Vascular Smooth Muscle Contraction and Regulation

Question 1

Excitation-Contraction coupling in smooth muscle

Answer 1

1) Most important: NE binding to alpha/beta receptor; less important:AP causes voltage-gated Ca2+ channels to open and influx of Ca2+
1a) Ligand-gated Ca2+ channels in sarcolemmal membrane may be opened by hormones or NTs, letting more Ca2+ in
1b) IP3-gated Ca2+ release channels in sarcoplasmic reticulum may be opened by hormones and NTs, letting more Ca2+ in
2) Ca2+ (4 molecules) binds calmodulin, and Ca2+/calmodulin complex binds and activates myosin-light-chain kinase (MLCK)
3) MLCK phosphorylates myosin light chain, changing the conformation of myosin head
4) Myosin binds actin, hydrolyzes ATP, and forms cross-bridge and produces tension

(Also Ca2+/calmodulin phosphorylates calponin and caldesmon which releases them from actin and allow activity of myosin ATPase and interaction between actin and myosin)

Question 2

During relaxation of smooth muscle, what causes fall of intracellular Ca2+

Answer 2

Activation of myosin-light-chain phosphatase (MLCP), which dephosphorylates myosin light chain

Hyperpolarization closes voltage-gated Ca2+ channels

Direct inhibition of Ca2+ channels by ligands like cAMP and cGMP

Inhibition of IP3 production and decreased release of Ca2+ from sarcoplasmic reticulum

Increased Ca2+ ATPase activity in SR

Question 3

Voltage-gated Ca2+ channels

Answer 3

In sacrolemmal (plasma) membrane of smooth muscle cell

Open when cell membrane potential depolarizes and cause AP

Question 4

Ligand-gated Ca2+ channels

Answer 4

In sarcolemmal (plasma) membrane of smooth muscle cell

Hormones and NTs bind to receptors that are coupled to Ca2+ channel via G-protein

Question 5

IP3-gated sarcoplasmic reticulum Ca2+ channels

Answer 5

NE binds alpha1 receptor in sarcolemmal (plasma) membrane –> G-protein coupled to phospholipase C –> PLC turns PIP2 to IP3 and DAG –> IP3 diffuses to SR and opens SR IP3 receptor Ca2+ channels (IP3R) –> Ca2+ flows out of SR and into cytoplasm

IP3-gated channels are in SR, so are similar to RyR

Question 6

Normal values for preload and afterload

Answer 6

RV preload: 2-8 mmHg

RV afterload: (15-30)/(5-15) mmHg (pulm artery syst/diast)

LV preload: 4-12 mmHg

LV afterload: (100-140)/(60-90) mmHg (arterial syst/diast)

Question 7

Normal LV contractility

Answer 7

55-70% ejection fraction

Question 8

Normal cardiac output

Answer 8

5-6 L/min

Question 9

What does epinephrine at low physiological levels do?

Answer 9

Vasodilation (by binding beta2 receptors)

(At HIGH epi concentrations, it binds alpha1 receptors and causes vasoconstriction)

Question 10

Vascular smooth muscle (VSM) cells

Answer 10

Contains thick (myosin) and thin (actin) filaments which are randomly aligned

Cells around vessels perpendicular to vessel axis

Dense bodies hold contractile proteins to each other and cell surface

VSM use same sliding filament mechanism but do it >10x slower

Have some titin but more collagen and elastin

Has tropomyosin but NO troponin

Sparse SR come in contact with caveolae

Question 11

How are VSM cells innervated?

Answer 11

ONLY by sympathetic fibers!

(except sex organs)

Question 12

Multiunit smooth muscle

Answer 12

Each cell (or every few) has own nerve varicosity

Often don’t require AP to contract

In large arteries, veins, sphincters

MOST VSM are multiunit

Question 13

Single unit smooth muscles

Answer 13

Electrical communication via gap junctions

In arterioles and helpatic portal vein

Question 14

Myosin light chain phosphatase (MLCP)

Answer 14

Dephosphorylates myosin light chain to inactivate is and promote relaxation

PKG activates MLCP by phosphorylating it (muscle relaxation)

PKC inhibits MLCP by phosphorylating it (muscle contraction)

Question 15

DAG

Answer 15

DAG activates PKC

PKC phosphorylates L-type Ca2+ channels so Ca2+ influx

PKC phosphorylates K+ channels, decreasing their conductance and depolarizing membrane

PKC phosphorylates and activates MLCP to cause contraction

Question 16

What do PKA, PKG, and PKC do to contraction?

