Page 14 - Module 1-Anatomy and Physiology of Growth Hormone Function Final (Interactive)

Module 1: Anatomy and Physiology of Growth Hormone

GHRH and Somatostatin

The two main regulators of release of growth hormone are: growth-hormone
releasing hormone (GHRH) and somatostatin also known as growth-hormone
inhibiting hormone (GHIH), which have stimulatory and inhibitory effects.

• GH release is also inhibited by IGF-1, which serves as part of the negative
feedback mechanism. IGF-1 suppresses secretion of GH not only by directly
suppressing GHRH but also by stimulating release of somatostatin (GHIH) from
the hypothalamus.
• Ghrelin, which is a peptide that is released predominantly from the stomach, is
also a stimulator for GH release.
The interaction of these factors likely contributes to the diurnal, pulsatile pattern of
GH release.
The hypothalamic releasing and inhibiting factors are delivered to the anterior
pituitary through a specialized capillary network. They travel down the hypophyseal
portal veins to the anterior pituitary where they activate intracellular messengers that
act to release or inhibit pituitary hormones.

The responsiveness of the anterior pituitary to the effects of
hypothalamic stimulators or inhibitors can be modified by several
factors, including: hormone levels, negative feedback inhibition, and
circadian rhythms.

Sleep and circadian rhythms likely interact to produce the overall rhythmic pattern of
pituitary hormone release and the associated biological responses.
Somatostatin

As previously mentioned, the release of GH is inhibited by somatostatin (aka, GHIH).
Somatostatin is a peptide synthesized in the brain, and is also produced in peripheral
organs, including the endocrine pancreas, where it plays a similar inhibitory role on
insulin and glucagon. The expression of somatostatin receptors is modulated by
hormones and also by the nutritional state of the individual.
Other regulators of GH secretion

In addition to regulation by GHRH and somatostatin, GH is regulated by other
factors:

• Catecholamines, dopamine, and excitatory amino acids increase GHRH and
decrease somatostatin release.

• Hormones such as cortisol, estrogen, androgens, and thyroid hormone can also
have an effect on responsiveness to GHRH and somatostatin.
• Glucose and amino acids (eg from nutrition) can affect GH release, as they are
metabolized and used by the body. Hypoglycemia, which is low blood glucose
concentrations, stimulate GH secretion in humans in an effort to increase
glucose.

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