G Protein Function and Adrenergic Receptors

Question 1

Classifying Hormones

Answer 1

• Hormones can be classified in a variety of ways.
• One of these is to distinguish them by the location of their target cell receptor; either intracellular (group I) or cell surface (group II).
• The group I hormones are lipophilic (hydrophobic) and readily diffuse across the plasma membrane to bind to cytoplasmic or nuclear intracellular receptors.
• The group II hormones are hydrophilic and hence do not diffuse across the plasma membrane.

Question 2

Group II Hormones

Answer 2

• The group II hormones are hydrophilic and hence do not diffuse across the plasma membrane.
• They have short half-lives in the circulation (minutes).
• Peptide and amine hormones (except thyroid hormone) bind to membrane spanning receptors that lie in the plasma membrane of target cells.
• These hormones communicate with intracellular metabolic processes through second messengers; (the hormone itself is the first messenger).
• Second messengers amplify the hormone signal without the hormone actually entering the cell. These messengers are generated as a consequence of the ligand-receptor interaction.
• Only hormones that bind to plasma membrane receptors can utilize such a second messenger system.
• Most hormones in this class employ either cyclic AMP or calcium/phosphatidyl-inositol/diacylglycerol as second messengers

Question 3

Group ii Subcategories

Answer 3

•IIA

-subdivided as stimulatory or inhibitory because they act via a G-protein-coupled mechanism that either positively or negatively regulates the activity of adenylyl cyclase. Adenylyl cyclase is a membrane-associated enzyme that converts ATP to cyclic AMP.

•IIB

-Group IIB hormones also act via a G-protein-coupled mechanism that activates phospholipase C (PLC) in the plasma membrane. PLC cleaves a membrane phospholipid to produce inositol trisphosphate (IP3), and diacylglycerol (DAG), as well as calcium as a third messenger.

•IIC

-Hormones in Group IIC bind to receptors that possess intrinsic (e.g., contained within the receptor structure) tyrosine protein kinase activity. This activity is stimulated on the inner surface of the target cell membrane when the hormone binds to the outer cell surface domain of the receptor.

•IIC’

-Hormones in group IIC’ also cause activation of tyrosine kinase but this JAK enzyme is in a soluble form that is recruited.

•IID

-Atrial natriuretic peptide (ANP) works through a unique second messenger cyclic GMP (Group IID).

Question 4

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Question 5

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Question 6

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Question 7

Answer 7

Question 8

Catecholamine Receptor Stimulation

Answer 8

• The catecholamine neurotransmitters are divided into different receptor subtypes: alpha and beta- (β) adrenergics and dopamine.
• These receptors dictate the discrete response of the effector organ to the agonist, an agonist being a hormone or drug that activates the receptor.

Question 9

The Alpha Receptors

Answer 9

• The alpha-receptors consist of two types, alpha1 and alpha2.
• The alpha1- receptors are located on innervated organs. They elicit a physiologic change in response to neurotransmitter stimulation of the receptor.
• In contrast alpha2-receptors are located on the presynaptic nerve terminal where activation of this receptor leads to a reduction of neurotransmitter release.
• Adrenergic alpha1-receptors after binding an agonist activate the alpha-subunit of the Gq-protein. The activated alphaq increases activity of membrane-bound phospholipase C that results in release of IP3 and DAG. IP3 triggers intracellular Ca2+ release

Question 10

The Beta Receptors

Answer 10

• Adrenergic beta-receptors, like alpha-receptors, are G-protein coupled.
• The beta- receptors are linked to Gs proteins that stimulate adenylyl cyclase activity to increase the concentration of cAMP.
• The cell type and its specific beta-receptor produces the variety of physiological responses, but all do it through the Gs-linked increase in cAMP.
• For example, activation of beta1-receptors increases heart rate and contractile force.
• In ventricular cells, cAMP activates PKA, which phosphorylates the voltage-gated Ca2+ channel, troponin 1 and phospholamban.
• When phosphorylated the opening of the Ca2+ channel is enhanced such that Ca2+ influx is increased.
• The entering Ca2+ triggers release of additional Ca2+ from the sarcoplasmic reticulum (SR). With increased Ca2+ availability, more cross bridges form and the force of contraction increases.
• The phosphorylation of phospholamban allows sarcoplasmic/endoplasmic reticulum Ca2+ to reduce cytosolic Ca2+ levels more rapidly.
• The phosphorylation of troponin facilitates the release of Ca2+ by troponin and facilitates relaxation.
• The phosphorylation of phospholamban and troponin decrease duration of contraction (although force is increased) as heart rate increases.
• Stimulation of beta2-receptors leads to the relaxation of smooth muscles in vascular and bronchial tissues resulting in a relaxation (vasodilation and bronchodilation). For this reason, beta2 agonists are a mainstay therapy for treating the acute bronchial spasms occurring in asthma.
• 3 receptors have been identified on adipose cells, and when stimulated increase cAMP, and lipolysis. These beta3-receptors are much more sensitive to norepinephrine than to epinephrine. Polymorphisms in the gene that encodes the beta3-receptor may be related to the risk of obesity or Type-2 diabetes.

