RECEPTORS – CLASSIFICATION

RECEPTORS – CLASSIFICATION

Q. 1

Which of the following hormones uses Phospholipase C as a messenger?

 A Angiotesin II
 B Glucagon
 C Calcitonin
 D All of the above
Q. 1

Which of the following hormones uses Phospholipase C as a messenger?

 A Angiotesin II
 B Glucagon
 C Calcitonin
 D All of the above
Ans. A

Explanation:

Angiotesin II


Q. 2

When activated by β-adrenergic receptors, the G protein:

 A

Activates phospholipase C

 B

Activates adenylate cyclase

 C

Activates protein kinase C

 D

Converts guanosine diphosphate (GDP) to guanosine triphosphate (GTP)

Ans. B

Explanation:

When norepinephrine binds to a beta-adrenergic receptor, it activates a G protein (i.e., a guanosine triphosphate (GTP) binding protein), which, in turn, activates adenylate cyclase. Adenylate cyclase catalyzes the formation of cyclic adenosine 3’,5’-monophosphate (cAMP), which activates a variety of kinases. One of these kinases (protein kinase A) phosphorylates phospholamban, which reduces the inhibition of the sarcoplasmic reticular Ca2+ pump, increasing sequestration of Ca2+ from the cytoplasm.
 
Ref: Biaggioni I., Robertson D. (2012). Chapter 9. Adrenoceptor Agonists & Sympathomimetic Drugs. In B.G. Katzung, S.B. Masters, A.J. Trevor (Eds),Basic & Clinical Pharmacology, 12e.

Q. 3

The following hormone does NOT have any intracellular receptor:

 A

Vitamin D3

 B

Cortisone

 C

Adrenaline

 D

Thyroxine

Ans. C

Explanation:

Cell membrane receptor proteins are located within the phospholipid bilayer of the cell membrane of target cells.

Binding of hormones (ie, catecholamines, peptide and protein hormones) to cell membrane receptors and formation of the hormone-receptor complex initiates a signaling cascade of intracellular events, resulting in a specific biologic response.

Intracellular receptors are transcription factors that have binding sites for the hormone (ligand) and for DNA and function as ligand (hormone)-regulated transcription factors.

Steroid hormones and the steroid derivative vitamin D3 fulfill this requirement.

Thyroid hormones must be actively transported into the cell.
Ref: Molina P.E. (2013). Chapter 1. General Principles of Endocrine Physiology. In P.E. Molina (Ed), Endocrine Physiology, 4e.

Q. 4 All are TRUE about protease activated receptors, EXCEPT:

 A

Family of three seven transmembrane protein

 B

Activated by thrombin

 C

Releases PGE2

 D

Protects epithelial cells

Ans. A

Explanation:

Protease activated receptors (PARs), a family of four seven-transmembrane G protein-coupled receptors (PAR1, PAR2, PAR3, PAR4), act as targets for signalling by various proteolytic enzymes (thrombin, trypsin). PARs are characterized by a unique activation mechanism involving the proteolytic unmasking of a tethered ligand that stimulates the receptor. They release PGE2 and this inturn protects the epithelium.

Ref: Ganong’s Review of Medical Physiology, 21st Edition, Page 667


Q. 5

Phospholipase C acts as second messenger for which of the following hormones?

 A

FSH

 B

LH

 C

TSH

 D

GnRH

Ans. D

Explanation:

GnRH receptors are a part of G protein coupled receptor superfamily and it functions primarily through the phospholipase C induced diacylglycerol and inositol triphosphate second messenger system.

Second messengers used by hormones:
 
CRH, ACTH, LH, FSH, TSH, PTH, calcitonin, glucagon, ADH (V2 receptor), hCG: increase cAMP.
Prolactin: decrease cAMP.
GHRH, GnRH, TRH, ADH (V1 receptor), oxytocin: DAG, IP3.
Cortisol, aldosterone, testosterone, estrogen, progesterone, calcitriol, thyroid hormones: Steroid mechanism.
NO: cGMP.
Insulin and IGF-1: Tyrosine kinase.
 
