-operated Ca2+ channels on the cell membrane. These Ca2+ channels, made form the protein Orai-1, then open to allow more Ca2+ into the cell. The Ca2+ ATPase pumps the cytosolic Ca2+ into the SR.
Where are actin molecules anchored in smooth muscle?
actin anchored at cell membrane structures called dense bodies in smooth muscle. Not attached to the Z lines as in skeletal muscle.
Where are caveolae located?
along the cell membrane
How are myosin light chains dephosphorylated?
by myosin light chain phosphotase
Which second messengers promote relaxation?
cAMP
slow wave potentials
cells that exhibit cyclic depolarization and repolarization of their membrane potentials. Sometimes the cell simply cycles through a series of subthreshold slow waves, however if the peak of the depolarization reaches threshold, APs fire, followed by contraction.
Where are myogenic contractions common?
common in blood vessels that maintain a certain amount of tone.
pharmacomechanical coupling
contractions initiated by chemical signals without a significant change in membrane potential.
MLCK
enzyme that adds a phosphate group, phosphorylates (kinase). Needs to be activated, is inactive in the cell.
How can paracrine signals alter smooth muscle contraction?
ex: Asthma --> smooth muscle of airways constricts in response to histamine release, reversed when administration of epinephrine which relaxes smooth muscle.
Voltage-gated Ca2_ channels
open in response to a depolarizing stimulus. AP maybe generated in muscle cell or may enter from neighboring cells via gap junctions. Subthreshold graded potentials may open a few Ca2+ channels, which opens additional voltage-gated Ca2+ channels. Sometimes chemical signal molecules open cation channels, resulting depolarization opens the Ca2+ channels.
What is the force of contraction proportional to in smooth muscle?
the amount of Ca2+ released
What determines the contraction state of smooth muscle?
the ratio of MLCK to MLCP activity. MLCP is always active to some degree in smooth muscle, so the activity of MLCK is the critical factor. MLCK activity depends on Ca2+/calmodulin