Biomedical Engineering Reference
In-Depth Information
giving a particular response in the presence of antagonist divided by the concentration of agonist
that gives the same response in the absence of antagonist. Typically one will chose the EC
50
values
to calculate the DR. In the Schild analysis the log (DR-1) is depicted as a function of the antago-
nist concentration (Figure 12.11B). When the slope of the curve equals 1 it is a sign of competitive
antagonism and the afi nity can then be determined by the intercept of the abscissa. When the slope
is signii cantly different from 1 or the curve is not linear it is a sign of noncompetitive antagonism,
which invalidates the Schild analysis.
As shown in the example in Figure 12.11, i ve concentration-response curves are generated to
obtain one antagonist afi nity determination, illustrating that the Schild analysis is rather work
intensive compared to, e.g., the transformation by the Cheng-Prusoff equation where one inhibi-
tion curve generates one antagonist afi nity determination. However, the latter cannot be used to
determine whether an antagonist is competitive or noncompetitive, which is the advantage of the
Schild analysis. When testing a series of structurally related antagonists one would thus often deter-
mine the nature of antagonism with the Schild analysis for a couple of representative compounds.
If these are competitive antagonists, it is reasonable to assume that all compounds in the series are
competitive and thus determine the afi nity of these compounds by using the less work intensive
Cheng-Prusoff equation.
12.3.5 C
ONSTITUTIVELY
A
CTIVE
R
ECEPTORS
AND
I
NVERSE
A
GONISM
Most receptors display no basal activity or only minor activity but some receptors display increased
basal activity in the absence of agonist that has been referred to as constitutive activity. Interestingly,
it has been shown that inverse agonists can inhibit this elevated basal activity, which contrast antago-
nists that inhibit agonist-induced responses but not the constitutive activity (Figure 12.12A).
The examples of important constitutively active receptors include the human ghrelin receptor
and several viral receptors that display constitutive activity when expressed in the host cell. This
latter group includes the ORF-74 7TM receptor from human herpesvirus 8 (HHV-8), which show
a marked increased basal response when expressed in recombinant cells (Figure 12.12B). ORF-74
is homologous to chemokine receptors and does indeed bind chemokine ligands. As shown in
Figure 12.12B, chemokines display a wide range of activities on the receptor from full agonism (e.g.,
GRO
) to full inverse agonism (e.g., IP10), which correlates with the angiogenic/angiostatic effects
of the chemokines. In 1994, it was demonstrated that HHV8 infection is the cause of Kaposi's
sarcoma, which is a multifocal angioproliferative cancer disease mainly affecting AIDS patients.
α
Angiogenic
chemokines
Full agonist
100
GROα
200
GROγ
Partial agonist
150
GROβ
Acute
inlammatory
chemokines
IL-8
50
100
Antagonist
NAP-2
SDF
Partial inverse agonist
Full inverse agonist
50
Angiostatic/
modulatory
chemokines
ORF74
HHV8
IP10
0
0
-10
-9
-8
-7
-6
-5
-11
-7
Log conc.CXC-chemokine (M)
-10
-9
-8
(A)
Ligand (log M)
(B)
FIGURE 12.12
(A) The nomenclature of ligand efi cacies and schematic illustration on their concentration-
dependent effects on constitutive activity. (B) Ligand regulation of the constitutively active ORF-74 receptor
from Human Herpesvirus 8 (HHV8). ORF-74 is a GPCR coupled to phosphatidylinositol (PI) turnover, which
is regulated by a variety of human chemokines ranging from full agonism by GRO
to full inverse agonism
by IP10. (Adapted from Rosenkilde, M.,
Neuropharmacology
, 48, 1, 2005. With permission.)
α
