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Flashcards in L5-8 Receptor Theory Deck (156)
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1

What is the major target of drugs

Proteins

2

What is a target of a select group of drugs, what type of drugs target this

DNA
Chemotherapy drugs

3

Define a receptor

Proteins whose function it is to recognise and respond to endogenous chemical signals

4

What are some examples of protein targets of drugs

Receptors
Enzymes
Transporters and Carriers
Ion channels

5

Define a drug target

Other macro molecules which drugs interact with in order to produce their effects

6

Receptors are classified by

Strucuture, pharmacology and signalling

7

Receptors show _______ in the classes of drugs which they recognise

Specificity

8

Are drugs 100% efficient

No, no drug is 100% specific

9

What arises as a result of a loss of receptors

Disease

10

Location of ligand gated ion channels

Membrane

11

How many tm domains in ligand gated ion channels

Variable

12

Where is the c' term in ligand gated ion channels

Extracellular

13

Where is the n' term in ligand gated ion channels

EC

14

What is the effector for a ligand gated ion channels

Ion channel

15

What type of coupling involved in ligand gated ion channels

Direct

16

Structure of ligand gated ion channels

Oligomeric assembly of subunits around a central pore

17

Location of GPCRs

Membrane

18

How many TM domains in GPCRs

7

19

Where is the c' term in GPCRs

IC

20

What is the effector for GPCRs

Channel or enzyme

21

Where is the n' term for GPCRS

EC

22

Coupling GPCRs

Via a G-protein

23

Structure of a GPCR

Mon/oligomeric assemblt of 7 TM helicies with an IC G-protein coupling domain

24

Location of kinase-linked receptors

Membrane

25

How many TM domaiins kinase-linked receptors

1

26

Where is c' term kinase-linked receptors

IC (catalytic domain)

27

Where is the n' term. kinase-linked receptors

EC (binding ligand domain)

28

Effector for kinase-linked receptors

Protein kinases

29

Coupling in kinase-linked receptors

Direct

30

Structure of kinase-linked receptors

Single Tm domain linked to EC receptor domain and IC catlytic domain

31

Location of nuclear receptors

Cytosol

32

Effector for nuclear receptors

Gene transcription via the DNA

33

Structure of a nuclear receptor

Monomeric structure with separate receptor and DNA binding domain

34

As downstream pathway

With GTP bound
Activation of adenylyl cyclases
Prod of cAMP
Act of PK-A

35

Ai downstream pathway

Inhibition of adenylyl cyclase

36

Aq downstream pathway

Act of PLC-B
Breakdown of PIP(4,5)2 to IP3 and DAG

37

Occupation is governed by

Affinity

38

Activation is governed by

Efficacy

39

What does high specifity mean for the binding of drug to receptor

Binding will be strong

40

What does high specifity mean for the rate of the forward and reverse reaction

Rate forward will be high
Rate backward will be low

41

A full agonist will have an efficacy of

E>0

42

An antagonist will have an antagonist of

E=0

43

Define occupancy

How many receptors are occupied by the drug and are forming the drug receptor comple

44

Occupancy will vary with

Drug concentration

45

Formula for occupancy

No receptors occupied / Total no. receptors

46

What scale is occupancy measure on

0 -1
0 no drug
1 fully occupied

47

Why is response NOT a good measure of occupancy

Response will vary with efficacy

48

Is occupancy directly proportional to occupancy

NO proportional - but not a 1:1 realationship

49

Define efficacy

The ability of the active form of the drug receptor complex to induce further downstream signalling

50

What is the method used to measure occupancy

A radioligand bidning assay

51

Method for accounting for non-specific binding in a radioligand binding assay

Two experiments in parrallel
In one - excess of cold ligand to displace the hot ligand from the specific bidning sites.
So any flourescence seen will be non specific binding (infinite number of binding sites)
So then specific binding = total bound - non specific binding

52

Conditions for purity in a radioligand binding assay

100% pure
Free from chirality (optical isomerism)

53

Methods to prevent degredation in a radioligand binding assay

Free radical scavenger
Store at low temps
Avoid light
Incorporation of an anti-oxidant

54

Example of a free radical scavenger

Ethanol

55

Example of an anti-oxidant

Ascorbic acid

56

Characteristics of a good lable in a radioligand binding assay

High specifity
Low concentration of tracer (safe)
Does not impact on normal fucntion

57

3H is

Tritium

58

Advantages of using tritium as a radiolable

Radioactive form is indistinguishable from the natural form
High specific activities
Good stability
Long half life (~67 years)