Answer 16

PKC increases force of contraction

PKA and PKG decrease force of contraction

Question 17

Phosphorylated L-type Ca2+ channels

Answer 17

Exist in sarcolemma

More likely to be open at any given voltage

So, promotes contraction

Note: phosphorylated by PKC

(Normal L-type Ca2+ channels threshold at -55mV, but phosphorylated would be lower)

Question 18

Voltage-gated K+ channels in smooth muscle (gK, v)

Answer 18

“Delayed rectifier”

g_K increases with depolarization more than 40mV

Has incomplete inactivation and speeds repolarization after cell depolarization

Question 19

ATP sensitive K+ channels in smooth muscle (gK, ATP)

Answer 19

Senses low ATP, high ADP, low pH, adenosine (things that happen during ischemia) –> closure of L-type Ca2+ channels –> hyperpolarization –> VSM dilates/relaxes

Channels in most arteries, arterioles, veins, coronary arteries

Phosphorylated by PKA, PKG –> relaxation

Phosphorylated by PKC –> constriction

Question 20

Calcium channel blockers

Answer 20

Nifedipine

Diltiazem

Verapamil

Question 21

Drugs for hypertension that increase gK, ATP (promote relaxation)

Answer 21

Minoxidil

Penacidil

Cromakalim

Question 22

What does PKA phosphorylation do?

Answer 22

Phosphorylated phospholamban can’t bind SERCA, so SERCA active and more Ca2+ pumped into SR

Phosphorylated gK, ATP channels more likely to be open, hyperpolarization of cell causes relaxation

Phosphorylated MLCK can’t bind calmodulin, so myosin light chain can’t be phosphorylated, so can’t bind actin, which causes relaxation

Question 23

Which receptors activate PKA? (and thus produce vasodilation)

Answer 23

Beta2 stimulated by epinephrine (in arterioles of skeletal muscle, myocardium, liver)

Alpha2 receptor by adenosine (in arterioles of myocardium, a little in skeletal muscle)

H2 receptor by histamine (in arterioles)

Question 24

How is PKG activated?

Answer 24

NO produced in vascular endothelial cells –> NO diffuses across cell membrane and binds and activates soluble guanylate cyclase (sGC) –> sGC produces cGMP –> cGMP activates PKG

Question 25

What does PKG phosphorylation do?

Answer 25

Phosphorylated phospholamban can’t bind SERCA, so SERCA active and more Ca2+ pumped into SR

Phosphorylated gK, ATP channels more likely to be open, hyperpolarization of cell causes relaxation

Phosphorylated MLCP inactivates myosin light chain and produces relaxation

Question 26

What stimulates production of NO?

Answer 26

Physical deformation of endothelium (trauma, burn, bites)

Ach

ATP

Question 27

Atrial naturetic factor (ANF, ANP?)

Answer 27

Secreted by cardiac atria in response to right atrial stretch –> ANF binds G protein type receptor NPR-A –> cGMP produced –> activates PKG

Question 28

Do vascular smooth muscle cells have voltage-gated Na+ channels?

Answer 28

No!!!

Question 29

What is a stretch-sensitive Ca2+ channel?

Answer 29

In the sarcolemma, and when sarcolemma (cell membrane) stretched, this stretch-sensitive Ca2+ channel opens and lets Ca2+ in. So pressure increases and muscle contracts

Question 30

Do vascular smooth muscle cells have vagus stimulation?

Answer 30

No, VSM cells only have sympathetic stimulation

Question 31

Do VSM cells have troponin?

Answer 31

No troponin, but do have tropomyosin, little titin, and more collagen and elastin

Question 32

Do VSM cells have T tubules?

Answer 32

No, VSM cells don’t have T tubules, but have invaginations called caveolae which interact with the SR

Question 33

Two types of innervation of smooth muscle

Answer 33

Multi unit: each cell separately activated by associated nerve varicosity; dependent on innervation for regulation; don’t require action potentials to contract; in most VSM cells, large arteries, veins, sphincters don’t produce APs

Single unit: phasic; have some rhythmic behavior; have extensive electrical communication via gap junctions; depol one then you depol the others; hepatic portal vein, arterioles

Many smooth musclses fall in between these categories

Question 34

What does relaxed VSM look like?

Answer 34

Myosin light chain (wrapped around lever arm) is not phosphorylated

Cross bridge head bound to catalytic center of other cross bridge which is bound to S2 coil

Myosin jaws closed (so can’t bind actin)

Question 35

What triggers VSM contraction?

Answer 35

MLC is phosphorylated at serine 23 –> cross bridge heads detach from each other and S2 coil –> myosin binds actin –> ATP to ADP produces force and shortening

This happens at a slow rate compared to other contractions

Question 36

How does Ca2+ enter the VSM cytosol

Answer 36

Weird!!

Sympathetic fibers on blood vessels spill NE right onto the vessel –> NE binds alpha/beta receptors on smooth muscle cell surface –> trigger intracellular cascades

Also can be triggered by depolarizaion????