Biology Reference
In-Depth Information
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A
E
Elimination
Elimination
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D
D
B
B
Elimination
Elimination
E
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FIGURE 10-23.
Two hypothetical networks, in which hormone E stimulates the secretion of B. E is either involved in
a delayed feedback (left panel), or perturbs the subsystem A-B (right panel). (Reprinted from Farhy,
L. S. [2004]. Modeling of oscillations in endocrine networks with feedback, Methods in Enzymology,
384, 54-81. Copyright 2004, with permission from Elsevier.)
establish whether E is involved in a delayed feedback loop with B.
Moreover, in both networks, constant high infusion of E (as proposed
above) removes the pulsatility and elicits constant secretion of B.
Therefore, a more sophisticated experiment is required to reveal whether
E is indeed involved in a feedback loop with B (Figure 10-23, left
panel) or acts by perturbing the A-B subsystem (Figure 10-23, right
panel). A possible approach would be to block the endogenous
secretion of E and then administer a single E bolus. The system response
would be either a single spike of B secretion, if the network were that
depicted on Figure 10-23 (left panel), or a waning train of several B
pulses, if the network is the one shown on Figure 10-23 (right panel).
F. Separating Synthesis from Secretion
Although fluctuations of hormone concentration levels in the
bloodstream reflect analogous secretion patterns, this does not
necessarily mean that hormone synthesis follows the same patterns. For
example, the hormone synthesis may be occurring at a low constant rate,
with the amount of synthesized hormone accumulating in a separate
pool until a physiological signal triggers its release into the bloodstream.
Thus, there is a difference between the secretion of a hormone and its
synthesis that should not be overlooked.
As a physiological example, consider one observation typical for the
growth hormone control axis. Recall that GH is synthesized and secreted
from the pituitary gland into the bloodstream. Two major hypothalamic
hormones are universally recognized as regulating this process: GHRH
is known to stimulate both synthesis and release of GH, while the GH-
inhibiting hormone somatostatin suppresses only the secretion. It has
been observed that constant, short-term (4 hours), systemic SRIF infusion
initially suppresses GH release, but upon its withdrawal stimulates a
