Smooth muscle physiology

Question 1

Four different types of smooth muscle

Answer 1

Single unit, multi-unit, phasic, tonic

Question 2

Caveolae

Answer 2

Represent invaginations of the sarcolemma; similar to T-tubules in smooth muscle

Question 3

Gap junctions in smooth muscle

Answer 3

Allow for electrical coupling and chemical communication

Question 4

Latch-bridge

Answer 4

Controversial structure that allows for muscles to be tonically activated for long periods of time with minimal effort

Question 5

Two enzymes involved in de/phosphorylating myosin

Answer 5

Myosin-light-chain kinase and myosin-light-chain phosphatase

Question 6

Variations of basal tone in smooth muscle

Answer 6

Normally contracted (sphincters)
Normally partially contracted (tone)
Phasically active (stomach, intestines)
Normally relaxed (esophagus, bladder)

Question 7

Difference between single unit and multi-unit smooth muscle cells

Answer 7

Single unit: they’re all connected and one receptor for neurotransmitters on one cell can propagate to many different cells.

Multi-unit: each individual cell has receptors that will act individually from the cell next to it

Question 8

What allows the calcium signal to be propagated from one cell to the next in a single-unit system?

Answer 8

Gap junctions between cells

Question 9

Function of IP3

Answer 9

Causes release of calcium from the sarcoplasmic reticulum via IP3 receptors, leading to MLCK-dependent contraction via Ca2+-calmodulin

Question 10

What does cAMP stimulate PKA to do?

Answer 10

Phosphorylate MLCK to decrease its Ca2+ sensitivity, and increases sarcoplasmic reticulum calcium pumping

Question 11

What does cGMP do in smooth muscle?

Answer 11

Stimulates MLC phsphatase, decreasing myofilament activation, which relaxes the muscle

Question 12

What is cGMP associated with in muscle relaxation?

Answer 12

Nitric oxide (especially in vascular smooth muscle)

Question 13

Describe the catch-22 of ACh in smooth muscle.

Answer 13

In endothelial cells, if ACh attaches, then the endothelial cells will release NO into the bloodstream and stimulate relaxation. However, if the ACh directly attaches to AChR’s in the absence of endothelium, then it will stimulate contraction.

Question 14

Pathways that do not require membrane depolarization

Answer 14

Hormone receptor stimulation leading to formation of IP3, cAMP, cGMP, or the activation of a ligand operated Ca2+ channel

Question 15

Pathways that do require alterations in membrane potential

Answer 15

Depolarization-induced Ca2+ entry into smooth muscle cells

Question 16

Response, receptor, and second messenger: norepinephrine and epinephrine

Answer 16

• Contraction predominantly with some relaxation
• Alpha/beta-1 AR
• IP3 and cAMP

Question 17

Response, receptor, and second messenger: acetylcholine from parasympathetic stimulation

Answer 17

• Contraction directly, relaxation indirectly
• Muscarinic receptors
• No second messenger

Question 18

Response, receptor, and second messenger: angiotensin II

Answer 18

• Contraction
• AT-II receptors
• IP3

Question 19

Response, receptor, and second messenger: vasopressin

Answer 19

• Contraction
• Vasopressin receptor
• IP3

Question 20

Response, receptor, and second messenger: endothelin

Answer 20

• Contraction
• Endothelin receptor
• IP3

Question 21

Response, receptor, and second messenger: adenosine

Answer 21

• Relaxation
• Adenosine receptor
• cAMP