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Introduction to hormones

Florence Papworth Physiology AHS1 English

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Learning objectives:
- overview of endocrine, paracrine and autocrine signals
- overview of peptide, steroid and small molecule hormones
- hormones of metabolic (glucose) homeostasis; glucagon, adrenalin, insulin and glucocorticoids
- importance of receptors as mediators of hormone action
- principle of signal transduction and amplification
Endocrinology
- Hormones are the chemical messengers of the endocrine system (greek-set in motion)
- study of communication within a living organisms by means of hormone
- Classic hormone description:
  - chemical messenger secreted from glands, emptied directly into circulation and transported by the blood some distance to alter function of the target organ
    - today's definition - blood-bourne communication = just one aspect of the endocrine system
Hormonal communication
- Hemocrine - often referred to as 'endocrine'
  - straight from cells to blood vessels - blood vessel transports to target cell
- Autocrine - produces hormone affecting itself/it's own receptors
- Paracrine (neurocrine if neuronal cells)
  - Produces hormone affecting cell directly adjacent to itself
- Solinocrine - lots of cells producing hormone into a luminal space
- General functions:
  - single hormone = single function
    - rare e.g. erythropoietin on HB SYNTHESIS in erythrocytes
  - single hormone = multiple actions
    - thyroid hormone on CHO METABOLISM, bone turnover, enzyme synthesis
  - multiple hormones = single function
    - regulation of LACTATION by prolactin, corticosteroids, thyroxin, sex steroids
- Hormone action:
  - concentration in ECF very LOW (10^-9 -> 10^-11)
  - unique recognition bu receptors at target tissue
    - receptors can be on CELL SURFACE, CYTOSOLIC or in NUCLEUS
  - Hormones can circulate either FREE or BOUND to a binding protein
    - resevoir/pool of hormone to avoid fluctuations
      - extend half lives
      - STEROIDS - insoluble so need carrier to circulate
Classes of hormones
- Peptide hormones
  - varying sizes - small peptides e.g. TRH - large protein e.g. INSULIN
- Amino acid derivatives
  - e.g. T3/T4 - tyrosine derived
    - T3 = 2x tyrosine + 3I. T4= 2x tyrosine + 4I.
- Steroid hormones
  - cholesterol-derived e.g. corticosterone
    - glucocortico-steroids, mineralo-corticosteroids, progesterone etc.
- Signal transduction
  - G-protein complex coupled to receptor - hormone binds to receptor = phosphorylation and activation of G-protein complex = alteration and activation of cell funcition
    - Initiates secondary messaging in cell -> cascade -> target cell carries out desired funciton
    - PEPTIDE
  - STEROID
    - Receptor internal - within cytoplasm or nucleus
      - Steroid diffuses into the cell - binds to receptor in cytosol, or nucleus - activates/inhibits actovation of genes and transcription of proteins
  - AMINO ACID DERIVED
    - similar to steroid hormones, but receptor is ALWAYS NUCLEAR
      - e.g. Thyroid hormone
Peptide vs Steroid hormones
- Property
  - Storage pools
    - Interaction w/in cell membrane
      - Receptor
        Action
        Response time
- Steroid
  - None
    - Diffusion through cell membrane
      - In cytoplasm or nucleus
        Regulation of gene transcription
        Hours to days
- Peptide
  - Secretory vesicles
    - Binding to receptor on cell membrane
      - On cell membrane
        Signal-transduction cascades affect a variety of processes
        Seconds to minutes
Control of release of hormones
- Feedback mechanisms
  - Sensory send hormone -> target -> hormone produced by target 1, activate target 2 and allosterically inhibits sensor
    - e.g. sensory = B-cell producing INSULIN. Target 1 = Store glucose producing GLUCOSE. Target 2 = brain.
- Tropic hormone, e.g. thyroid releasing hormone
- Neuronal control, e.g. adrenaline
  - Neuronal control
