Biology Reference
In-Depth Information
B. Secretion of One Hormone Controlled by the Concentration of Another
Up to this point, the events of secretion and elimination of a single
hormone have been considered independent from any other hormone. In
reality, there is considerable interaction between different hormones. We
now consider the case when the secretion rate of hormone A is controlled
by the concentration of hormone B. Figure 10-9 represents this situation.
B
(
±
)
A
The secretion of hormone A can be divided into two components, basal
and system-regulated (Figure 10-10). The basal secretion represents the
amount of hormone secreted independently of other system (model)
components (e.g., hormone B, in our case). The system-regulated
component corresponds to that part of the secretion exclusively related
to other system hormones. Within this paradigm, if B is a stimulator, the
basal secretion is what would remain if B were removed from the
system. However, if B were an inhibitor, the basal secretion would be the
release of A remaining after the action of B is applied to its full potency
(e.g., by constant infusion of high, pharmacological doses of B).
1
Elimination
FIGURE 10-9.
Schematic presentation of a
hormone network wherein the
secretion of A is regulated by the
concentration of hormone B (and
also by elimination).
The above concept implies that the rate of change of the concentration
C
A
of hormone A from Eq. (10-1) can be written as:
dC
A
dt
¼a
B
C
A
ð
t
Þþ
S
A
ð
t
Þ;
where the rate of secretion S
A
can be presented as the following sum:
(
±
)
S
A
¼
S
A
;
basal
þ
S
A
;
system
ð
C
B
Þ:
(10-4)
A
System-controlled
A
Basal
Here, S
A
;
basal
and S
A
;
system
ð
represent the basal and system-controlled
secretion of A, respectively. The system component S
A
;
system
ð
C
B
Þ
will
depend explicitly only on the concentration of B (and not on the time t).
We shall assume the basal secretion S
A
;
basal
is constant. S
A
is called a
control function, and we discuss below the choice of its components
S
A
;
basal
and S
A
;
system
C
B
Þ
elimination
FIGURE 10-10.
Formal separation of the secretion of A into basal
and system-controlled.
. This choice, although arbitrary to some extent,
should conform to a set of general rules. For example, S
A
;
basal
and
S
A
;
system
ð
C
B
Þ
must be non-negative, because the secretion rate is always
non-negative. Also, in most cases the system component of the control
function will be monotone. The presence of hormone B can serve to
either stimulate or inhibit the secretion of hormone A, but in either case
we shall assume the effect is monotone. That is, if the effect of hormone
B is to stimulate the secretion of A, then a higher concentration of
B will cause a stronger stimulus (and higher secretion rate) of A. The
function S
A
;
system
ð
ð
C
B
Þ
will be monotone increasing as a function of
C
B
. Likewise, if the effect of B is to inhibit the production of A, the
C
B
Þ
1. The removal of hormone B could be performed by infusion of antibody to B,
suppression of its secretion, or down-regulation or blockage of the receptors
that mediate its action. For additional detail, see Section IV, part C. Note that this
use of the term basal secretion is model-dependent and differs from the definition
provided in Chapter 9.
