Hormones are chemical substances,secreted by cells into the extracellularfluids, that regulate the metabolic functionof other cells in the body.Most hormones are amino acid-based, butsome are steroids.
Eicosanoids are biologically active lipidsthat include leukotrienes andprostaglandins, which are released frommost cell membranes.
Amino acid-based hormones andsecond-messenger systems Second messengers are intracellular molecules generated by the binding of a hormone to a plasma membrane receptor. Cyclic AMP is an important 2nd messenger that mediates hormonal effects.
A first messenger, such as a hormone, bindsto a receptor, which prompts the G proteinto signal adenylate cyclase, which generatescyclic AMP from ATP.
GTP hydrolysis provides the energy toconvert the first messenger into the second.Protein kinases are enzymes, activated bycAMP, that catalyze a variety of reactionthrough phosphorylation.Phosphodiesterase is an enzyme thatdegrades cAMP.
Phospholipase is membrane-bound enzymethat splits PIP2 into diacylglycerol and IP3(inositol triphosphate), which act as secondmessengers.
IP3 triggers the release of of Ca+2 from theER, which may act as a third messenger bybinding to the intracellular regulatoryprotein calmodulin.
Target cells are the tissue cells whoseactivity are influenced by a particularhormone.Target cells respond when hormones bind tospecific protein receptors on their plasmamembranes.
When target cells form more receptors inresponse to rising blood levels of theaffecting hormone, it is known as up-regulation.When prolonged exposure to high hormoneconcentration desensitizes the target cells sothat the respond less vigorously to hormonalstimulation, it is known as down-regulation.
Half-life is the persistence of a hormone inthe blood.The synthesis and release of most hormonesare regulated by some type of negativefeedback system.
Endocrine gland stimuli The secretion of hormones as a result of changing blood levels of ions and nutrients is called humoral stimulus. The secretion of hormones as a result of neural stimulation is called neural stimulus. The secretion of hormones in response to hormones produced by other endocrine organs is known as hormonal stimulus.
Nervous system modulation The nervous system can also affect the homeostasis of circulating hormones.
Endocrine Gland Major Hormones Target Organs Primary EffectsAdrenal cortex Glucocorticoids Liver and muscles Cortisol promotes gluconeogenesis Mineralocorticoids Kidneys Aldosterone promotes Na+ retention, K+ excretionAdrenal medulla Epinephrine, Heart, bronchioles, Causes adrenergic stimulation norepinephrine blood vesselsHeart Atrial natriuretic Kidneys Promotes excretion of Na+ in hormone the urineHypothalamus Releasing & inhibiting Anterior pituitary Regulates secretion of anterior hormone pituitary hormonesSmall intestine Secretin and Stomach, liver, and Inhibits gastric motility and stimulates bile cholecystokinin pancreas and pancreatic juice secretionIslets of Langerhans Insulin Many organs Insulin promotes cellular uptake of glucose and formation of glycogen and fat Glucogon Liver and adipose Glucagon stimulates hydrolysis of glycogen and fat
Endocrine Gland Major Hormones Target Organs Primary EffectsKidneys Erythropoietin Bone marrow Stimulates red blood cell productionOvaries Progesterone, Female repro. tract Maintains structure of reproductive tract, estrogen & mammary glands promotes and maintains secondary sexual characteristicsParathyroid glands Parathyroid hormone Bone, small intestine, Increases Ca+ concentration in blood and kidneysPineal gland Melatonin Hypothalamus and Anti-gonadotropic effect, anterior pituitary affects sleep cyclesPituitary, anterior GH, TSH, ACTH, Endocrine glands Stimulates growth and development of FSH, LH, PRL target organs, stimulates secretion of other hormonesPituitary, posterior Antidiuretic hormone Kidneys and blood ADH promotes water retention and vessels vasoconstriction Oxytocin Uterus and Oxytocin stimulates contraction of uterus mammary glands and mammary secretory units
Endocrine Gland Major Hormones Target Organs Primary EffectsStomach Gastrin Stomach Stimulates acid secretionTestes Testosterone Prostate and Promotes and maintains secondary seminal vesicles sexual characteristicsThymus Thymosin Lymph nodes Stimulates white blood cell productionThyroid gland Thyroxin and Most organs Thyroxine and triiodothyronine promote triiodothyronine growth and development and stimulate basal rate of cell respiration Calcitonin Calcitonin lowers blood calcium and phosphate levels