Biology Reference
In-Depth Information
Example 10-3
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The Growth Hormone Network. Growth hormone (GH), or
somatotropin, is a major hormone regulating growth and metabolism.
GH is secreted by the pituitary gland under the control of the following
substances released by the hypothalamus: (1) The
GH-releasing hormone (GHRH) that triggers the production and
secretion of GH, and (2) somatostatin, or somatotropin release-inhibiting
factor (SRIF), which is a GH secretion inhibitor. Numerous other
substances could impact GH behavior. However, they are of secondary
importance to the GH network and, for the sake of simplicity, will be
omitted in the discussion that follows.
In a simplified view, GH secretion increases with increased
hypothalamic GHRH secretion but is inhibited by SRIF (which acts as a
suppressor for both the GHRH and GH). All three hormones are
subject to exponential elimination with certain half-lives. Laboratory
data in the adult male rat show that elevated concentrations of GH act by
way of time-delayed feedback (D
60-120 minutes) to stimulate
SRIF release from the hypothalamus, which antagonizes GH release
from the anterior pituitary gland and represses GHRH secretion.
Schematically, this minimal GH network can be represented by the
three-node diagram in Figure 10-5. The ellipses marked ''elimination''
signify that, in addition to the secretion rates of GH, GHRH, and SRIF
controlled by the feedback mechanisms, the dynamic behavior of the
system also depends upon the rates of continuous ongoing elimination
for all three hormones.
ΒΌ
Schematic representations (such as Examples 10-1 through 10-3) are
based on significant prior knowledge of the functional connectivity of
the system. Such knowledge is usually acquired through experimental
work, data analysis, and, as we shall see below, mathematical
modeling. Data analyses may also reveal certain specifics, such as
periodicity, fluctuations, or time patterns. Once the schematic diagram is
developed, it could be used as a basis for creating a dynamic model
utilizing difference equations or differential equations. The model
should be capable of reproducing and explaining key experimental
observations.
Consider the schematic representation of the GH network in Figure 10-5,
which is based on the physiological links between GH, GHRH, and SRIF
described in Example 10-3. In this system, frequent measurements of GH
concentrations in the bloodstream have unmasked complex patterns of
gender-specific and developmentally regulated patterns of GH release in
rats, sheep, and humans. In particular, GH secretion evolves as
infrequent clusters of large pulses in adult male rodents, pubertal
children, and young fasting or sleeping men and women, but
unfolds as frequent, low-amplitude bursts in female rats and older,