59

Disadvantages of using tritium as a radiolabel

Hard to get homogenus group (i.e. getting the sam number of Hs labelled on each compound)
Specialist labs are required
Labelling is expensive and difficult

60

Advantages of using 125 I

If aromatic hydroxyl group (tyrosine residues in peptides) can be incorporated at high specific activities
Easy to use in labs
Cheap

61

Disadvantages of 125 I

More readily degraded
Biological activity of the ligand can be reduced
Short half-life (~67 years)

62

Characteritics of the tissue selected

Selected to contain the recog sites
Can be isolated membrane or whole organ
Dependent on the receptor and hypothesis

63

Incubation conditions for a radioligand binding assay

Preserve the integrity of the ligand and receptor
High enough protein content for assay volume
Additives added for protection
Temp close to 0

64

Why must temp be close to 0 during incubation

When cells homogenated, lysosomes ruptured, lysozyme enzyme released which would breakdown proteins
At a low temperature activity is low enough to protect the proteins

65

Two techniques usually used for spearation of bound from free

Filtration or centrifugation

66

What soluble binding techniques may be used to separate bound from free

Dialysis
Column chromatography
Precipitation/adsorption

67

The time available to separate depends on what

The KD

68

A low KD means

High affinity

69

A high KD means

Low affinity

70

If KD is low will you have long to separate

Yes

71

If the KD is high will you have long to separate

No

72

How to calculate specific binding from total bound and non specific

Specific = total - non specific

73

What is the shape o the total bound curve

Rectangular hyperbola

74

Why is specific binding saturable

Only a certain number of binding sites, eventually these will be full

75

How many non specific binding sites

Infinite

76

What is Bmax

The maximum occupancy

77

What is Kd

The disociation constant - concentration required for 50% occupancy

78

What is the equation linked to occupancy

Langmuir equation

79

Kd is a measure of

Affinity

80

Kd =

(k-1)/(k+1)

81

Lower KD means a

Higher affinity

82

Give the equation for bound drug at X concentration

(Bmax X Xa) // (Xa + Kd)

83

Give the equation for b/f from a scatchard plot

(Bmax - B) // Kd OR

B/F = 1/KD . (B-Bmax)

84

What does B represent

Bound radioligand

85

What does F represent

Free radioligand

86

What is the free radioligand equvilant to?

The ammount of drug added, concentration change due to some binding will be neglidgeable

87

What is Kd

The dissociation constant, concentration at which there is 100% occupancy

88

Bmax

Maximum ammount of drug which can specifically bind if one drug molecules binds to each receptor

89

Low Kd means

High affinity

90

High Kd mean

Low affinity

91

On a scatchard plot what is given by the slope of the graph

-1/KD

92

On a scatchard plot what is given by the x intercept

A ratio of B/F of zero => no bound

93

What is known if Kd is different

Must be a different receptor

94

Kd can be used to ___________ the ___________ of receptors

Quantify number

95

What disease involves a loss of receptors

Myasthenia gravis

96

What would be used to measure efficacy

A bioassay

97

Define EC50

Drug concentration which gives half of the maximum response capable of a tissue

98

Describe how Kd and EC50 values differ, why>

EC50 lower than Kd ==> 50% of the response is seen which less than 50% occupancy of the receptors
Due to receptor reserve or spare receptors

99

Why is there a receptor reserve

Signal amplification --> 1 activated receptor causes activation of many second messengers
MAX response may have been achieved - i.e. a muscle will only be able to contract to a certain degree

100

What is the hill equation used to determine

Response of a tissue to an agonist

101

What is the hill equation

(MAX . [Xa]^n) // ([Xa] + [EC50]^n)

102

What is MAX

Max response of a tissue

103

Xa =

Concentration of agonist

104

What is n

The slope factor -
Determined by how many molecules of agonist need to bind to a receptor in order to activate it

105

What would the n number of a muscarinic receptor be

1

106

What would the n number of a nicotinic receptor be

2

107

EC50 is a measure of a drugs

Potency

108

What are the three things that potency depends on

Affinity, efficacy and the receptor reserve

109

What does a lower EC50 indicate

A greater potency

110

What does a higher EC50 indicate

A lower potency - more receptors need to be activated to induce 50% of the MAX response

111

What can be said about agonists A B and C in respect to their maximum responses and their EC50 value

