Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5843_Библиотеки_им_академика_М_И_Перельмана.pdf
82 ESSENTIAL PHARMACOLOGY
for receptor activation. The phenomenon is the result of complex
coupling mechanisms between the initial binding event and the
actual measured effect in the system controlled by the receptor. One
teleological explanation for the evolution of this characteristic in a
biological signaling system is that it allows the system to be turned
on and off quickly, as might be needed, for example, for rapid con-
trol of tissue functions by the nervous system. When the receptors
are in excess, the target system responds when the concentrations
of the neurotransmitter (agonist) are lower than would be expected
on the basis of actual receptor affinity. This allows a rapid activation
of the system before very many neurotransmitter molecules have
had time to be released or diffuse across the synaptic cleft separat-
ing the presynaptic terminal from the postsynaptic membrane con-
taining the receptors. Then, since the affinity of the receptor for the
neurotransmitter is actually fairly low, at the end of the signal, the
transmitter is released from the receptor readily (a high off- rate),
permitting rapid relaxation of the system.
Here we will develop a model for a receptor signaling system
that exhibits this spare receptor behavior. Note that although the
term “spare receptors” may suggest some of the receptors are not
used, we will see that all of the receptors are the same and are
participating in the equilibrium phenomenon– in that sense none
of the receptors are really spare because they are not expendable.
We will postulate that there is some type of coupling between the
receptors and their effector systems. Then we assume that there
are more receptors present than are necessary to activate all of the
effectors. An exact formulation would depend on the molecular
nature of the coupling between the receptor and the effector, and
any intermediate coupling proteins that might be involved. For
the purpose of providing a relatively simple, hypothetical exam-
ple, let us suppose that the activated receptors have to bind to
something else– the effectors– to cause an effect. The percentage
of active receptors we will assume is given as in our simple occu-
pancy model:
Ra activereceptors DDKd
%/
100
Pharmacodynamics III 83
Then assume that the activated receptors (and only the acti-
vated receptors, not the inactive receptors) bind to the effector pro-
teins with a dissociation constant Kr. When the effector proteins are
bound by the activated receptors, they become active themselves,
again following the simple occupancy model so that
%/
maximum effect Ra Ra Kr100
We can generate a dose–response curve for this system by com-
bining these equations to yield
%/
/maximum effect DDKr DKd100 100
If we take Kd=10
−6
M as we did with the simple occupancy model,
and we set Kr = 0.1, so that half of the effectors are activated when 0.1%
of the receptors are activated. (For simplicity we are using 0–100% as
the numerical range for the concentration of active receptors). This
construct leads to the titration curve illustrated in Figure9.12.
This figure compares the dose–response curves that would be
generated if the system followed the simple occupancy model with
the curve obtained with the spare receptor model. We see that the
10
–10
0
20
40
60
80
100
120
10
–9
10
–8
10
–7
10
–6
[D], molar
10
–5
10
–4
10
–3
10
–2
% of maximal effect
Simple occupancy
model
“Spare receptors”
Figure9.12
84 ESSENTIAL PHARMACOLOGY
spare receptor phenomenon causes the dose–response curve to shift
to the left, in this case by three orders of magnitude. The size of the
shift can be determined by calculating the concentration of D at
which the expression above yields 50%. Inspection of the equation
indicates this occurs when
DKrD Kd / 100
Solving forDyields DKrKdKr
M
M
/
./.
~
100
01 10 99 9
10
6
9
Thus the apparent potency of this drug is in the nanomolar
range, even though its affinity for its receptor is only micromolar.
The existence of the spare receptor phenomenon has important
consequences for the way a number of our most useful antagonists
behave invivo. We saw before that noncompetitive antagonists are
particularly effective in a biological context because the body can-
not overcome their effect by releasing more of the endogenous ago-
nist. This property of noncompetitive antagonists was due to their
ability to make some of the receptors unavailable for activation.
However, when “spare” receptors are present, noncompetitive
antagonists act more like competitive antagonists until the excess
receptors have been blocked by the antagonist. We can illustrate
this effect by considering the model above with the introduction
of a noncompetitive antagonist I. We modify the expression for the
number of active receptors according to the formula we derived
before for noncompetitive antagonists:
RACTIVERECEPTONS
D
DK
I
K
a
d
i
% 100
1
1
Pharmacodynamics III 85
In Figure9.13, dose–response curves for this system are plotted,
where the Ki=10
−6
M and the concentrations of the antagonist are
I=0, 10
−6
, 10
−5
, 10
−4
, and 10
−3
M.
