Hormone Classification lec 2

Alteration of Sensitivity to Other Chemical Messengers

• Estradiol (E) increases the synthesis of cellular receptors for progesterone and some neurotransmitters.

Classes of Hormones

• A. Peptides: Water-soluble
• B. Steroids: Lipid-soluble
• C. Amines:
  • 1. Catecholamines: Water-soluble (NE, EPI, dopamine)
  • 2. Indoleamines (Serotonin, melatonin)
  • 3. Thyroid hormones: Lipid-soluble (Triiodothyronine, tetraiodothyronine)

Peptide Hormones

• Structure:
  • Includes most hormones
  • Amino acid sequence can vary among species
  • Peptide hormones: ~3-40 amino acids
  • Polypeptide/protein hormones: ~50-200 amino acids
  • Glycoproteins: protein + carbohydrate
• Synthesis & Storage:
  • Water-soluble
  • Synthesized as preprohormone
  • Signal peptide is cleaved off N-terminal → prohormone
  • Enzymatic processing of prohormone → ≥1 hormone
  • Hormone (or prohormone + enzyme) is stored in secretory vesicle & eventually expelled from the cell
• Secretion, Transport, & Metabolism:
  • Released in bursts by exocytosis
  • Dissolve in plasma
  • Bind to receptors in a target cell’s membrane
  • Rapidly degraded (by peptidases) or excreted
  • Half-life is around several minutes (short-lived)
• Receptors:
  • Embedded in plasma membrane of target cell
  • Binding of hormone leads to changes inside cell
  • Modify existing proteins (e.g., membrane channels, metabolic enzymes, transport proteins) → rapid response
  • May affect synthesis of new proteins → slow response
  • Two major types of receptor complexes:
    1. Receptor-enzymes
    2. G protein-coupled receptors (GPCRs)

Peptide Hormone Receptors

• 1. Receptor-enzymes:
  • Have intrinsic enzymatic activity
  • Binding of hormone activates enzyme on cytoplasmic side → series of reactions within cell
• 2. G protein-coupled receptors (GPCRs):
  • Act via a G protein and intracellular second messenger (e.g., cAMP)

Activation and Termination of Peptide Signaling

• 1. Receptor is activated by ligand; hormone has its effect on the cell
• 2. Receptor is phosphorylated by kinases (inactivation, desensitization)
• 3. Hormone-receptor complex is endocytosed (internalized) into cell
• 4. Receptor is recycled or broken down

Steroid Hormones

• Transport:
  • Not soluble in plasma; circulate in blood bound to carrier proteins/binding proteins (CPs)
  • Thought to be inactive while bound to CPs
  • Free Hormone Hypothesis: Only unbound (free) hormone molecules can enter target cells
• Metabolism:
  • Carrier proteins protect steroids from degradation
  • Inactivated in liver
  • Half-life is about several hours (comparatively long-lived hormones)
• Entry into Cells:
  • Conventional Model: simple diffusion across plasma membranes
  • New Hypothesis: facilitated diffusion via transporters
• “Classical” Actions:
  • Bind to receptors in target cell cytoplasm or nucleus
  • Affect gene transcription (slow response)
  • Co-activator proteins may be necessary for transcriptional effects

The Hypothalamus & the Pituitary

• Hypothalamus:
  • At base of brain; comprises many distinct nuclei
  • Integrates info from many parts of brain
  • Important in behavioral drives, emotions, biological rhythms, homeostasis, & endocrine control
• Pituitary Gland(s):
  • Connected to hypothalamus by pituitary stalk (infundibulum)
  • Anterior and posterior pituitary are developmentally, anatomically, and functionally distinct organs

Posterior Pituitary (neurohypophysis/pars nervosa)

• Develops as an outgrowth of the brain
• Made of brain tissue (neural)
• Contains axon terminals of neurosecretory cells originating in two nuclei of hypothalamus:
  • Paraventricular nucleus (PVN)
  • Supraoptic nucleus (SON)
• Stores and secretes two peptide hormones synthesized in PVN and SON:
  • Vasopressin (AVP), or antidiuretic hormone (ADH)
  • Oxytocin (OT)

Posterior Pituitary: Hormones Secreted

• 1. Vasopressin (AVP) = Antidiuretic Hormone (ADH):
  • Acts on kidney to increase water retention (decrease diuresis)
  • Acts on blood vessels to increase vasoconstriction
  • Released in response to increase blood osmolarity, decrease blood volume, or decrease blood pressure
  • Also released within brain to increase aggression, paternal behavior, and social bonding
• 2. Oxytocin (OT):
  • Acts on uterus to trigger smooth muscle contractions which induce/progress labor
  • Acts on breasts to trigger smooth muscle contractions and induce milk letdown
  • Also released within brain to increase social bonding, trust, empathy, and maternal behavior

Learn about the different classes of hormones, including peptides, steroids, and amines, and how they interact with chemical messengers and receptors.