Different maximum responses
Same EC50 values

112

What is efficacy a measure of

A single agonist-receptor complexes ability to cause a response

113

What can be said about the occupancy for a partial agonist to elicit it's maximum response

All of the receptors have to be occupied

114

What are the three properties that determine the effect of a drug

Specificty
Affinity
Efficacy

115

Define specificity

The ability of a drug to interact with a structurally defined site/receptor

116

Define affinity

The ability of a drug to bind to a receptor

117

Define efficacy

The ability of a drug to cause activation of a receptor

118

What is the efficacy of a full agonist

E = 1

119

What is the efficacy of an antagonist

E = 0

120

What is an inverse agonist

A drug that when it binds to the receptor stabilises the resting state of that receptor reducing any constitutive activity

121

What is constitutive activity of a receptor

Where no agonist is bound but the receptor becomes spontaneously active

122

Define antagonist

A drug which prevents the response of an agonist - there are many types

123

Describe chemical antagonism

Substances which can combine in solution to chemically alter the agonist so that the active drug is lost

124

Example of a chemical antagonist

Heavy metal poisoning treating with chelating agents

125

What is a pharmacokinetic antagonist

Drug that causes a change in rate of metabolism, rate of absoprtion or rate of excretion

126

How do opiates act as pharmacokinetic antagonists

Affect GI tract motility, will reduce the effectiveness of any drugs which are taken by the oral route

127

What is warfarin used to treat, how does it show pharmacokinetic antagoism and why is this dangerous

Blood thinner (anti-coagulant) - taken in order to prevent heart attacks
Care must be taken when treated with some a.biotics as stimulate the metabolism of warafin and all of the effects are lost

128

What is physiological antagonism

Two drugs which act on different receptors to cause opposing effects within the body

129

How is arterial BP affected by physiological antagonism

NA --> Increases arterial BP
Histamine --> Causes vasodilation thus lowering arterial BP

130

What is a non competitive antagonist

Substances which cause a block between receptor activation and response so no response is seen

131

How do dihydropyridines show non competitive antagonism

By blocking voltage gated calcium channels, preventing smooth muscle contraction

132

Define competitive antagonism

Directly compete with the agonist for occupancy of the receptor

133

What are the two types of competitive antagonists

Reversible and irreversible

134

What would happen to the dose response curve if reversible antagonist added, what about if more agonist was added ontop of this

Curve would shift (translate) right

135

What would happen to the max response and EC50 values if reversible antagonist added, what about if more agonist was added ontop of this

Max response would remain the same but ammount of agonist required would increase
EC50 would also increase

136

FOR COMP REVERSIBLE ANTAG
If concentration of agonists is ______ sufficiently the ______________________ increases and the effect of the antagonist ___________. This type of antagonism is _____________

Increased
Probability of an agonist binding specifically
Decreases
Surmountable

137

Define dose ratio

How many more times agonist is needed in the presence of an antagonist for the same response to be seen

138

Equation of dose ratio (DR)

[Agonist in presence of antagonist] // [Agonist without the antagonist]

139

A shild analysis is used to measure

Antagonist affinity

140

In a schild analysis what is DR given by

DR = (Xb/Kd) + 1

141

What is Xb

Concnetration of antagonist

142

What is Kd

Antagonist affinity contastnt

143

Whar is the equation of the line in a schild analysis

log(DR-1) = Log[Xb] - Log(Kd)

144

In a shild analysis what is plotted on the y axis

Log(DR-1)

145

In a schild analysis what is plotted on the x axis

Log(Xb)

146

The x intercept of a schild analysis is

- log(Kd)

147

Define Pa2

The negative log of molar concentration od the antagonist which will reduce the effect of double dose of the agonist drug to that of a single dose or -1*logKd

148

What can be said when DR

Double ammount of the receptor agonist is required for the same response to be seen

149

Pa2 =

logKd

150

What can be said about a higher Pa2 value

Higher the affintiy of the antagonist

151

If Pa2 is 9 what is KD

10^-9

152

Can irreversible competivie antagonism be reversed by washing

No

153

irreversible competivie antagonism is

Time dependent

154

Give an example of irreversible competitive antagonism

The effects of the alkylating drug bidenamine on histamine receptors in the guinea pig ileum

155

What condition cause desensitisation

When a drug is given continuously or repeatedly over a period of time

156

Desensitiation occurs due to

Loss of receptors from the cell surface
Change in the receptor - e.g. due to phosphorylation
Exhaustion of mediators
Increased metabolic degredation or extrusion of the drug
Physiological adapation