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100

50

0

Response (% maximum)

10 –9 10 –8 10 –7 10 –6 10 –5

Concentration of agonist (log scale)

(b)

Full agonist

Full agonist

and partial

agonist

Partial agonist

100

50

0

Response (% maximum)

10 –9 10 –8 10 –7 10 –6

10 –7M10–6M10–5M

10 –5 10 –4

Concentration of agonist (log scale)

(a) Increasing antagonist concentration

Fig. 17.2Dose–response curves for receptor agonist. (a) In the presence of a competitive reversible antagonist.

In the presence of a reversible antagonist, the dose–response curve for a full agonist is shifted to the right

because the full agonist competes for receptor binding. The higher the concentration of antagonist the greater

the shift, but the maximum response remains unchanged. below (b) In the presence of a partial agonist,

the dose-response curve of the full agonist is shifted to the right. (Reproduced from Maxwell, S. R. J. and

Webb, D. J. (2008). Receptor functions.Medicine, 36 , 344–349, by permission of Elsevier Science.)

666 Cell membrane receptors and cell signalling