Step I Non-competitive antagonist VS. partial agonist

[LichenPlanus]

Got a uworld question saying you add Drug Y to a current drug, the system has spare receptor -> and graph showed:

- 0.01M Drug Y added -> right shift in Km (less potency)
- then 1.0 M Drug Y added-> decreased Efficacy (Vmax)

I understand that it makes sense this could be a noncompetitive antagonist, since as you run out of receptors @ higher concentration -> decreased efficacy. But, why can it NOT be just a partial agonist?

 

[Focused1]

Got a uworld question saying you add Drug Y to a current drug, the system has spare receptor -> and graph showed:

- 0.01M Drug Y added -> right shift in Km (less potency)
- then 1.0 M Drug Y added-> decreased Efficacy (Vmax)

I understand that it makes sense this could be a noncompetitive antagonist, since as you run out of receptors @ higher concentration -> decreased efficacy. But, why can it NOT be just a partial agonist?

For partial agonists the activity would depend upon the PREEXISTING TRANSMITTER/LIGAND concentration..

If preexisting ligand + partial agonist = Antagonist
If NO preexisting ligand + partial agonist = Agonist

Hope it helps

 

[Jabbed]

Got a uworld question saying you add Drug Y to a current drug, the system has spare receptor -> and graph showed:

- 0.01M Drug Y added -> right shift in Km (less potency)
- then 1.0 M Drug Y added-> decreased Efficacy (Vmax)

I understand that it makes sense this could be a noncompetitive antagonist, since as you run out of receptors @ higher concentration -> decreased efficacy. But, why can it NOT be just a partial agonist?

Partial agonists act as competitive antagonists assuming excess agonist concentration. There is no decrease in efficacy.