Note that the effect of the antagonist at lower concentrations is
to shift the dose–response curve for the agonist to the right with-
out reducing its efficacy. This effect is very similar to the behavior
of competitive antagonists; that is, at lower concentrations of the
antagonist, its effect can be surmounted completely by higher con-
centrations of the agonist. However, eventually, if the antagonist
concentration is further increased, the true nature of the antago-
nist as a noncompetitive agent is revealed, and the antagonism
is no longer surmountable. Notice that this becomes clear just as
the apparent potency of the agonist is shifted to the vicinity of its
10
–10
0
20
40
60
80
100
120
10
–9
10
–8
10
–7
10
–6
[D], molar
10
–5
10
–4
10
–3
10
–2
% of maximal effect
I = 10
–3
I = 0
Figure9.13
−6
−
−4
−3
−6
−9
86 ESSENTIAL PHARMACOLOGY
true Kd for the receptor (10
−6
M). Clinically, then, the body can to
some extent overcome the effect of noncompetitive antagonists
by producing more of the agonist if the signaling system has
“spare” receptors.
9.8 Displacement ofthe Activation Curve
tothe Right ofthe Binding Curve: Fractional
Occupancy Threshold
There may be other anomalous relationships between receptor
occupancy and effect, including a “threshold” phenomenon that is
depicted in Figure9.14. If the receptors with which a drug combines
do not have a rate- limiting role in the overall reaction sequence
leading to the characteristic biological response, then no drug effect
at all will be seen until a certain fractional occupancy threshold is
exceeded and the step being regulated by the drug now becomes the
rate- limiting step in the overall process (see Figure9.14).
10
–10
0
20
40
60
Simple
occupancy
model
Fractional
occupancy
threshold
80
100
120
10
–9
10
–8
10
–7
10
–6
[D], molar
10
–5
10
–4
10
–3
10
–2
% of maximal effect
Figure9.14
Pharmacodynamics III 87
A mechanistic model for a requirement for fractional occupa-
tion is given by the following example. Consider a garden hose with
two clamps in series that restrict flow through the hose and are each
controlled by independent valves (“receptors”), as in Figure9.15. If
the upstream clamp (CLAMP A) is partially closed, flow through the
hose will not be further restricted by the second clamp until the sec-
ond clamp (CLAMP B) is closed to a point that provides additional
restriction to the overall flow. The flow in the hose will show a frac-
tional occupation requirement for the second receptor to alter the flow
through the hose. The ”flow” in the hose could be representative of
any series of events in a biochemical pathway regulated by multiple
receptor- controlled steps, one or more of which become rate- limiting.
9.9 Review: TheUse ofthe Dose–Response Curve
toCharacterize Drugs andReceptors
The dose–response curve is a valuable research tool for the charac-
terization of receptors in complex systems. In particular, the effects
antagonists have on the curves can reveal information, as we have
seen, about the nature of the antagonists as well as the number and
types of receptors present. By way of review, we will examine here a
Flow
Clamp A,
Receptor A
Clamp B,
Receptor B
Figure9.15
88 ESSENTIAL PHARMACOLOGY
set of hypothetical titration curves for a number of agents acting on
a complex tissue, for example, the contraction of a smooth muscle
or the secretion of a hormone from a gland (Figure 9.16).
Let us first review the terms potency and efficacy. In this system
it is clear that agent A is more potent than agent B or C. It is also
more efficacious than agents D or C. Agent D, however, is more
potent than agents B or C.
Agent E appears to be unable to activate this system (notice the
flat line at the bottom of the diagram). However, the presence of a
fixed amount of agent E does cause a shift in some of the titration
curves: The curves for A and D are shifted to the positions A + E2
and D + E2, respectively. Since the curves are shifted in a parallel
fashion, it appears that agent E is a competitive antagonist.
The presence of the antagonist E does not appear to shift the
dose–response curve for agent C. This is very good evidence that
agent C acts through a different receptor in this system than agents
A or D. This type of analysis is frequently used to determine how
D
100
50
% maximal response to Δ
log [agent, A to E]
B
(A + E
2
)
C
E
(C + E
2
)
(D + E
2
)
Figure9.16
Pharmacodynamics III 89
many receptor types may be controlling a particular tissue or pro-
cess in an organism. What can we say about the receptor for agent
B? Nothing!– because we are presented with no information about
the effect of agent E on the dose–response curve for agent B.