Question 11

Phorbol Esters

Answer 11

• activation of PKC mimicked by phorbol esters
• originally detected in oil prepared from seeds of Croton tiglium
• structurally similar to DAG, but only slowly degraded in the cell
• permanently activate PKC
• act as tumor promoters - promote growth and cell division by interfering with normal regulation of these processes

Question 12

Answer 12

Question 13

Answer 13

Question 14

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Question 15

Answer 15

Question 16

Epinephrine and Beta Receptors

Answer 16

• Epinephrine-induced stimulation of the beta-receptor terminates upon dissociation of epinephrine from the receptor.
• However, is epinephrine presence continues with binding to the receptor, inactivation can be initiated by the enzyme beta-adrenergic receptor kinase (beta-ARK).
• This kinase phosphorylates the serine and threonine residues on the carboxy terminus of the receptor.
• Phosphorylation then leads to binding of beta-arrestin to the phosphorylated sites to decrease sensitivity to further catecholamine stimulation.
• The end result is a decreased coupling to Gs and decreased stimulation of adenylyl cyclase.

Question 17

The main naturally occurring adrenergic agonist is […].

Answer 17

The main naturally occurring adrenergic agonist is epinephrine.

Question 18

Epinephrine

Answer 18

• Epinephrine can be used to raise blood pressure and heart rate in shock, treat an acute life threatening asthmatic episode, or reverse an anaphylactic reaction to an adverse drug reaction or insect sting.
• Individuals susceptible to these conditions often are prescribed an EpiPen.
• Epinephrine also is contained in some local anesthetics to prolong their duration of action by constricting blood vessels (alpha1-effect).

Question 19

Norepinephrine

Answer 19

• Norepinephrine is the principle mediator for maintaining total peripheral resistance in the vasculature (alpha1-effect), and along with epinephrine to increase heart rate (beta1-effect).
• Norepinephrine is systemically administered to treat sepsis induced hypotension where fluid therapy has not worked or when systemic vascular resistance is low.
• Norepinephrine is a potent vasoconstrictor with no off-setting beta2 agonist effects.
• Epinephrine, via presynaptic beta2-receptors, can enhance release of norepinephrine.
• Norepinephrine, via presynaptic autoreceptors, can inhibit release of norepinephrine.

Question 20

Answer 20

• Phenylephrine – contracts (shrinks) nasal blood vessels
• Clonidine – slows heart rate to lower BP

Question 21

Answer 21

Question 22

Answer 22

• Dobutamine – sympathomimetic to treat heart failure
• Albuterol - bronchodilation to increase air flow to lungs

Question 23

Answer 23

• Dopamine – improves blood flow to kidneys
• Fenoldopam – antihypertensive agent

Question 24

Answer 24

• Phenoxybenzamine – antihypertensive; reduces vasoconstriction of epi and norepi
• Prazocin – causes vasodilation

Question 25

Answer 25

• Phenoxybenzamine – antihypertensive; reduces vasoconstriction of epi and norepi
• Yohimbine – vasodilation for erectile disfunction/impotence

Question 26

Answer 26

• Metoprolol – treats angina; chest pain by decreasing heart contraction
• Propranolol - treats angina, chest pain by decreasing heart contraction and increasing smooth mus relax

Question 27

Answer 27

•Propranolol - treats angina, chest pain by decreasing heart contraction and increasing smooth mus relax