    - e.g. epinephrine (adrenaline) release from medulla
      - CNS synapses with peripheral nerve and proximal ganglia of ADRENAL MEDULLA = stimulated epinephrine production
- CIRCADIAN or PULSATILE release
  - Circadian = dependant on daylight, e.g. cortisol
  - Pulsatile release, e.g. growth hormone during sleep
Tropic hormones
- stimulate release of another hormone from other endocrine glands
- e.g. TRH - thyroid releasing hormone produced by hypothalamus, stimulates anterior pituitary to produce TSH - thyroid stimulating hormone
  - TSH in turn stimulates thyroid gland to produce T3/T4
- have a growth effect on cells/tissues
  - Hyperplasia - increasing cell number or hypertrophy - increasing cell size or both
Nervous system vs Endocrine system
- Property
  - Nervous system
    - Endocrine System
      - Range of molecules/receptors
        Slow
        Slow
        Slow
        Limited
        None
        Yes
    - Location, Range of transmitters
      - Short to long range
        Slow to fast
        Fast
        High
        High
        High
  - Specificity
    - communication over a range of distances
      - speed of communication
        speed of activation/termination
        integration with other responses
        Acquisition of new responses or behaviours
        Susceptibility to injury or disease
Hypothalamus-pituitary axis
- hypopophyseal portal circulation
  - Neuronal and glandular tissue
    - Anterior, posterior and intermediate lobe
    - optic chasma
- ANTERIOR LOBE: Hypothalamus = Hypothalamic hormone -> HYPOPOPHYSEAL PORTAL circulation -> anterior lobe hormone-> circulation
- POSTERIOR LOBE: Hypothalamus = posterior lobe hormone -> AXONAL transport -> posterior lobe
- Nuclei of hypothalamus - Neuroendocrine function
  - neuroendocrine cells reside in the nuclei of the hypothalamus
  - Control of levels of circulating:
    - Hormones
    - Metabolites
    - Nutrients
    - Electrolytes
  - Stress, cold, trauma, hunger, pain, neural reflex -> sensory information provided
    - The hypothalamus is a focus point for information on internal well being
      - Hormone production and the pituitary gland
Hypothalamus hormones
- GnRH
  - stimulatory effect on pituitary = FSH/LH produced by gonadotrophs
- GHRH
  - stimulates somatotroph = GH production in pituitary
- SS
  - inhibitory effect on pituitary= somatotroph produces GH
- TRH
  - stimulatory effect on thyrotoph in pituitary = TSH production
- dopamine
  - inhibitory effect on lactotroph of pituitary = PROLACTIN release
- CRH
  - stimulatory on pituitary = corticotroph porduces ACTH
- Axons -> posterior pituitary = AVP and OXYTOCIN
  - POST PIT
- ANT PIT
Posterior pituitary: OXYTOCIN
- 1. during labour to cause contraction of uterus
- 2. during lactation essential for milk let-down
- Activation of stretch-sensitive sensory cells feeds back to hypothalamus via spinal cord - NOT HORMONAL feed back but NEURAL
- Gravid uterus = stretch receptors = spinal cord = PVN & SON = posterior pituitary = OXYTOCIN to UTERUS
  - Hypothalamic nuclei
    - PVN - paraventricular nucleus
    - SON - supraoptic nucleus
Posterior pituitary hormones
- Regulation - secretory stimulus, two REFLEX MECHS
  - Increased = plasma osmolarity. Decreased = plasma volume
- Clinical abnormalities
  - DIABETES INSIPIDUS
    - chronic excretion of large volumes of dilute urine
    - Thirst caused by hyperosmolarity
    - Disruption of AVP production (pituitary tumours or traumatic injury to the skull)
- Volume receptors = veins, atria, carotids
  - osmo-receptors = hypothalamus
EG: control of thyroid production
- Hypothalamus produces TRH - thyroid releasing hormone
  - TRH - hypophyseal portal system - ANTERIOR pituitary
    - Thyrotrophs in ant pit produce TSH - thyroid stimulating hormone
      - TSH leaves pituitary -> Thyroid gland

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Description: AHS1. Endocrine physiology. Dawn Davies 1/6
Created by: Florence Papworth
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