Ref: Physiology By Robin R. Preston, page 439

Q. 6 The following hormone does not have any intracellular receptor:

 A

Vitamin D3

 B

Cortisone

 C

Adrenaline

 D

Thyroxine

Ans. C

Explanation:

C i.e. Adrenalin


Q. 7

Intracellular receptors are found in:

 A

Insulin

 B

Glucagon

 C

Corticosteriods

 D

All

Ans. C

Explanation:

C i.e. Corticosteroids

Classification of Hormones by Lipid/Water Solubility

Group I Hormones

Feature

Group II Hormones

Are lipophilic (i.e. lipid or steroid

soluble)Q

Solubility

Are hydrophilic (i.e. water soluble)

Being insoluble in water, they are

Transport (Carrier) protein

Carrier protein is not required as they

transported in blood bound to a plasma

protein; their association with carrier

protein (commonly globulin) circumvents

the problem of solubility

 

themselves are water soluble

They are not stored & secreted on demand

Synthesis & storage

They are synthesized in endoplasmic

(except thyroid hormones; thyroxine

 

reticulum (ER), like all proteins and stoed

stores can last for several weeks)

 

in granules or vesicles.

Prolonged (hours-days) d/t transport

protein

Plasma half life (ti/2)

Short (minutes)

In target cells, they easily penetrate

Site of action & Receptor

Being lipid in soluble, they act on plasma

lipophilic plasma membrane to act

 

membrane receptorsQ, and do not enter the

intracellularly in cytosol or nucleus

 

cell to act.

Slower in onset and prolonged in

Action

Rapid in onset and shorter in duration

duration. Their actions are terminated in

liver.

 

(bcoz cell entery is not required)

Hormone -receptor complex undergoes

Mediator or Intra cellular messenger

Use intracellular second messengers like

activation reaction and moves into the

 

c-AMP, c-GMP, IP3-DAG, calcium ‑

nucleus to bind with high affinity to a

specific DNA sequence k/ a hormone

response element (HRE).

 

calmodulin and kinases etc.

All steroid hormones & thyroid (T3, T4)

Examples

–   Catecholamines (adrenaline &

hormonesQ

 

noradrenaline)


Q. 8 True about intracellular receptors:

 A Mainly on nuclear surface

 B

GH act on it

 C

Estrogen does not act on it

 D

All

Ans. A

Explanation:

A i.e. Mainly on nuclear surface


Q. 9

True about G-protein receptor complex:

 A

It interacts transmembrane domain.

 B

GTP to GDP

 C

Adenyl cyclase activation leads to increased cAMP

 D

All

Ans. D

Explanation:

A i.e. It interacts transmembrane domain; B i.e. GTP to GDP; C i.e. Adenyl cyclase activation leads to increased cAMP


Q. 10

True statement about the G-protein coupled receptor is:

 A

It binds to ligands at the cell surface

 B

Association of all the three subunits (alpha, beta and gamma) is essential for its action

 C

G-protein acts as inhibitory and excitatory as alpha subunit exists in two forms

 D

GTP binds to G protein in resting state

Ans. A

Explanation:

A i.e. It binds to ligands at the cell surfac

G protein coupled (linked) receptor (GPCRs)

  • These cell surface receptors bind to the ligand at the cell surfaceQ. And have 7 trans membrane segments that loop in and out of the cell membraneQ. So the receptor is also k/a seven transmembrane domain (7TM) receptors, heptahelical or serpentine receptor.
  • Part of receptor that protudes into cytoplasm (cytoplasmic tail of receptor) is coupled to G protein that has 3 different subunits (hetrotrimeric) – a, y. This G protein has an ability to bind guanosine nucleotides.

–                   In resting (inactive) state the a, 13, y subunits form a complex that binds GDP on

a subunitQ.

When receptor gets activated (after ligand binding) it causes this GDP bound trimeric G protein complex to associate with cytoplasmic tail of receptor and to exchange GDP by GTP. This exchange causes a subunit to dissociate

from combined fl & r subunits. This separated a subunit brings about biological effectQ. The 0 y subunits do not separate from each other and activates variety of effectors. (So dissociation of subunits 1/t action).

When ligand is removed from receptor, the intrinsic GTP ase activity of a subunit inactivates itself by converting its bound GTP into GDPQ.

In the absence of hormone, the heterotrimeric (a,13,y) G protein complex is in an inactive GDP bound form anchored to plasma membrane through prenylated groups on 13 y subunits and by myristoylated groups on a-subunit , but not associated with the receoptor. Binding of hormone (H) to receptor causes conformational change of receptor and activation of G protein complex d/t exchange of GDP with GTP on a subunit.

