5. Unknown Drug

Question 1

its potency can be expressed as a dissociation constant (KA) which is = 1 / affinity constant if it is what type of antagonist

Answer 1

a competitive antagonist

Question 2

Is the KA value independent or dependent on the agonist

Answer 2

This value will be independent of the agonist concentration (or type of agonist used to activate the receptor) and is a measure of the potency of the antagonist.

Question 3

A competitive antagonist will produce what change in the graph

Answer 3

parallel shifts in the sigmoidal agonist curves TO THE RIGHT, with no change in the maximum response.

Question 4

In addition, the slope of the SCHILD PLOT can be calculated (for a true competitive antagonist) and should be what

Answer 4

1 / close to unity

Question 5

Atropine is an antagonist at what receptors

Answer 5

at muscarinic acetylcholine receptors

Question 6

Hexamethonium is an antagonist too at what receptors

Answer 6

the nicotinic (acetylcholine) receptors

Question 7

Acetylcholine is an agonist for what receptors

Answer 7

the muscarinic acetylcholine receptors

Question 8

nicotine is an agonist for what receptors

Answer 8

the nicotinic (acetylcholine) receptors

Question 9

the addition of atropine led to no response when acetylcholine was being measured, which indicated that atropine blocked what

Answer 9

muscarinic receptors where acetylcholine (ACh) would have acted (as if did act, there would be a response).

Question 10

Since atropine is a muscarinic antagonist, this also confirmed to me that ACh-induced contraction is mediated what receptors

Answer 10

by muscarinic receptors in the rat ileum.

Question 11

when hexamethonium was added, there was still a little response, which indicated what about ACh

Answer 11

that ACh is not inhibited by that drug as acetylcholine can bind to both nicotinic and muscarinic receptors, therefore showing even though the nicotinic receptors were blocked ACh was still able to bind and illicit a response.

Question 12

If ACh’s contraction was due to nicotinic receptors, hexamethonium would have blocked it, but it didn’t. What does this tell us about the role of nictonic receptors in the contraction response

Answer 12

nicotinic receptors aren’t significantly involved in the contraction response.

Question 13

Nicotine activates what receptors where

Answer 13

the nicotinic receptors in the autonomic ganglia

Question 14

what effect does activating the nicotinic receptors in the autonomic ganglia have

Answer 14

leads to the release of ACh from postganglionic neurons. The ACh would then binds to muscarinic receptors, triggering a response.

Atropine blocks this response by preventing ACh from activating the muscarinic receptors, while hexamethonium would blocks it by inhibiting nicotinic receptor activation in the ganglia.

Question 15

what other organ has receptors for nicotine

Answer 15

the brain

Question 16

what do we understand about pA₂?

Answer 16

pA₂ is a measure of antagonist potency in competitive antagonism. It’s defined as:

The negative logarithm (base 10) of the molar concentration of an antagonist that requires a two-fold increase in the concentration of agonist to produce the same effect.

Formula:
pA 2 = −log 10 [antagonist]
when the antagonist causes a two-fold shift in the agonist dose-response curve.

Question 17

What does pA₂ tell us about atropine?

Answer 17

Atropine is a competitive antagonist at muscarinic receptors. The higher the pA₂ value, the more potent the antagonist.

If atropine has a high pA₂, it means a very small concentration is enough to significantly block muscarinic receptor responses (e.g., to acetylcholine).
It does not tell you the efficacy (because atropine doesn’t activate the receptor), but how strongly it binds to and blocks the receptor.