Smooth Muscle
What ion channels and pumps can result in smooth muscle relaxation
A) CA+-ATPase pumps in SR or plasma membrane B) Na+/Ca+-exchanger (3 Na+ for 1 Ca+) in plasma membrane uses Na+ gradient to pump Ca+ out of cell
When would cAMP and cGMP rise inside a cell
Gs protein coupled events Beta agonists
Termination of smooth muscle contraction
Intracellular Ca+ is pumped back into SR and outside cell. This activates a myosin light chain phosphatase which will dephosphorylate the myosin light chain and myosin can no longer bind to actin
How does opening or closing of smooth muscle ion channels can lead to contraction or relaxation of smooth muscle.
Relaxation • Atrial natriuretic peptide binds receptor --> activates Guanyl cyclase --> increase cGMP --> activates a protein kinase --> phosphorylation of ion channels that increases K+ channels and decreases Ca+ channels --> relaxation o cGMP also increases phosphatase --> relaxation Contraction • angiotensin II binds receptor --> activates PLC --> increased DAG --> increased PKC --> increased Ca+ channels --> contraction • PLC also leads to increased IP3 --> increased Ca+ from SR --> contraction
How do changes in cGMP alter smooth muscle contraction
Rise in cGMP --> dephosphorylation of myosin light chain --> smooth muscle RELAXATION o Nitric Oxide activates guanyl cyclase, which results in increased cGMP o Viagra acts to increase intracellular cGMP which results in vasodilation of penile blood vessels and increase blood flow to the penis
Phasic smooth muscle contraction
o Intestine—muscles contract with a regular periodicity in order to propel the GI contents through the intestine o Bladder - muscles contract periodically when needed to empty Similar to a skeletal muscle twitch o Ca+ levels rise and then fall back down to baseline o The rise and fall in intracellular Ca+ first activates the contractile machinery and then turns it off o Consequently the force generated by the muscle rises and then falls o The time course is slower than for skeletal muscle
What channel is responsible for the upstroke in smooth muscle?
v-g Ca+ channel that has 2 gates, and immediately inactivates
What channel is responsible for the repolarization of smooth muscle?
v-g K+ channels that open more slowly than the Ca+ channels, and have only one gate (do not inactivate)
What ion channels and pumps can result in smooth muscle contraction
*From SR* 1. IP3 pathway : cell surface receptor activation --> increase IP3 --> activates IP3-gated (ligand gated) Ca+ channel in SR 2. Ca+-Induced-Ca+ Release: Ca+ entry into cell can trigger Ca+ release from SR *From outside* 1. V-G Ca+ channels: Depolarization of smooth muscle (via diff mechanisms) can open v-g Ca+ channels 2. Voltage-Independent store Operated channels (SOC) : ♣ Depletion of Ca+ from the SR can lead to activation of voltage-independent channels in the plasma membrane ♣ These channels are Store Operated Channels (SOCs) ♣ Activation of SOCs allows intracellular Ca+ to remain elevated even after the SR has been depleted of Ca+ 3. Agonist Activated Channels: ♣ Activated by binding of different hormones to different receptors found on smooth muscle cells ♣ Activation of the receptor triggers and intracellular cascade which ultimately leads to a change in activity of the plasma membrane Ca+ channels ♣ Aka Receptor Operated Channels (ROCs) 4. Stretch activated Channels: ♣ Some smooth muscle cells such as that around blood vessels contain mechanosensitive channels that respond to deformations of the plasma membrane Ex: mechanosensitive channels in smooth muscle cells that surround blood vessels open in response to stretch when the diameter of the blood vessels increase
What are 3 ways to control smooth muscle
1) ANS 2) Hormones 3) Stretch of the muscle
Maximum velocity in smooth muscle is dependent on what 2 factors
1) Myosin ATPase rate - SIMILAR to skeletal muscle 2) % Phosphorylation of cross-bridges -DIFFERENT from skleletal muscle
What are the 2 types of smooth muscle
1) Unitary/Synctial/Visceral 2) Multi-unit
What are the types of ion channels involved in a smooth muscle action potential
1) Upstroke of AP = V-g Ca+ channels that have 2 gates and are inactivated immediately after the inactivation gate opens -- Nernst for Ca+ is 132—when v-g Ca+ channels open, the membrane depolarizes towards 132 mV 2) Repolarization of AP = V-g K+ channels open with a time delay o As Ca+ channels inactivate, more K+ channels open o K+ channels have only one gate and repolarization towards the K+ Nernst closes the K+ channels
what is alpha actinin?
