6 - Intracellular Receptors, NO Signalling - Gray

Question 1

Describe the ligands for cell surface receptors and what they effect

Answer 1

• hydrophilic
• eg peptide hormones eg insulin OR charged molecules eg adrenaline (PICA)
• affect enzyme activity and gene expression (fast acting)

Question 2

describe the ligands for intracellular receptors, how they reach their receptors and where and give examples

Answer 2

• often small hydrophobic molecules that can diffuse across plasma membranes and interact with their receptors in cytoplasm or nucleus
• eg NO - interacts with its receptor which is an enzyme therefore directly affecting its activity (granulate cyclase)
• steroid hormone receptor complexes- affecting transcription of genes through binding to enhancer regions
• auxin sends TFs to be degraded

Question 3

draw a diagram showing how NO enters the cell and interacts with its receptor

Answer 3

• NO = dissolved gas that diffuses through membrane

Question 4

what is the function of NO and what is the result of NO binding to guanylate cyclase (GC)?

Answer 4

• NO causes relaxation of smooth muscle cells surround blood vessels, expand, increase of blood flow and reducing blood pressure
• NO binding to GC causes activation of enzyme catalysis cGMP from GTP

Question 5

what are the 2 main ways that GC can be activated?

Answer 5

• activation of the membrane bound receptor with intracellular GC domain. binding of atrial naturetic peptide to extracellular domain
• binding of NO to the soluble GC receptor found in cytoplasm

Question 6

what is the result of cGMP presence?

Answer 6

activation of the cGMP-dependent protein kinase G

Question 7

draw the structure of the GC receptor and explain its binding sites and how it is activated.

Answer 7

• at low [NO] only one biding site occupied. binds to the high affinity haem group which stimulates low levels of catalytic activity
• at higher [NO] both binding sites can be occupied -> higher levels of catalytic activity. activity increased few 100 fold
• easily reversible as soon as [NO] drops

Question 8

draw a diagram and explain how NO causes a drop in blood pressure

Answer 8

• ACh released from blood vessel lumen interacts with GPCR on endothelial cell membrane
• activation of ACh GPCR, activation of Gqa protein which activates PLC (B iso form)
• PLC cleaves PIP2 -> IP3
• causes release of intracellular Ca2+
• Ca2+ binds to calmodulin to active Nitric oxide synthase
• arginine + O2 -> citrulline + NO
• NO = paracrine signal to nearby muscle cells. diffuses locally. short half life (2-30secs)
• NO enters muscle cells and binds to guanylate cyclase R and activating it. GTP -> cGMP and PPi
• cGMP activates PKG, drop in intracellular Ca
• relaxation of muscle cell

Question 9

draw a diagram (showing the enzymes etc) and explain outline how smooth muscle cells contract and relax

Answer 9

• myosin light chain kinase activation involves Ca and Calmodulin (CaM) binding
• results in P of the myosin light chain causing relocation of the myosin head
• activation of a MLC phosphatase (by PKG activity) and a reduction in intracellular Ca2+ and IP3 (also through PKG inhibiting Ca channels) meaning MLCK inactive causes deP of the MLC -> relaxation

Question 10

how does phosphodiesterase link to smooth muscle cell contraction and NO signalling? name the specific PDE invovled in this

Answer 10

• PDE5
• PDE converts cGMP -> GMP therefore inactivation of PKG
• therefore we get contraction of SMC

Question 11

what is the result (in terms of bodily function) of NO release?

Answer 11

NO release by nerve terminals in penis responsible for local blood vessel dilation in penis

Question 12

how can erectile dysfunction be treated?

Answer 12

sildenafil citrate is a reversible inhibitor of PDE5 that prevents cGMP hydrolysis so blood vessels can remain dilated and blood flow occurs

Question 13

describe 2 other NO signalling pathways

Answer 13

• oxygen depravation at high altitude can cause NO signalling to cause blood vessel dilation -> increased blood flow therefore more O2 to muscles
• HDL cholesterol stimulates NO production. blood vessel dilation helping to prevent ischaemia and angina

Question 14

describe 2 conditions that viagra can be used to treat

Answer 14

• cardiac hypertrophy
• pulmonary hypertension
  both invovle high blood pressure. in heart this can cause thickening of the heart walls (bad)

Question 15

describe the discovery of NO signalling (Nobel prize - 1998)

Answer 15

• ACh applied to blood vessels. showed variation in responses (sometimes dilated/constricted). dependent upon whether endothelium layer was present. showed that signal released from endothelium
• showed that the endothelium relaxation factor showed the same spectral shift as NO when added to haemoglobin
• showed that nitroglycerine response to relate SMC via GC invovled NO release