Final answer:
Growth hormone (GH) is a protein hormone from the anterior pituitary gland that stimulates tissue growth and protein synthesis through direct and indirect actions, including the use of insulin-like growth factors (IGFs). Direct actions include lipolysis and the glucose-sparing effect, while indirect actions involve promoting the uptake of amino acids and the production of new proteins. GH secretion is regulated by GHRH and GHIH from the hypothalamus.
Step-by-step explanation:
Growth hormone (GH), also known as somatotropin, is a protein hormone produced and secreted by the anterior pituitary gland, playing a crucial role in the endocrine system. GH primarily functions in an anabolic capacity, promoting protein synthesis and tissue building. GH activity can be categorized into two mechanisms: direct and indirect.
Direct Mechanisms of GH: The direct action of GH includes stimulating triglyceride breakdown, also known as lipolysis, which frees fatty acids into the bloodstream and encourages tissues to utilize fatty acids over glucose for energy, a process called a glucose-sparing effect. Additionally, GH directly provokes glycogen breakdown in the liver to increase blood glucose levels, an effect known as diabetogenic because it mimics high blood sugar levels found in diabetes mellitus.
Indirect Mechanisms of GH: The indirect mechanism of GH action is mediated by insulin-like growth factors (IGFs), or somatomedins, which are growth-promoting proteins mainly produced by the liver, subsequently stimulating tissue growth. IGFs increase the uptake of amino acids from the blood into skeletal muscle cells, cartilage cells, and other target cells, fostering new protein formation. This action is critical for post-meal times when blood concentrations of glucose and amino acids are elevated.
The regulation of GH levels in the body is controlled by two hypothalamic hormones: growth hormone-releasing hormone (GHRH), which stimulates the release of GH, and growth hormone-inhibiting hormone (GHIH, also referred to as somatostatin), which inhibits the release of GH.