Endocrine System
from multiple web sites and BIOLOGY: The Science of Life by Wallace, King and Sanders 2nd Edition Scott, Foresman and Co. 1986

• Hormones and the chemical messengers, are chemically active substances that cause changes in cells and tissues.
• The hormonal system often shares regulatory functions with the nervous system.

Hormones and the Invertebrates

• The source of hormones in many invertebrates is the nervous system.

Molting Activity in Crustaceans

• Molting or ecdysis, begins with preedysis (absorption of exoskeletal materials), followed by ecdysis (loss of old cuticle, stretching of the new), postecdysis (hardening of new cuticle), and interecdysis (variable amount of time between molts.
• In crayfish, the Y organ secretes crustecdysone, the molting hormone, while nearby X organ secretes molt inhibiting hormone (MIH).  Light is an essential part of regulating ecdysis.
• Establishing the validity of a hormonal substance includes
• removing the organ and observing deficiency symptoms
• isolating the substance in pure form
• removing deficiency symptoms by introducing the substance
• withdrawing the substance and observing the reappearance of the deficiency symptoms.

Hormonal Activity in Insects

• Many insects pass through a complete metamorphosis, including egg, larva, pupa, and adult states. Insect metamorphosis is influenced by the corpus cardiacum, corpus allatum, and the or the prothoracic, or ecdysal gland. The juvenile state is maintained by the juvenile hormone (JH). Final molting and the end of the larval state occur when brain hormone  from the corpus cardiacum stimulates the prothoracic gland to secrete ecdysone.
• Transplanted larva corpora allata cause an extension of the larval state. If the transport of hormones is stopped, parts of the body mature while others remain in a larval state.

Vertebrates: The Human Endocrine System

• The human endocrine system consists of about nine distinct glands and a number of less organized tissues. The endocrines are ductless glands, whose secretions enter the blood (as opposed to endocrine glands, whose secretions empty through ducts). Hormones act in minute amounts and are rapidly degraded
• Vertebrate hormones are classified as steroid, proteins, polypeptides, peptides, modified amino acids, or prostaglandin.

Pituitary

• The pituitary gland influences other endocrine glands and body tissue through negative feedback loops. The hypothalamus of the brain acts os an intermediary, monitoring feedback activity and stimulating the pituitary.
• The pituitary includes two major lobes, the anterior and posterior pituitary . The anterior lobe receives releasing hormones (releasing factors) from the hypothalamus via the blood, while the posterior lobe receives hormones from the hypothalamus, transported within connecting neurons.
• hypothalamus influences the anterior pituitary through secretions of six releasing hormones, and two inhibiting hormones. The hormones are peptides that are secreted in trace amounts.
• The anterior pituitary hormones include the following:
• Adrenocorticotropic hormone (ACTH) regulates fat metabolism and stimulates the adrenal cortex to secrete its glucocorticoids, which are involved in glucose metabolism. Negative feedback begins when rising levels of the adrenal hormones are sensed by the hypothalamus, which responds by lessening its pituitary stimulation.
• Growth hormone (also, GH or somatotropin) accelerates the cellular uptake of amino acids, decreases carbohydrate metabolism, and stimulates insulin release. Excesses of GH in childhood result in giantism (pituitary giants), while insufficiencies cause dwarfism (pituitary dwarfs). Excesses in the adult cause acromegaly, growth in cartilaginous areas of the body.
• Thyroid-stimulating hormones (TSH or thyrotropin) stimulates the thyroid to release its hormones.
• Prolactin promotes milk production, while oxytocin promotes milk release. Prolactin increases in males and females during sexual excitement.
• Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are gonadotropins that stimulate gamete and sex hormone production in the gonads.
• The posterior pituitary releases the hormones oxytocin and antidiuretic hormone (ADH or vasopressin). In addition to milk release, oxytocin stimulates the uterine muscles to contract during sexual excitement and during labor. ADH, a diuretic, regulates water reabsorption in the collecting ducts of the kidney.

Thyroid

• Two of the three thyroid hormones, thyroxin (T3) and triiodothyronine (T4) differ only in their iodine content. In amphibians thyroid hormones stimulates metamorphosis, while in humans it influences carbohydrate metabolism.
• Hyperthyroidism produces weight loss and nervousness, while a shortage of iodine produces a goiter, a visable enlarged thyroid gland. Hypothyroidism causes myxedema-sluggish behavior, body puffiness, and fluid retention. In childern, hypothyroidism leads to cretinism, severe mental and physical retardation.
• Calcitonin, the thrird thyroid hormone, works with the parathyroid glands to regulate calcium.