Question 28

Pheochromocytoma

Answer 28

• Most common tumor of adrenal medulla
• Secretes norepi, epi, dopamine
• Elevated metanephrine and normetanephrine in blood
• Urinary excretion of vanillylmandelic acid (VMA) from epi/norepi deg
• Urinary excretion of homovanillic acid (HVA) from dopamine deg
• Clinical signs: sudden hypertension; postural hypotension; hypertensive retinopathy, pulmonary edema; cardiomyopathy
• Associated with other syndromes
• Prior to surgery treat:
• 1 st with alpha-blocker (e.g., phenoxybenzamine) to dilate vessels; increase blood flow
• then by beta-blocker (e.g., propranolol) decreases heart rate
• this order essential to prevent hypertensive crisis

Question 29

Dopamine

Answer 29

• Dopamine needs to be considered as a therapeutically useful catecholamine.
• At low doses (<2 µg/kg/min) it stimulates dopamine receptors and increases cAMP and PKA.
• Dopamine receptors have been identified in renal tissue, which when stimulated affects arteriolar blood flow, distal tubule sodium reabsorption, renin secretion, and cortical collecting duct cells.
• Dopamine also has the ability to stimulate beta1-receptors at intermediate doses (5-10 µg/kg/min) and activate alpha1-receptors at high doses (10-20 µg/kg/min).
• Dopamine also can cause the release of norepinephrine from nerve terminals.

-In turn, this effect can activate both beta1 and alpha1 receptors.

•Because of these multiple actions, dopamine can increase renal blood flow (vasodilation of renal blood vessels), increase cardiac output (beta1) and increase total peripheral resistance (alpha1) that can be useful in treating hemorrhagic shock

Question 30

Cholera

Answer 30

• Ganglioside GM1 on the intestinal mucosal cells provides a site for binding of cholera toxin that is secreted by Vibrio cholerae.
• The clinical symptoms of cholera include severe diarrhea with loss of blood electrolytes. These symptoms are a consequence of the excessive secretion of chloride ions into the intestinal lumen due to the overproduction of cyclic AMP, which normally regulates the chloride channel via a Gs-protein mechanism.
• Cholera toxin irreversibly activates adenylyl cyclase by locking the alphas-subunit in its active state. Normally, the alpha-subunit is inactivated by hydrolysis of bound GTP catalyzed by the inherent GTPase.
• Cholera toxin catalyzes the cleavage of NAD+ into nicotinamide and ADP-ribose. The toxin then attaches the ADPribose portion onto an arginine residue of the alphas-subunit.

-This ADP-ribosylation inactivates the GTPase to prevent inactivation of the alphas-subunit.

• Sustained activity of adenylyl cyclase leads to a marked increase in the intracellular concentration of cAMP resulting in loss of both water and salt from intestinal epithelial cells.
• The process can only be reversed by antibiotics destroying the pathogen to allow time for unmodified -subunits to be generated.

Question 31

Pertussis

Answer 31

• Pertussis toxin permanently activates adenylyl cyclase by a different mechanism.
• The toxin is generated in the lungs from inhaled Bordetella pertussis.

-The pathogen is induced to produce its toxin by the higher temperature environment in the lungs.

• Pertussis toxin, like cholera toxin, catalyzes cleavage of NAD+ with subsequent ADP-ribosylation of a cysteine residue on the alphai-subunit.
• This modification prevents interaction of Gi with the hormone-receptor complex so that dissociation of the alphai-subunit cannot occur in response to agonists.
• Consequently this ribosylation prevents inhibition of adenylyl cyclase leading to overproduction of cyclic AMP.

Question 32

Mutations in the alpha subunit of G-proteins

Answer 32

•Pseudohypoparathyroidism Type 1A (Albright Hereditary Osteodystrophy)

• defective alphas in the kidney
• Normally parathyroid hormone receptor in the kidney activates Gs protein in the membrane there. This defect results in an end-organ resistance to PTH.
• Individuals with pseudohypoparathyroidism exhibit physical exam features simiar to pseudohypoparathyroidism but lacks the end-organ PTH resistance

•McCune-Albright Syndrome

–Gs protein mutation that consitutively activates adenylyl cyclase.

-This mutation causes overproduction of several hormones that results in abnormal bone growth and flat pigmeneted birthmarks (café au lait spots) with ragged edges.