Hormones are chemical messengers that regulate bodily functions. They come in different types, like peptides, steroids, and amines, each with unique structures and properties. These hormones interact with specific receptors on or inside cells, triggering complex signaling pathways.

The endocrine system uses hormones to maintain balance in the body. Hormone production is tightly regulated through feedback loops, neural stimuli, and other factors. Understanding how hormones work helps us grasp how the body coordinates its many functions.

Types and Functions of Hormones

Types of hormones by structure

• Peptide and protein hormones consist of chains of amino acids vary in length from a few to hundreds synthesized by ribosomes (insulin, growth hormone, antidiuretic hormone (ADH))
• Steroid hormones derive from cholesterol lipid-soluble allowing them to diffuse through cell membranes bind to intracellular receptors (cortisol, testosterone, estrogen)
• Amine hormones derive from the amino acids tyrosine or tryptophan include catecholamines and thyroid hormones (epinephrine, norepinephrine, thyroid hormones (T3 and T4))

Intracellular vs membrane receptors

• Intracellular hormone receptors reside inside the cell in the cytoplasm or nucleus bind to lipid-soluble hormones (steroid hormones, thyroid hormones)
  • Hormone-receptor complex directly influences gene expression by binding to DNA regulating transcription
• Cell membrane hormone receptors span the surface of the target cell bind to water-soluble hormones (peptide hormones, protein hormones, catecholamines)
  • Hormone binding initiates intracellular signaling cascades involving second messengers (cAMP, IP3) leading to cellular responses
  • These receptors are found on target cells, which are specific cells that respond to particular hormones

Hormone Signaling and Regulation

Hormone signaling pathways

1. cAMP (cyclic adenosine monophosphate) pathway

   • Hormone binds to a G protein-coupled receptor (GPCR) activates the associated G protein
   • Activated G protein stimulates adenylyl cyclase which converts ATP to cAMP
   • cAMP activates protein kinase A (PKA) which phosphorylates various proteins modifying their activity leading to cellular responses (glycogen breakdown, lipid metabolism)

2. IP3 (inositol triphosphate) pathway

   • Hormone binds to a GPCR activates the associated G protein
   • Activated G protein stimulates phospholipase C (PLC) which cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) into IP3 and diacylglycerol (DAG)
   • IP3 diffuses through the cytoplasm binds to receptors on the endoplasmic reticulum triggering calcium release
   • Increased intracellular calcium along with DAG activates protein kinase C (PKC) which phosphorylates various proteins leading to cellular responses (neurotransmitter release, smooth muscle contraction)

Factors in hormone response

• Hormone concentration in the blood higher concentrations generally lead to greater cellular responses until saturation is reached
• Receptor availability and affinity on the target cell determine the magnitude of the response
  • Number of receptors present on the cell surface
  • Strength of hormone-receptor binding (affinity) influences signal transduction
• Post-receptor events influence the efficiency of intracellular signaling cascades
  • Availability of necessary enzymes (adenylyl cyclase, PLC, PKA, PKC)
  • Presence of substrates (ATP, PIP2) for signal amplification
• Desensitization and downregulation occur with prolonged exposure to a hormone reducing the cell's sensitivity
  • Receptor internalization or degradation decreases the number of receptors on the cell surface attenuating the response

Regulation of hormone production

• Negative feedback loops maintain hormone levels within a narrow range
  • Rising hormone levels inhibit further hormone production and release by the endocrine gland (thyroid-stimulating hormone (TSH) and thyroid hormones (T3 and T4))
  • Falling hormone levels stimulate increased production and release
• Positive feedback loops are less common in endocrine regulation
  • Rising hormone levels stimulate further hormone production and release creating a self-amplifying cycle (oxytocin during childbirth stimulates uterine contractions)
• Neural stimuli from the nervous system can trigger hormone release
  • Hypothalamic control of pituitary hormone secretion via releasing and inhibiting hormones (growth hormone-releasing hormone (GHRH), somatostatin)
• Humoral stimuli involve changes in blood levels of nutrients, ions, or other hormones influencing hormone release
  • Blood glucose levels affect insulin and glucagon secretion from the pancreas maintaining glucose homeostasis

Endocrine System Overview

Components and Functions

• The endocrine system consists of hormone-producing glands and tissues that work together to maintain homeostasis
• Endocrine glands secrete hormones directly into the bloodstream for transport to target tissues
• Hormone synthesis occurs within specialized cells of endocrine glands, involving various biochemical pathways
• Hormone transport in the bloodstream can occur freely or bound to carrier proteins
• Hormone metabolism and clearance by the liver and kidneys regulate hormone levels in the body