Endocrine Exert influence by a second messenger. • Hormones that are lipid soluble can enter the cell membrane; thus, their receptors are located within the cell. – These hormones
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Hormones: chemical substances produced at one site which initiates or regulates the activity of an organ or group of cells in another part of the body.
Neurotransmitters: chemical substances which modifies or results in the transmission of nerve impulses at synapses.
• Both systems rely on release of chemicals– Nervous: synaptic (mostly) and produces a rapid
response– Endocrine: circulating hormones which may last from a
few minutes to several hours• Both systems share many chemical messengers
– examples: norepinephrine (NE) and epinephrine (E)• Both systems respond primarily by negative feedback• Both systems preserve homeostasis by regulation of
activity of other cells, tissues, organs, and systems
• Catecholamines are classified as biogenic amines (other biogenic amines include neurotransmitters, the indolamines, which include serotonin (from trytophan) and histamine (from histidine)
– either increase or decrease what the cell normally does
• Actions include changes with influence– permeability of plasma membrane– electrical state (ion channels)– protein synthesis; structural and/or regulatory molecules– enzyme activity: activates or deactivates enzymes– secretory activity– mitotic division
HORMONE FUNCTION• The activity of the target cell can be changed by
– Changes in membrane permeability can alter electrical activity of the cell; functions include -nerve conduction, release of neurotransmitter, and muscle contraction
– A new enzyme or a protein may be produced by the activation of genes
– The rate of synthesis of enzymes or proteins may be altered by changing the rate of transcription
– An existing enzyme can be turned “on” or “off” by a hormone changing the enzyme’s structure
• Hormones that are not lipid soluble can not enter the cell; thus, their receptors are located on the cell membrane. – These hormones are mostly the amino acid based
hormones and include ACTH, FSH, LH, Glucagon, PTH, TSH, Calcitonin, Catecholamines)
– Exert influence by a second messenger.• Hormones that are lipid soluble can enter the cell
membrane; thus, their receptors are located within the cell.– These hormones are mostly the steroid based hormones
and involve gene activation/inactivation, enzyme activation/inactivation, etc.
• Hormone acts as a first messenger and results in the appearance of at least one second messenger in the cytoplasm.– Second messenger may function as enzyme
inhibitor, activator, or cofactor. Net result is a change in the metabolic reactions of the cell
• Amplification - binding of hormone may cause the release of many second messengers
• Several types of second messengers may be released in response to one hormone
• Membrane bound proteins are a link between the first messenger and the second messenger
• Membrane proteins are activated when the first messenger (hormone) binds to its receptor and commonly results in the appearance of the second messenger.
• Common second messengers are cyclic-AMP and calcium ions
– Examples include:– Testosterone promotes production of enzymes
and structural proteins which promotes development of skeletal muscle.
– Thyroid hormones bind to mitochondria and chromatin receptors. Mitochondria binding increases rate of ATP production and chromatin receptors alters transcription of enzymes.
Thyroid Gland• A follicle consists of a colloid filled cavity which is
surrounded by cuboidal follicular cells– produce thyroid hormones (mostly thyroxine)– controlled by TSH (anterior pituitary)– targets most cells of the body (except brain, spleen, testes, uterus, and
itself)
– effect to increase glucose oxidation and promote growth and development
• Parafollicular cells are located among the follicles– produce thyroid hormone calcitonin– stimulus for secretion is high blood levels of ionic calcium– targets (1) osteoblasts (deposition of calcium) and (2)
PARATHYROID GLANDS• Two parathyroid glands located on posterior
surface of each lobe of thyroid• Endocrine cells are called chief cells• Produce parathyroid hormone (PTH)• Stimulus for secretion of PTH is decreased blood
levels of ionic calcium (hypocalcemia)• Targets
– (1) osteoclasts - remove calcium from bone – (2) kidneys - reabsorb calcium and conversion of
vitamin D to vitamin D3, calcitriol, which promotes intestinal absorption
• Mineralocorticoids - function in the electrolyte composition of body fluids.
• Aldosterone is the principal mineralocorticoid; it maintains sodium balance. Its primary role is the reabsorption of sodium from the forming urine. Along with sodium water is reabsorbed (osmotic). Thus, blood pressure and volume are maintained.
• Secretion is regulated by • (1) low blood levels of sodium, • (2) rising levels of potassium• (3) low blood pressure and/or volume.
• Effects - Cortisol (hydrocortisone) – Promotes gluconeogenesis (formation of glucose from
noncarbohydrate sources). – Mobilizes fatty acids from adipose and promotes their
catabolism as a primary source of energy, – Promotes the break-down of proteins to conserve available
glucose for the brain.• Downside of Glucocorticoids
– Stress triggers increase of ACTH and, thus cortisolproduction increases. Anti-inflammatory and anti-immune effects are associated with excess of cortisol.
• Gonadocorticoids include both androgens and estrogens. Androgens (which include testosterone) are the primary gonadocorticoids.
• Hormones may be involved in outset of puberty• Androgens for females are thought to influence sex drive
and may be converted to estrogens after menopause• Hypersecretion of gonadocorticoids produces
masculinization. At a young age females produce facial hair, male pattern of hair distribution, and enlarged clitoris. Males develop early secondary sexual characteristics and an intense sex drive.
Glucagon and Insulin• Glucagon promotes hyperglycemia (it produces an increase
in blood sugar)– promotes breakdown of glycogen into glucose– promotes neoglucogenesis– promotes the release of glucose into blood
• Insulin promotes hypoglycemia (it produces a decrease in blood sugar)– promotes oxidation of glucose for energy– promotes the formation of glycogen– promotes the formation of lipids from glucose
• Renin is released in response to decreased blood pressure and increased sympathetic (crisis) stimulation of renal nerves.
• Functions as an enzyme which converts inactive plasma protein, angiotensinogen, into cascading reactions leading to production of angiotensin (angiotensin II).
Kidney : Renin-Angiotensin Pathway• Primary purpose of the renin-angiotensin
pathway is to increase blood pressure• Angiotensin
– is a powerful constriction of vascular smooth muscle (increases peripheral resistance, and thus blood pressure)
– targets adrenal cortex to increase secretion of aldosterone. Aldosterone increases sodium ion reabsorption by the kidneys. Thus, the osmotic reabsorption of water. Blood volume increases, thus so does blood pressure.