Tadenyl cyclase, Ca”/Na+ /C1- channels for a-s

–         TIC* channels for al and ao

–         T Phospholipase C[It for aq and phospholipase CI32 for an

–         T C GMP phosphodiesterases for at and CI channel for cti2

  • Depending on the coupling of ligand (hormone) to inhibitory G protein(Gi) or stimulatory G protein (Gs) receptors, these can either decrease or increase the activity of intracellular enzyme. (so it is not dependent on a subunit).
  • Mechanism of action: Ligand (photon, protein, amine hormone, & neurotransmitter) binding —> conformation change & activation of G protein —> exchange of GDP for GTP and dissociation between a- and 13 y subunits which interact with effectors.
  • a – subunit act on Na, Ca, K- channels and phospholipse – C (PLC). p y subunit acts on adenyl cyclase, phospholipase A2, DLC, Ca – ATPase etc.

Adenyl cyclase – cAMP pathway activation (i.e. adenyl cyclase activation lit T cAMP) causes stimulatin of cAMP dependent prtein kinase (protein kinase A) which phosphorylates the receptorQ.

G protein coupled receptor kinase are protein kinase that phosphorylates only active GPCRs Phosphorylation of receptor 1/t translocation & arrest linking.

  • GPCRs are found only in eukaryotes, including yeast, plants & choanoflagellates. These are grouped in 6 classes: A (rhodopsin like); B (secretin receptor family); C (pheromone/ metabotropic glutamate); D (Fungal mating pheromone receptor); E (cyclic AMP receptor); and F (frizzled or smoothened).



Q. 11 Which one of the following statements best describes the mechanism of action of insulin on target cells?

 A

Insulin binds to cytoplasmic receptor molecule and is transferred as a hormone receptor complex to the nucleus where it acts to modulate gene expression.

 B

Insulin binds to a receptor molecule on the outer surface of the plasma membrane and the hormone receptor complex activates adenylate cyclase through the Gs protein.

 C

Insulin binds to a transmembrane receptor at the outer surface of the plasma membrane, which activates the tyrosine kinase that is the cytosolic domain of the receptor.

 D

Insulin enters the cell and causes the release of calcium ions from intracellular stores.

Ans. C

Explanation:

Ans is ‘c’ i.e. insulin binds to a receptor molecule on the outer surface of the plasma membrane which activates the tyrosine kinase that is the cytosolic domain of the receptor.

o Insulin receptors are membrane receptor of tyrosine kinase enzyme linked type. Their hormone binding site is extracellular and effective domain (i.e. tyrosine kinase) is intracellular.

o Insulin binds at extracellular site (outer surface of plasma membrane) and this binding activates intracellular (cytoso­lic) domain, i.e., tyrosine kinase


Q. 12 Inotropic drugs acts on which receptors:

March 2013

 A Alpha1

 B

Alpha2

 C

Beta1

 D

Beta2

Ans. C

Explanation:

Ans. C i.e. Beta1

Beta adrenergic receptors

  • Activation of beta-1 receptors induces positive inotropic, chronotropic output of the cardiac muscle, leading to
    increased heart rate and blood pressure, secretion of ghrelin from the stomach, and renin release from the kidneys.
  • Activation of beta-2 receptors induces smooth muscle relaxation in the lungs, gastrointestinal tract, uterus, and various blood vessels. Increased heart rate and heart muscle contraction is also associated with the beta-2 receptors.
  • Beta-3 receptors are mainly located in adipose tissue. Activation of the beta-3 receptors induces the metabolism of lipids.

Q. 13 Intracellular receptors are present for

 A Estrogen

 B

Vitamin D

 C

Thyroxine

 D

All of the above

Ans. D

Explanation:

Ans. D. All of the above

Intracellular receptors are receptors located inside the cell rather than on its cell membrane. Classic hormones that use intracellular receptors include thyroid and steroid hormones. Examples are the class of nuclear receptors located in the cell nucleus and cytoplasm and the IP3 receptor located on the endoplasmic reticulum.


Q. 14

Intracellular receptor hormone ‑

 A

Insulin

 B

T3

 C

T4

 D

b and c both

Ans. D

Explanation:

Ans. is `b & c’ i.e., T3 & T4


Q. 15

Intracellular receptors are used by

 A

ACTH

 B

TSH

 C

Glucocorticoids

 D

Insulin

Ans. C

Explanation:

Ans. is ‘c i.e., Glucocorticoids



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