Endocrine System - BIO 126

• Endocrine System - Involves the hormone-secreting tissues

  and organs of the body, secretory cells which lack outflow ducts
• The endocrine system can be compared with the nervous system because chemical signals are transmitted more slowly and effects are longer-lasting
• Hormones are chemical messengers
• Half-life of a hormone - duration of time required to decrease the concentration of a circulating hormone by half
• Growth hormone has a long half-life while epinephrine (adrenaline) has a short half-life
• Two Factors Influencing the Hormone Concentrations in Blood - Rate of hormone secretion into the blood and the rate of removal of the hormone from the blood (metabolic clearance rate)
• Clearance rate of hormones from the plasma include: Metabolic destruction (enzymes), Binding with the tissues, Excretion by the liver into the bile, Excretion by the kidneys into the urine
• Chemical messenger - Any substance produced by a cell and

  plays a physiological role in the control of the activity of another cell
• Hormone - Any substance elaborated by one cell to regulate another cell
• Tropic hormone - Hormone which regulates the secretions of other endocrine tissues or organs
• Neurohormone - A hormone produced by a nerve cell
• Neuropeptide - A peptidergic (neurotransmitters are short peptide chains) neurohormone
• Neuromodulator - A hormone that modules the response of a neuron to a neurotransmitter or neuromodulator
• Neuroregulatory - A generalized term for any neurohormone that acts as a neurotransmitter or neuromodulator
• Pheromones - Chemical messengers released to the exterior of an animal to stimulate a response in another member of the same species
• Lumones - Chemical messengers released into the lumen of the gastrointestinal tract
• Chalones - Putative cellular mitotic inhibitors (inhibits cell division)
• Growth factors - Mitogenic (induces a cell to begin cell division) substances
• Modes of Hormone Delivery - Endocrine, Neuroendocrine, Paracrine, Neurocrine, Autocrine
• Endocrine - hormones are directly released into the bloodstream
• Neuroendocrine - hormones are released directly into the bloodstream but comes from a nerve cell
• Paracrine - Target cells are adjacent to the source cells, so need for hormone to be released into the bloodstream
• Neurocrine - neuron to neuron release, hormones are not released in the bloodstream
• Autocrine - source cell is the target cell, regulated by negative feedback
• Chemical Classes of Hormones - Peptide hormones, Amines, Steroid hormones, iodothyronines
• Peptide Hormones - composed of polypeptide chains, ends in -IN

  ex. insulin and corticotropin
• Amines - carbon ring + amine group, usually fast-acting, ends in -INE

  ex. epinephrine, norepinephrine
• Steroid Hormones - cholesterol base + R-group, ends in -OID, -ONE

  ex. glucocorticoid, progesterone, testosterone
• Iodothyronine - thyroid hormones with iodine in its structure
• RST of Amines and Peptides - (Activation) hormone binds to receptors on plasmalemma -> (Transduction) Hormone-receptor complex activates nucleotide regulatory protein -> NRP binds with adenyl cyclase to convert ATP to cAMP -> (Action of Second Messenger) cAMP combines with specific cyclic nucleic-dependent kinase -> interaction of cAMP with regulatory subunit releases the catalytic subunit -> catalytic subunit phosphorylates proteins leading to a cascade phenomenon -> cellular response -> (Termination) cAMP is rapidly metabolized and activating proteins are dephosphorylated
• GDP - inactivated G-protein
• GTP - activated G-protein
• Steroids and Iodothyronines are lipophilic / hydrophobic; They require carrier proteins (steroid-binding proteins) to keep the molecule stable and prevent degradation
• RST of Steroids and Iodothyronines - Lipid-soluble hormones pass through the cell membrane -> The hormone molecules bind to specific

  cytosolic receptors -> The formed complex bind to specific regions on genes/DNA -> Transcription of mRNA is initiated and gene expression (primary or secondary) is thus controlled
• Primary response in gene expression - the direct regulation of the

  transcription of only a small number of genes by the hormone-receptor complex in a target cell
• Secondary response in gene expression - the activation of other genes by the protein products of the primary response which results in another round of transcription and translation and production of another protein
• Hormones in Combination - Permissive Action, Antagonistic Effects, Synergism
• Permissive Action - the requirement of the presence of a hormone for other hormones to exert their effects
• Permissive Action of GH on TH: TRH acts on pituitary gland -> secretion of TSH -> TSH influences thyrocytes to secrete TH (T4) -> 5' deiodase peripherally activates T4 to T3 -> T3 elicits metabolism, growth, and maturation response but ONLY with GH
• Antagonistic Effects - describe the opposing effects of two different hormones on the same target cell or tissue