Growth Hormone | How does it work?
Sunday, March 20, 2005
It is always prudent to have a basic understanding of how a supplement, hormone or drug works to build and/or preserve muscle before considering its use. The knowledge of how a hormone acts in the body is necessary to make your own decisions and manage your own regimens if you plan on utilizing it. Without this understanding you will no doubt end up wasting a lot of money and perhaps put your health at risk.
It has been long believed that GH exerts its anabolic effects on peripheral tissues through IGFs, also known as somatomedins ("mediator of growth"). Binding proteins play an important role in moderating the anabolic effects of both GH and IGF-1. IGF-1 is controlled by at least 6 different binding proteins and there may others waiting to be elucidated. To date there are a couple theories as to just how GH causes growth in target tissues. The first theory is called the somatomedin hypothesis (Daughaday, 1972).
The Somatomedin hypothesis states that GH is released from the pituitary and then travels to the liver and other peripheral tissues where it causes the synthesis and release of IGFs. IGFs got there name because of there structural and functional similarity to proinsulin. This hypothesis dictates that IGFs work as endocrine growth factors, meaning that they travel in the blood to the target tissues after being released from cells that produced it, specifically the liver in this case. Indeed, many studies have followed showing that in animals that are GH deficient, systemic IGF-1 infusions lead to normal growth. The effects were similar to those observed after GH administration. Interestingly, additional studies also followed that showed IGF-1 to be greatly inferior as an endocrine growth factor requiring almost 50 times the amount to exert that same effects of GH (Skottner, 1987). Recently rhIGF-1 has become widely more available and is currently approved form the treatment of HIV associated wasting. This increased availability allowed testing of this hypothesis in humans. Studies in human subjects with GH insensitivity (Laron syndrome) has consistently validated the somatomedin hypothesis (Rank, 1995; Savage, 1993).
The second theory as to how GH produces anabolic effects is called the Dual Effector theory (Green, 1985). This theory states that GH itself has anabolic effects on body tissues without the need of IGF-1. This theory has been supported by studies injecting GH directly into growth plates. Further evidence supporting this theory lies in genetically altered strains of mice. When comparing mice who genetically over express GH and mice who over express IGF-1, GH mice are larger. This evidence has been sited by some to support the dual effector theory. Interestingly, when IGF-1 antiserum (it destroys IGF-1) is administered concomitantly with GH, all of the anabolic effects of GH are abolished.
The Somatomedin theory and the Dual Effector theory are not all that different. One simply asserts that GH can produce growth without IGF-1. From the research I am inclined to believe in the Somatomedin theory. This only becomes an issue when one decides whether or not to use just GH or to combine it with IGF-1 or insulin.
From the evidence currently available you can count on three major mechanisms by which GH leads to growth (Spagnoli, 1996).
The effects of GH one bone formation and organ growth are mediated by the endocrine action of IGF-1. As stated in the Somatomedin hypothesis, GH, released from the pituitary, causes increased production and release of IGF-1 into the general circulation. IGF-1 then travels to target tissues such as bones, organs, and muscle to cause anabolic effects.
GH regulates the activity of IGF-1 by increasing the production of binding proteins (specifically IGFBP-3 and another important protein called the acid-labile subunit) that increase the half-life of IGF-1 from minutes to hours. Circulating proteases then act to break up the binding protein/hormone complex thereby releasing the IGF-1 in a controlled fashion over time. GH may even cause target tissues to produce IGFBP-3 increasing its effectiveness locally.
IGF-1 not only has endocrine actions, but also paracrine/autocrine actions in target tissues. This means that as GH travels to my muscles, the muscle cells increase there production of IGF-1. This IGF-1 may then travel to adjacent cells (especially satellite cells) leading to growth and enhanced rejuvenative ability of cells that didn’t see any GH. This is as suggested by the Dual Effector theory.
It has been long believed that GH exerts its anabolic effects on peripheral tissues through IGFs, also known as somatomedins ("mediator of growth"). Binding proteins play an important role in moderating the anabolic effects of both GH and IGF-1. IGF-1 is controlled by at least 6 different binding proteins and there may others waiting to be elucidated. To date there are a couple theories as to just how GH causes growth in target tissues. The first theory is called the somatomedin hypothesis (Daughaday, 1972).
The Somatomedin hypothesis states that GH is released from the pituitary and then travels to the liver and other peripheral tissues where it causes the synthesis and release of IGFs. IGFs got there name because of there structural and functional similarity to proinsulin. This hypothesis dictates that IGFs work as endocrine growth factors, meaning that they travel in the blood to the target tissues after being released from cells that produced it, specifically the liver in this case. Indeed, many studies have followed showing that in animals that are GH deficient, systemic IGF-1 infusions lead to normal growth. The effects were similar to those observed after GH administration. Interestingly, additional studies also followed that showed IGF-1 to be greatly inferior as an endocrine growth factor requiring almost 50 times the amount to exert that same effects of GH (Skottner, 1987). Recently rhIGF-1 has become widely more available and is currently approved form the treatment of HIV associated wasting. This increased availability allowed testing of this hypothesis in humans. Studies in human subjects with GH insensitivity (Laron syndrome) has consistently validated the somatomedin hypothesis (Rank, 1995; Savage, 1993).
The second theory as to how GH produces anabolic effects is called the Dual Effector theory (Green, 1985). This theory states that GH itself has anabolic effects on body tissues without the need of IGF-1. This theory has been supported by studies injecting GH directly into growth plates. Further evidence supporting this theory lies in genetically altered strains of mice. When comparing mice who genetically over express GH and mice who over express IGF-1, GH mice are larger. This evidence has been sited by some to support the dual effector theory. Interestingly, when IGF-1 antiserum (it destroys IGF-1) is administered concomitantly with GH, all of the anabolic effects of GH are abolished.
The Somatomedin theory and the Dual Effector theory are not all that different. One simply asserts that GH can produce growth without IGF-1. From the research I am inclined to believe in the Somatomedin theory. This only becomes an issue when one decides whether or not to use just GH or to combine it with IGF-1 or insulin.
From the evidence currently available you can count on three major mechanisms by which GH leads to growth (Spagnoli, 1996).
The effects of GH one bone formation and organ growth are mediated by the endocrine action of IGF-1. As stated in the Somatomedin hypothesis, GH, released from the pituitary, causes increased production and release of IGF-1 into the general circulation. IGF-1 then travels to target tissues such as bones, organs, and muscle to cause anabolic effects.
GH regulates the activity of IGF-1 by increasing the production of binding proteins (specifically IGFBP-3 and another important protein called the acid-labile subunit) that increase the half-life of IGF-1 from minutes to hours. Circulating proteases then act to break up the binding protein/hormone complex thereby releasing the IGF-1 in a controlled fashion over time. GH may even cause target tissues to produce IGFBP-3 increasing its effectiveness locally.
IGF-1 not only has endocrine actions, but also paracrine/autocrine actions in target tissues. This means that as GH travels to my muscles, the muscle cells increase there production of IGF-1. This IGF-1 may then travel to adjacent cells (especially satellite cells) leading to growth and enhanced rejuvenative ability of cells that didn’t see any GH. This is as suggested by the Dual Effector theory.