Dense bodies contain alpha-actinin, which is also found in skeletal muscle
Ca+ triggers contraction in skeletal, cardiac, and smooth muscle. However, there are important differences in the way in which Ca+ triggers contraction
1. In smooth and cardiac: Ca+ enters cell from both SR and from outside the cell. Sometimes, extracell Ca+ is main source of Ca+. In skeletal, Ca+ only enters from SR. 2. Smooth muscle does NOT contain troponin, so there is no inhibition by a troponin/tropomyosin complex. Instead, the myosin light chain needs to be activated by MLCK which is activated by Ca+-Calmodulin. 3. Contraction is activated in smooth muscle by MLCK which phosphorylates the myosin light chain. In smooth muscle, myosin and actin normally do not bind to each other; Ca+ triggers the binding of myosin to actin via the Kinase. In skeletal muscle, myosin and actin are prevented from binding to each other, and Ca+ removes the inhibition In smooth, myosin must be activated to want to bind actin 4. Smooth muscle contains a phosphatase which removes the phosphate from the myosin light chain
___ agonists cause vasodilation of smooth muscle
Beta 2 agonists
Sliding Filament Mechanism for smooth muscle
During contraction, myosin cross-bridges attach to actin and they move relative to each other This is the same as in skeletal muscle. What is different is the regulation and initiation
In tonic smooth muscle contraction, what keeps Ca+ levels from falling down to baseline?
Fist, Ca+ influx from SR causes initial rise. Then Ca+ influx from Ca+ channels in plasma membrane maintain levels above baseline
What does it mean that smooth muscle is very heterogeneous?
In each organ, smooth muscle is composed of cells that have both unitary and multi-unit characteristics
How can the velocity of contraction in smooth muscle can be regulated.
In skeletal muscle, one AP always increases intracell Ca+ by the SAME amount, and this always generates the same twitch force (Due to one AP). Thus, skeletal muscle contraction is all or none In smooth muscle: o Intracell Ca+ can rise to different values depending on the stimulus (depending on how many open Ca+ channels) --> this in turn controls the # of phosphorylated cross-bridges • A larger intracell Ca+ will activate more MLCK which will phosphorylate more cross-bridges increased shortening velocity Thus, maximum velocity in smooth muscle is dependent on 2 factors 1) Myosin ATPase rate - SIMILAR to skeletal muscle 2) % Phosphorylation of cross-bridges -UNLIKE skleletal muscle
What activates contraction in smooth muscle?
In smooth, there is Ca+-activation of contraction just like in skeletal. But the mechanism of activation is DIFFERENT! 1) Ca+ enters cell from BOTH extracell space AND from SR (in skeletal, its only from SR) 2) Ca+ binds CALMODULIN, a cytoplasmic Ca+ binding protein 3) Ca+-Calmodulin complex binds and activates MLCK 4) The Ca+-Calmodulin-MLCK complex phosphorylates the myosin light chain this allows the myosin head to bind to actin 5) Cross-bridge cycling and then contraction follows
What is the latch mechanism?
