9 - Smooth muscles physiology Flashcards
G actin molecules form a chain of ________,
and 2 of them will form a double-helical actin chain.
F actin
_________ is absent from the thin filament of the smooth muscle. Instead, ______ is used as the regulatory protein that binds Ca2+.
Troponin;
Calmodulin
- in skeletal muscles, TnC complex is formed (Troponin bound to Ca2+)
In smooth muscles, there is
A. regular arrangement of protein filament
B. sarcomere
C. no striation
C only
In smooth muscles,
A. there is more actin than myosin
B. actins are much longer and therefore myosin can slide a longer distance without encountering the end of sarcomere.
C. the degree of shortening is greater than skeletal muscles
All of them are correct
In smooth muscles, why is extracellular Ca2+ so important?
Absence of T tubule system, thus less extensive sarcoplasmic reticulum.
A large number of ___________ are prevent for increasing SA for Ca2+ influx.
Caveolae
small membrane infoldings
What are the 4 extracellular sources and 2 intracellular sources of Ca2+ for contraction?
Extracellular
1. Voltage-gated Ca2+ channel (L-type Ca2+ channel)
- Ligand-gated Ca2+ channel
- Stretch-sensitive Ca2+ channel (non-selective cation channel)
- Stored-operated Ca2+ channel
Intracellular
5. IP3-gated Ca2+ channel
- Ca2+-gated Ca2+ channel (Ryanodine receptor for Ca2+ -induced Ca2+ release)
(ryanodine is a drug that opens this channel)
What is the mechanism of smooth muscle contraction?
5 steps
- Intracellular Ca2+ concentrations increase when Ca2+ enters the cell / released from SR
- Ca2+ binds to calmodulin
- Ca-calmodulin activates myosin light chain kinase (MLCK)
- MLCK phosphorylates light chains in myosin heads to trigger binding of myosin head to actin(by increasing affinity of myosin head to actin) and increases myosin ATPase activity
- Active myosin cross bridges along actin and create muscle tension
Why does smooth muscles have a slower onset time of contraction?
Phosphorylation of the myosin light chain is kinase is needed.
Considering muscle relaxation, transport proteins responsible for lowering cycstolic ca2+ include:
- Plasma membrane Ca2+ - ATPase (PMCA)
- Sarcoplasmic reticulum Ca2+ -ATPase (SERCA)
- Na+/Ca2+ exchanger (NCX)
The dephosphorylated cross-bridge is known as _______.
Latch bridge
Slower onset rate and offset rate of smooth muscles compared to cardiac and skeletal muscles due to?
- More complicated electro-mechanical transduction system
(requires Ca2+ calmodulin to activates MLCK and phosphorylation of MLCK ) - Slower ATPase activity
- Slower cycling rate (ADP)
Tonic contraction: A. contract most of the time B. Rhythmic contraction in response to stimuli (AP) C. Effectors: GI muscles, uterus D. Multi-unit muscles E. Pharmacomechanical coupling
A, D and E only
B: relax in responses to inhibitory signals
C. Effectors: Sphincter, arterial smooth muscles, air way
Phasic contraction: A. relax most of the time B. Rhythmic contraction in response to stimuli (AP) C. Effectors: GI muscles, uterus D. Single unit muscles E. Electro-mechanical coupling
All correct
What is pharmacomechanical coupling (by tonic contraction)
and
electromechanical coupling (by phasic contraction)
and
mechano-mechanical coupling
respectively?
Electromechanical:
Slow wave potential > open L-type Ca2+ channel >Ca2+ induced Ca2+ release
Pharmacomechanical:
Cytosolic Ca2+ increases due to
a) release from intracellular store (PLC-IP3)
Binding to GPCR linked to Gq protein
- > activation of phospholipase C (PLC)
- > generation of IP3 and DAG.
- > IP3 stimulates Ca2+ release from intracellular store.
;
b) entry through ligand-gated non-selective cation channels
PLC could be induced by Gq protein (adrenaline/NA/Ach)
Mechano-mechanical coupling:
Muscle is excited by stretch which opens stretch-sensitive Ca2+ channels
