Page 18 - Module 1-Anatomy and Physiology of Growth Hormone Function Final (Interactive)

Module 1: Anatomy and Physiology of Growth Hormone

Physiology and regulation of IGF-1

In Part 1, we discussed that IGF-1 is produced primarily in the liver in response to
stimulation by GH. GH produced by the pituitary gland is transported by GH-binding
protein which then binds to its receptors in different tissues, including the liver.

Figure 7. Effects of GH and IGF-1 on growth and metabolism

(+), stimulatory effect; (-), inhibitory effect; AN, anabolic effect; GLU, glucose utilization; LIP,
lipogenesis.

In Figure 7, two sources of IGF-1 are represented. One source is systemic (hepatic)
IGF-1 and the second is local IGF-1 produced as a direct result of local hGH
stimulation.

There are six known insulin-like growth factor-binding protenins (IGFBPs) which
regulate the availability of IGF-1 to its receptor in target tissues. In humans, almost
80% of circulating IGF-1 is carried in a trimeric complex consisting of IGF-1, IGFBP-
3 and ALS (acide labile subunit), which collects IGF-1 in the vascular system,
increasing its half-life and preventing excess IGF-1 from binding to the insulin
receptor.

Effects of IGF-1

IGF-1 exerts its physiologic effects by binding to specific cell surface receptors.
Although IGF-1 binds primarily to the IGF-1 receptor, some effects may be facilitated
through the IGF-2 and insulin receptors. IGF-1’s primary functions are regulations of
somatic growth, cell proliferation, transformation, and apoptosis.
At a more detailed level, this involves development, as well as, other biological
actions in the central nervous system, cardiovascular system, musculoskeletal
system, thymus, kidneys, reproductive system and has effects on fetal growth and
differentiation.

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