Are agents A and D acting at the same receptor, even though
the maximum effect is very different for the two agents? We can
assess this by looking at the effect of agent E on the titration curves
for agents A and D. In both cases the titration curves are shifted to
the right. Notice that the magnitude of the shift is the same in both
cases. We demonstrated in our analysis of the action of competitive
antagonists that the apparent Kd of the agonist is increased by the
factor (1 + I/Ki) by the presence of the antagonists. Since the same
amount of agent E is used in both cases, and the shift is the same
magnitude, the affinity of the agent E (Ki) for the receptor for A is
the same as the affinity it has for the receptor for agent D. We can
take this as very good pharmacological evidence that E, A, and D
are all acting on the same receptor.
Since A and D are acting on the same receptor, but D is less effi-
cacious than A, we can define agent D as a partial agonist at this
common receptor. We could not make the same statement about
agent C acting at its receptor even though it is less efficacious than
agent A. Since C is apparently acting at a different receptor than A,
C may indeed be a full agonist at this receptor, even though this
receptor cannot drive this system as effectively as A (or B) can.
As a final point of review, we would want to be cautious about
our hypothesis that E is acting as a competitive antagonist at the
receptor for A. Although the dose E2 causes a parallel shift of the
dose–response curve for A to the right, what might happen if we
added a higher dose of E, say E3? If we then found that the curve
of A + E3was coincident with the curve for C, we would see that
agent E was beginning to reduce the efficacy of agent A. This is
the type of behavior that we found is shown by a noncompetitive
antagonist acting on a system in which there are spare receptors:
first the antagonism appears competitive, but when tested at higher
doses of the antagonist, the effect is revealed to be noncompetitive.
Sometimes it pays to do just one more experiment!
90
Chapter 10
Biological Receptor-
Effector Mechanisms
A
s mentioned earlier, it is an axiom of pharmacology that
drugs do not create new processes in the body; rather,
they alter the rates of existing processes. Typically they
do this by acting through the body’s own control mechanisms.
Therefore, the study of how processes in the body are normally
regulated by endogenous ligands is an important area of current
pharmacological research. Current and future drugs will likely
“follow the same rules.”
Receptors have traditionally been identified as high-affinity
binding sites for hormones or neurotransmitters that control
cellular processes. Upon isolation they typically have turned
Biological Receptor- Effector Mechanisms 91
out to be protein molecules large enough to have specific pock-
ets to bind the small molecule ligands. We classify them by the
name of the ligand, for example, the “insulin receptor,” rather
than by the process they control, e.g., not the “glucose uptake
controller.” However, this system of nomenclature quickly
becomes inadequate as more receptor subtypes are discovered.
For example, the neurotransmitter acetylcholine binds to two
major classes of receptors, which can be distinguished phar-
macologically in that one class binds the natural alkaloid from
tobacco, nicotine, while the other class binds an alkaloid from
a poisonous mushroom, muscarine. Accordingly, these receptors
are called nicotinic cholinergic and muscarinic cholinergic
receptors, respectively.
A similar multiplication of receptor subtypes characterizes the
pharmacology of the neurotransmitter noradrenaline– there are
alpha and beta subtypes of adrenergic receptors, and among these,
alpha1, alpha2, beta1, beta2, etc.
Initially our ability to recognize the existence of so many
classes of receptors was limited by the availability of specific com-
pounds that had differential binding affinities for one or several
subtypes of receptors. However, molecular cloning and genome
sequencing technologies have now surpassed our pharmaco-
logical approaches. Now we recognize there are more receptor
subtypes in many classes than we can distinguish with the small
molecule ligands currently available. In some cases these sub-
types are expressed in unique patterns in the body that suggest
if new drugs can be found to bind them specifically, we would
be able to alter physiological or pathophysiological processes in
novel patterns.
Genome sequencing has also brought about the embarrassing
situation that we now know of entire new classes of receptor- like
molecules, but we don’t know what the natural ligand in the body
is for these receptors. However, these so- called “orphan recep-
tors” may really be an embarrassment of riches as they hold out the
promise of new drugs and novel approaches to controlling normal
and abnormal physiology.