Endocrine System Overview

Endocrine Organs

• Primary Endocrine Organs: Hypothalamus, Pituitary Gland, Thyroid Gland
• Secondary Endocrine Organs: Heart, Liver, Stomach, Kidney

General Functions of Hormones

• Metabolism: Regulation of metabolic processes
• Reproduction: Control of reproductive functions
• Development and Growth: Regulation of growth and development
• Adaptation: Adaptation to stress and environmental changes
• Gastrointestinal Function: Regulation of digestion and absorption
• Other Diverse Functions: Regulation of various bodily functions.

Chemical Classes of Hormones

• Polypeptide and Protein Hormones:
  • Small peptide hormones: TRH (Thyrotropin-releasing hormone), Vasopressin
  • Protein Hormones: Insulin, Growth Hormone
  • Glycoprotein hormones: LH (Luteinizing hormone), FSH (Follicle-stimulating hormone), TSH (Thyroid-stimulating hormone)
• Steroid Hormones:
  • Cholesterol-derived: Cortisol, Aldosterone, Sex Hormones (Estradiol, Testosterone)
• Amine Hormones:
  • Amino Acid-derived: Epinephrine, Norepinephrine, Thyroxine
• Eicosanoids:
  • Derived from Arachidonic acid: Prostaglandins, Thromboxanes, Leukotrienes

Peptide Hormones

• Many protein hormones undergo modifications during packaging and secretion:
  • Pre-pro-insulin → Pro-insulin → Insulin

Mechanisms of Action of Hormones

• Hormone Receptors:
  • Specificity: Receptors bind to specific types of chemical messengers
  • Affinity: Receptors bind to messengers with varying strength

Receptor-Mediated Signaling

• Hormone Binding: Hormone binds to the receptor
• Transducer Activation: Activated receptor activates the transducer
• Second Messenger Generation: Transducer activates the second messenger generator (e.g. cyclic AMP, IP3)
• Effector System Activation: Activated second messenger triggers effector system activation
• Desensitization: Receptors desensitize or downregulate in response to prolonged exposure to hormones

Various Mechanisms of Hormone Action

• Cyclic AMP Mechanism: Hormones (ACTH, LH, FSH, TSH, ADH (V2R), HCG, MSH, CRH, b1 and b2 receptors, Calcitonin, PTH, Glucagon)
• IP3 Mechanism: Hormones (GnRH, TRH, Angiotensin II, ADH (V1R), Oxytocin)
• Steroid Hormone Mechanism: Hormones (Glucocorticoids, Estrogen, Testosterone, Progesterone, Aldosterone, Vitamin D)
• Other Mechanisms:
  • Activation of Tyrosine Kinase: Insulin, IGF-1
  • Cyclic GMP: ANP (Atrial natriuretic peptide), EDRF (Endothelium-derived relaxing factor)
  • Thyroid Hormone: Activates specific nuclear receptors

Regulation of Hormone Action

• Regulation of Hormone Secretion:
  • Control by Plasma Concentrations of Mineral Ions or Organic Nutrients: Plasma levels of minerals or nutrients affect hormone secretion.
  • Control by Neurons: Hormones can be regulated by neuronal inputs.
  • Control by Other Hormones (Tropic Hormones): Hormones can be regulated by other hormones.
  • Feedback Control:
    • Negative Feedback: Products of a pathway inhibit earlier steps.
    • Positive Feedback: Product of a pathway stimulates its production.
  • Circadian Regulation: Hormone secretion follows a daily cycle.
• Regulation of Receptors:
  • Downregulation: Decrease in number of receptors in response to prolonged hormone exposure.
  • Upregulation: Increase in number of receptors in response to reduced hormone exposure.
• Hormonal Interaction:
  • Antagonism: Two hormones oppose each other's effects.
  • Synergism: Combined effect of two hormones is greater than the sum of their individual effects.
  • Permissiveness: One hormone requires the presence of another hormone to exert its full effect.