Latch mech explains how tonic smooth muscle can contract for extended periods of time with minimal use of ATP • This is important in vasculature smooth muscle which must remain contracted for extended periods of time • The myosin light chain phosphatase can remove a phosphate at any point in the cross bridge cycle • Thus, instead of 4 states there are really 8 states o 4 with the myosin light chain phosphorylated (1, 2, 3, 4) o 4 with the myosin light chain dephosphorylated (1*, 2*, 3*, 4*) • the 4 states with the phosphate attached (1, 2, 3, 4) cycle at a much faster rate than the 4 states without the phosphate (1*, 2*, 3*, 4*). Thus, removal of a phosphate moves the cycle to a set of 4 states with slower cross-bridge cycling rate • If the phosphatase removes a phosphate when the cross-bridge is STILL ATTACHED, the cross-bridge thin filament complex will move to the slower cycling state (From 2 to 2*). • Because the cycle is now slower, the myosin head will remain attached to the thin filament for LONGER and thus continue to be able to MAINTAIN force • In this way, smooth muscle can maintain force for longer periods of time WITHOUT USING UP ATP, although the cycling rate will be slower • Thus smooth muscle maintains force at expense of velocity
Contraction/Relaxation without Rise in Intracellular Ca+
NT or Hormone --> GPCR --> AC activated --> increased cAMP --> inactivates MLCK --> relaxation
Is an AP required for smooth muscle contraction
No
What is Unitary/Synctial/Visceral muscle
Smooth muscle cells are connected in a sheet, both electrically and mechanically The sheet acts as a single unit -- all cells contract together ex= some blood vessels, smooth muscle around alimentary tract, smooth muscle around ureter
What attaches to Z disks
Thin filaments from 2 different smooth muscle cells connect to the same dense body --> allows smooth muscle cells to contract as a unit
Tonic smooth muscle contraction
o Sphincters - muscle remain contracted for long periods of time and periodically relax. Their maintained state of contraction is referred to as tone o Blood vessels - muscles remain contracted (tone) and contract or relax slightly relative to this baseline, in order to increase or decrease the blood vessel diameter o The Ca+ levels rise but fall back to a level above baseline o The initial rise in Ca+ is due to Ca+ influx from SR o The sustained level of Ca+ above baseline is likelt a constant influx of Ca+ through Ca+ channels in the plasma membrane o During this sustained Ca+ plateau, the contractile machinery remains activated and the muscle continues to generate force
What is multi-unit muscle
smooth muscle cells are NOT connected and function independently. Ex = eye muscles (ciliary, iris), and pilorector muscles of the skin
The dense bodies in smooth muscle are analogous to
the Z disks of skeletal muscle
Describe the organization of smooth muscle
• Both skeletal and smooth have thin and thick filaments (Actin and myosin) • In skeletal, they are arranged in Z-lines. No Z-lines in smooth muscle. • In smooth, there are dense bodies that attach to thin (Actin) filaments o Thin filaments from 2 different smooth muscle cels are connected to the same dense body o This provides a mechanical linage between smooth muscle cells and allows the smooth muscle cells to contract as a unit • Intermediate filaments link the dense bodies into a cytoskeletal unit
Contraction of Smooth Muscle with AP
• Hormone or NT binds to receptor on surface of smooth muscle cell triggers AP by activating both v-g Ca+ and v-g K+ cells o Mech of opening of channels is via a cascade of events after the NT or Hormone bind to their receptor ♣ 1) opening of Ca+ channel in plasma membrane (Receptor Operated Channel ROC) depolarization opening of v-g Ca+ channels and v-g K+ channels AP ♣ 2) Closing plasma membrane K+ channel (not a v-g K+ channel). Closing a K+ channel will depolarize the membrane due to decreased K+ outflow open v-g Ca+ and v-g K+ channels AP ♣ 3) opening of Na+ channel (not v-g) depolarization open v-g Ca+ and K+ channels AP *note: the NT or hormone can have the opposite effect on the ion channels listed above and close a Ca+ channel, open K+ channel, close Na+ channel, thus causing membrane potential to be further from threshold and make the cell less likely to fire AP
Describe the cross-bridge cycling steps in smooth muscle contraction
