Physiology of musculature (2)

Question 1

What are the 3 main stimuli that lead to VSMC relaxation?

Answer 1

cGMP, cAMP, K+ channels

Question 2

Mechanism - effect of cGMP leading to muscle relaxation?

Answer 2

Increased production of NO, NO enters cell;
Increased levels of cGMP occurs as a result of activation of guanyl cyclase. This causes activation of PKG which activates myosin phosphatase which dephosphorylates myosin to its inactive form

Question 3

Mechanism - effect of cAMP leading to muscle contraction?

Answer 3

increased cAMP causes by activation of adenylyl cyclase, causes decrease in intracell ca2+, which inhibits MLCK, stimulating relaxation of smooth muscle cells.

Question 4

Mechanism - effect of K+ channels leading to muscle contraction?

Answer 4

Activation of K+ channels leads to efflux of K+ from vascular smooth muscle cells. This leads to hyper polarisation (as no Ca2+ is released from the S.R) which causes a decrease in I.C [Ca2+] and results in relaxation

Question 5

What are the main regulators of VSMC relaxation and where to they bind

Answer 5

NO - diffuse through the membrane from the endothelium causing an increase in cGMP
cAMP produced when ligands bind to GPCRs, including b-agonists, Adenosine and Prostoglandins
Via K+ channels

Question 6

K+ channels on VSMCs - types?

Answer 6

BK channels (large conductance)
SK channels (small conductance)
b-agonists (via bg G-protein)

Question 7

WHat is the importance of PDE?

Answer 7

Hydrolyses second messengers cAMP and cGMP to inhibit the relaxation response

Question 8

Mechanism of action of NO (via eNOS)?

Answer 8

Activation of GPCRs causes an increase in Ca2+ which combines with calmodulin.
This complex binds to Nitric oxide synthase which is now converted to its active form
eNOs converts arginine to NO and citruline
NO enters VSMC and activates guanylyl cyclase –> acivates cGMP which activates PKG –> myosin phosphatase

Question 9

Where is NO localised to and why?

Answer 9

Localised to caveoli structures so that it can act on smooth muscle cells as soon as N.O is produced

Question 10

Features of caveoli?

Answer 10

specialised cholesterol rich microdomains in the plasma membrane, derived from golgi
Have palmotylation and acetylation sites that allow them to be anchored to the membrane
Can be phosph. at several residues (partic. for AKT) allowing futher activation

Question 11

What ligands bind to GPCRs to induce relaxation by increased production of NO?

Answer 11

Ach, Bradykinin, Substance P

Question 12

What impairs eNOs??

Answer 12

• Smoking
• increased LDL production (depletes cholesterol so less eNOs localised to caveoli)
• high levels glucose/insulin —> diabetes!!!

Question 13

What additional regulatory mechanism occurs at the level of phosphorylation of specific residues on eNOS

Answer 13

phosphorylation regulates its sensitivity to calcium-calmodulin

Question 14

What are Prostanoids and what is there mechanism of action?

Answer 14

Prostanoids are produced in the endothelium in response to an increase in calcium or increased ROS production.
This activates cyclo-oxygenase 1/2 enzymes to convert Arachidonic acid to PGH2.

Question 15

What is Thromboxane A2 (produced from PGH2) and what is its mechanisms of action?

Answer 15

Thromboxane A2, acts on thromboxane receptor (TP) also known as the prostanoidTP receptor.
This TP GPCR receptor couples to PLC activation and IP3 production, then intracellular calcium release, and muscle contraction

Question 16

What is Prostoglandin E2 (produced from PGH2) and what is its mechanisms of action?

Answer 16

prostaglandin E2 (PGE2) which acts on prostaglandin E2(PGE2)receptors (EP1-4).
- activate adenylyl cyclase
- others inhibit (via GPCR Gs or Gi, respectively).
For increased cAMP there is a relaxation response.
For reduced cAMP there is a contraction response. effect depends on relative level of expression of EP receptor subtypes on the VSMCs, varies in diff areas of vasculature

Question 17

What is Prostoglandin I2 (produced from PGH2) and what is its mechanisms of action?

Answer 17

• acts on the IP receptor on VSMCs.
• The IP GPCR receptor couples to activation of adenylyl cyclase to cause increased cAMP and therefore relaxation.
  Prostacyclin is a powerful vasodilator

Question 18

What is big endothelin and what is it upregulated by?

Answer 18

The precursor of ET-1 = “Big endothelin”, which is upregulated by a variety of stimuli including: IL-1, Thrombin, Glucose, OxLDL, Insulin, Angiotensin II, Cortisol, Adrenaline, Hypoxia

It is cleaved to ET-1 (by ECE which is inhibited by NO) to act on the VSMC at both the ETA and B GPCR Gq receptors

Question 19

What is the impact of ET-1 on ET-A and ET-B receptors on VSMCs?

Answer 19

Increase production of IP3 which stimulates release of Ca2+ from the SR to stimulate contraction