Biology Reference
In-Depth Information
We turn now to GH. Like LH and FSH, GH is produced by the
anterior pituitary in response to a hypothalamic releasing factor, the
growth hormone-releasing hormone (GHRH). GHRH reaches the
pituitary via the same circulatory system pathway taken by GnRH,
where it binds its target cells and stimulates the release of GH into the
bloodstream. The hypothalamus also produces an inhibitory substance,
growth hormone-inhibiting hormone (GHIH, also called somatostatin),
that decreases the production of GH by the pituitary. Generally
speaking, if the levels of GH are low, the hypothalamus will produce
GHRH. If the levels of GH are high, the hypothalamus will produce
GHIH.
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200
400
600
800
Minutes
1000 1200 1400
FIGURE 9-5.
A typical example of the variation of serum LH
levels over 24 hours in a healthy woman of
reproductive age sampled every 10 minutes.
Individual data points are represented as vertical
error bars corresponding to one standard
error of the mean value (SEM), where the SEM is
a function of the hormone concentration.
Hormone levels rise and fall multiple times over
the course of a single day. The SEMs were
evaluated with Eqs. (9-1) and (9-2) introduced
below, assuming an MDC ¼ 1.0 and CV ¼ 5.0%.
The units for LH are mIU/ml. The upper
sawtooth pattern represents peaks in the
concentration time series as evaluated by the
CLUSTER algorithm we shall describe later.
In summary, the endocrine system can be described as a complex of
signaling mechanisms, directing and coordinating multiple functions in
the organism. A remarkable feature of the system is its critical
dependence upon the pattern of hormone release that encodes
information necessary for the signaling mechanisms. Although the
average hormone levels are essential to the performance of the endocrine
system, achieving these levels through secretion events with exact
frequencies and amplitudes is of crucial importance. For example,
the same average concentration of a hormone can be achieved by
a few secretion events with large amplitudes, by numerous frequent
pulses of small amplitudes, or by an appropriate mix of large and
small hormone releases. Although the average hormone concentration
in all of these cases may be the same, the different profiles of the
secretion events would represent different examples of endocrine
signaling, only one of which would be functional in any given
situation.
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Serum levels of the pituitary hormones FSH and LH, as well as the
ovarian hormones estrogen and progesterone, vary considerably over a
normal 28-day menstrual cycle. Pituitary hormones, such as GH,
prolactin, thyrotropin, adrenocorticotropic hormone, FSH, and LH,
are secreted in a pulsatile manner (Veldhuis et al. [1987]), with their
levels rising and falling multiple times per day because of bursts of
secretion by the pituitary followed by periods of secretory inactivity.
The number of secretory bursts per day varies with the hormone in
question, the age, the gender, and the health of the individual. Figure 9-5
shows an example of the variation of blood serum LH levels over
24 hours in a healthy woman of reproductive age. LH levels rise and
fall repeatedly each day, and this variation is vital to the performance
of reproductive functions. Serum levels of GH for a healthy adult
exhibit similar behavior over the course of a 24-hour period (see
Figure 9-6).
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1
200
400
600
800
Minutes
1000 1200 1400
FIGURE 9-6.
A typical example of how serum growth
hormone (GH) concentration changes over a 24-
hour period in a normal healthy adult. The GH
was measured every 10 minutes. The individual
data points are represented with vertical lines of
length corresponding to one SEM. Note that
the SEM is a function of the hormone
concentration. The SEMs were evaluated with
Eqs. (9-1) and (9-2) introduced below, assuming
an MDC
8.23%. The units for
GH are ng/ml. The levels of GH rise and fall
multiple times over the course of a single day.
The large pulses of GH occur during the sleep
periods. The upper sawtooth pattern represents
peaks in the concentration time series as
evaluated by the CLUSTER algorithm we shall
describe later.
¼
0.0265 and CV
¼
The pulsatile nature of hormone release may easily be overlooked if
hormone levels are measured on a timescale with nonoptimal resolution.
Almost any introductory biology text would likely have a figure
