Peripheral Endocrine Organs

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Of peripheral sources of protein hormones, the pancreas, the source of insulin and glucagon, has received the most emphasis. Deficiency of insulin action, due to lack of or response to insulin, results in diabetes. Insulin is produced by pancreatic islets of Langerhans. Cells within the islets also secrete glucagon, SRIH, pancreatic polypeptide, and amylin. Cellular uptake of glucose and amino acids is stimulated by insulin. Insulin and glucagon act in concert in the liver to maintain energetic and glucose ho-meostasis. Increased blood concentrations of insulin result in reduced blood concentrations of glucose. Low blood glucose induces secretion of pancreatic glucagon, which activates hepatic gluconeogenesis.

Insulin is a member of a family of structurally similar hormones that comprise two peptide chains bound together by disulfide bonds. Other members of this hormone family are insulin-like growth factor (IGF)-I, IGF-II, relaxin, and nerve growth factor (NGF). Relaxin is produced by late pregnant corpora lutea and its principal action in mammals is to soften the cervix and pelvic ligaments in preparation for parturition. IGF-I and IGF-II have endocrine, paracrine, and autocrine actions, are produced by a plethora of tissues, respond to GH stimulation, and are generally considered anabolic. IGF-

I and -II can exert insulin-like endocrine effects on blood glucose in sufficient doses. Most IGF in the circulation are bound to specific binding proteins (IGFBP) that modify their biological activity and clearance. While IGF-I is important in postnatal growth, evidence from exogenous administration[1] and transgenic studies has not established whether actions are endocrine, paracrine, and autocrine. IGF-II is important for fetal growth and has a role in myoblast differentiation.

The gastrointestinal (GI) tract is a set of tissues with numerous secretory activities. Among the protein hormones produced by the GI tissues are secretin, gastrin, motilin, cholecystokinin, glucose-dependent intestinal polypeptide, galanin, vasoactive intestinal polypeptide, gastric inhibitory peptide, neurotensin, TRH, SRIH, glicentin, and ghrelin. Some GI hormones influence aspects of digestive tract function including motility, blood flow, and excretory functions. Others coordinate digestive processes with systemic metabolic and anabolic processes.

Liver is a major organ of the endocrine system. It is a site of glucagon and insulin action and produces IGF, IGFBP, and hormone-binding globulins. Hormone-binding globulins are important in the transport of steroid hormones.

Many tissues produce and receive hormonal signals. The heart produces atrial natriuretic hormones; lungs produce vasoactive intestinal peptide, SRIH, and sub-stance-P; the thymus produces thymulin and thymosins; the spleen produces splenin; kidneys produce renin, erythro-poietin, and angiotensins; platelets produce growth factors, e.g., platelet-derived growth factor (PDGF), hepatocyte growth factor, and others; macrophages produce interleu-kins and interferons; and muscle produces IGF.

Adipocytes are targets of many hormones and secretors of the hormones leptin, resistin, and adipsin, as well as sites where energy is stored as fat. Leptin has satiety effects and has received much attention as a potential treatment for obesity. Blood concentrations of leptin correlate with fatness and may be means by which adipocytes communicate information about body condition to higher centers, suppressing appetite and stimulating reproductive processes. Exogenous leptin has positive actions on some reproductive processes.

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