Parathyroid and Parathormone

• Parathormone (parathyroid hormone or PTH), a polypeptide, cooperates with calcitonin in regulating the distribution of body calcium. Parathormone increases calcium levels in the blood by increasing intestinal absorbtion and kidney reabsorption, and by stimulating the bone decalcifying activity of osteoclasts. Calcitonin performs roughly the opposite actions including the stimulation of bone calcifying activity by osteoblasts.
• Low levels of PTH produce blood calcium deficiencies resulting in severe muscle spasms and loss of muscle control. High PTH levels cause severe bone calcium loss (osteoporosis) and irregularities in muscle contraction.

Pancreas and Islets of Langerhans

• The islets of Langerhans are clusters of insulin-secreting beta cells, glucagon-secreting alpha cells, and delta cells that are believed to secrete somatostatin, or growth hormone inhibiting hormone (GHIH).
• Glucagon stimulates the liver to convert glycogen to glucose, which elevates the blood level. Insulin stimulates cells to take up glucose, thus lowering blood levels.
• Deficient insulinlevels cause hyperglycemia (high blood sugar), commonly called diabetes mellitus. Diabetes is most often caused by a shortage of or defects in insulin receptor sites, but it can be caused by a shortage of insulin or overabundance of the insulin-deactivating enzyme insulinase.
• The symptoms of diabetes are varied, but glucose in the urine is common. It is often successfully controlled by diet and insulin injection.
• Hypoglycemia (low blood sugar) is often a result of insulin overproduction. Commonly, the blood regulating feedback mechanism goes awry, and a time lag occurs between increased blood sugar and insulin release.

Adrenal Glands

• The adrenal glands have two layers-the outer cortex and the inner medulla. The adrenal cortex secretes mineralocorticoids, including aldosterone and cortisol (hydrococrtisone).
• Aldosterone increase potassium excretion by the kidney and slows sodium excretion. Hyperactivity causes alkalosis, whose symptoms include edema (fluid retention). Aldosterone hypactivity causes excessive salt and water loss and potassium retention, causing a potentially fatal acidosis.
• Coryisol increases blood levels of glucose, amino acids, and fatty acids and is an anti-inflammatory agent, suppressing and killing histamine-releasing lymphocytes.
• Adrenal steroid sex hormones include androgens (male)and estrogens (female). They play a role in fatal sex development.
• Adrenal medullary hormones include epinephrine and norepinephrine (adrenalin and noradrenalin). Their general effect is to prepare the body for emergencies (for example, increasing heart and breathing rate,elevating blood sugar, and shunting blood away from areas such as the digerstive tract).

Gonads: Ovaries and Testis

• The ovaries produce two kinds of steroids, estrogens (estradiol, estriol, and estrone) and progesterone, while the testes produce the steroid hormone testosterone. Sex hormones influence reproduction, but also influence body development and bone growth.

Thymus and Pineal Body

• The thymus secretes thymosin, which stimulates T-cell lymphocyte development.
• The only knownsecretion of the pineal body is melatonin, which causes color change in frogs and inhibits gonadal development. It may inhibit sexual activity in mammals and control circadian (daily) rhythms.

Other Chemical Messengers

• Other hormones include those that control digestive enzymes and those of the placenta.
• prostaglands are produced almost universallyin the body.  Some are involved in inflammation, others cause uterine contractions;one (thromboxane) aids blood clotting, while another (prostacyclin), slows clotting. Aspirin inhibits thromboxane synthesis, and new studies on aspirin suggest that this may be related to reduced incidences of heart and arterial disorders.

THE MOLECULAR BIOLOGY OF HORMONE ACTION

Cyclic AMP: The Second Messenger

• Adenosine 3,5-cyckic monophosphate (cyclic AMP or cAMP) is called second messenger (hormones are the first messengers.)
• Epinephrine attaches to a specific receptor site on its target cell, activing the membranal enzyme adenylate cyclase converts ATP to cAMP, which begins a series of reactions that converts glycogen to free glucose.
• Other hormones use the cAMP mechanism. Wide use is made possible by specific cell receptors for different hormones and preorganization of specific enzyme pathways involving the protein kinases (enzyme-activating enzymes).

Other Cell Activating Mechanisms

• receptor mediated endocytosis may be responsible for insulin and mitogens entering the cell. When enough are bound to membrane receptor sites, they are taken by phagocytosis. Once inside the cell, they are able to stimulate activity.
• Steroid hormones are lipid-soluble and may pass right through the plasma membrane. Inside the cell, they join cytoplasmic binding proteins. Upon entering the nucleus, the complex joins chromatin proteins and activates genes.