• In smooth muscle, the myosin-ADP-Pi complex and actin do not readily interact. This is in contrast to skeletal muscle where myosin and actin will readily interact but are prevented from doing so by the troponin-tropomyosin complex—as soon as the inhibition is removed, they interact 1) Ca+ Regulation -- ADP+Pi are bound to myosin= myosin has LOW affinity for actin -- Ca+ influx --> Phosphorylation of the myosin light chain --> myosin-ADP-Pi complex bind actin ♣ * the ONLY step DIFFERENT than skeletal muscle* 2) Powerstroke -- When ADP, Pi, and myosin are bound to actin, a conformational change is induced= rotates the head 45 degrees = moves the thin filament relative to the thick filament. -- This movement results in a dissociation of ADP+Pi from myosin - SIMILAR TO SKELETAL MUSCLE! 3) The actin-myosin complex has a high affinity for ATP. When ATP binds to the actin-myosin complex, myosin dissociates from actin—SIMILAR TO SKELETAL MUSCLE 4) ATP hydrolysis by the myosin head rotates the myosin-ADP-Pi complex by 45 degrees - SIMILAR TO SKELETAL MUSCLE • **cross-bridge cycling will continue until the intracellular Ca+ concentration decreases and the phosphatase removes the phosphate from the myosin light chain
How do changes in cAMP alter smooth muscle contraction
• Rise in intracell cAMP occurs by activation of GPCR such as Beta receptor • This increases rate of phosphorylation and INACTIVATION of the MLCK --> smooth muscle RELAXATION • Beta 2 is common receptor in smooth muscle o Compare this to cAMP in heart muscle where increased cAMP had the OPPOSITE effect: ♣ In heart, increased cAMP activates PKA which phosphorylates the L-type Ca+ channel in plasma membrane and Ca+ channel in SR cardiac muscle CONTRACTION ♣ Beta 1 is common receptor in heart
How can smooth muscle contraction be controlled by the autonomic nervous system and by hormones
• Smooth muscle can be controlled by the ANS and by hormones and by stretching of the muscle cell • Neurotransmitters or hormones bind to different receptors Autonomic control o AP in autonomic nerve that innervates the muscle can elicit an AP in the muscle o The AP travels throughout the muscle cell and to any muscle cells connected by gap junctions o Smooth muscle does not have T tubule system o Can be excitatory or inhibitory Hormonal control o Hormone can bind to smooth muscle cell, elicit AP and cause cell to contract o Hormone can also bind to smooth muscle cel and increase intracellular Ca+ without an AP and thus cause contraction without cell firing an AP
What determines the maximum velocity of contraction in smooth muscle, cardiac muscle and skeletal muscle
• The velocity of shortening vs load for smooth muscle has same basic shape as velocity vs load for skeletal muscle • The difference is the y-intercept - the max velocity of shortening with no load • The maximum shortening velocity at is much SLOWER for smooth muscle than for skeletal muscle. This is because the max velocity of shortening is DEPENDENT ON THE RATE OF ATP HYDROLYSIS by the myosin head o The rate of ATP hydrolysis by the myosin head is slower in smooth muscle than in skeletal muscle note: • Slow skeletal muscle = type 1 slow twitch muscle fibers - myosin has low ATPase rate • Fast skeletal muscle - type II fast twitch muscle fibers - myosin has high ATPase rate
Contraction of Smooth Muscle without AP
• steps are identical to above, except that the depolarization cuasied b opening of Ca+ channel or Na+ channel or closing of K+ channel does not depolarize above threshold to open enough Ca+ channels to trigger AP. Instead, the influx of Ca+ through v-g Ca+ channels activates the contractile apparatus but the cell does not fire AP • 1) Hormone or NT can bind to its receptor and cause contraction by opening (by phosphorylation) a Ca+ channel, which lets enough Ca+ in to activate the contractile apparatus • 2) Hormone or NT can activate a GPCR which via cascade leads to increase IP3 and opens Ca+ channel in SR
What is the significance of voltage-independent store operated channels in the PM?
♣ Depletion of Ca+ from the SR can lead to activation of voltage-independent channels in the plasma membrane ♣ These channels are Store Operated Channels (SOCs) ♣ Activation of SOCs allows intracellular Ca+ to remain elevated even after the SR has been depleted of Ca+
