Unit 45 Bio 1201

Chapter 45: Hormones and the Endocrine System

Endocrine Signaling Mechanisms

  • Endocrine signaling: Hormones are secreted into the bloodstream and affect distant target cells throughout the body.

  • Paracrine signaling: Hormones are released into the interstitial fluid and affect nearby target cells.

  • Autocrine signaling: A cell releases hormones that bind to receptors on its own surface, causing direct effects on itself.

  • Synaptic signaling: Neurons release neurotransmitters into synapses that affect adjacent cells.

  • Neuroendocrine signaling: Neurosecretory cells release hormones into the blood, affecting distant cells.

Endocrine Glands

  • Key endocrine glands include:

    • Thyroid gland

    • Pineal gland

    • Hypothalamus

    • Pituitary gland

    • Parathyroid glands

    • Adrenal glands

    • Ovaries (in females)

    • Testes (in males)

    • Pancreas

Hormone Classification

  • Water-soluble hormones (hydrophilic):

    • Examples: Polypeptides like insulin, amines like epinephrine.

    • Mechanism: Bind to receptors on the plasma membrane of target cells.

  • Lipid-soluble hormones (hydrophobic):

    • Examples: Steroids like cortisol, amines like thyroxine.

    • Mechanism: Pass through the plasma membrane and bind to intracellular receptors in the nucleus or cytoplasm.

Action of Water-soluble vs Lipid-soluble Hormones

  • Water-soluble hormones:

    • Receptor location: Plasma membrane.

    • Result: Activation of second messengers inside the target cell leading to a rapid response.

  • Lipid-soluble hormones:

    • Receptor location: Nucleus or cytoplasm.

    • Result: Directly influence gene regulation, leading to long-term changes in cellular function.

Signal Transduction Pathways

  • Epinephrine (Adrenaline) signaling pathway:

    • Binding of epinephrine to its receptor activates a G protein.

    • G protein activates Adenylyl cyclase, converting ATP to cAMP (cyclic AMP), which acts as a second messenger.

    • cAMP activates Protein Kinase A (PKA), which then phosphorylates target proteins.

  • Different effects on different target cells:

    • Liver cells: Glycogen breaks down to glucose, releasing it into the bloodstream.

    • Smooth muscle cells in blood vessels: Varying responses to epinephrine binding (relaxation vs contraction).

Local Regulators

  • Types of local regulators include:

    • Cytokines and growth factors

    • Nitric oxide (NO)

    • Prostaglandins

Endocrine Pathways

  • Simple endocrine pathway example:

    • Stimulus: low pH in the duodenum.

    • Secretin released by: Endocrine S cells of the duodenum into circulation.

    • Target cells: Pancreatic cells.

    • Response: Release of bicarbonate.

  • Simple neuroendocrine pathway example:

    • Stimulus: Suckling.

    • Neurosecretory cell: Hypothalamus releases oxytocin into the posterior pituitary.

    • Target cells: Smooth muscle in mammary glands.

    • Response: Milk release.

Blood Glucose Regulation

  • Homeostasis Pathway:

    • STIMULUS: Blood glucose level rises (e.g., after a meal).

    • Insulin released by: Beta cells of the pancreas.

    • Body cells take up more glucose.

    • Liver takes up glucose and stores it as glycogen.

    • Response: Blood glucose level declines.

    • Opposite response to low blood glucose levels (e.g., between meals):

    • Alpha cells of pancreas release glucagon to raise blood glucose levels.

Neurohormonal Coordination

  • Hypothalamus and pituitary gland interactions:

    • Neurosecretory cells in the hypothalamus secrete neurohormones:

    • Posterior pituitary releases ADH (Antidiuretic Hormone) and Oxytocin.

    • Anterior pituitary hormones and their effects:

    • Tropic effects: FSH (Follicle Stimulating Hormone), LH (Luteinizing Hormone), TSH (Thyroid Stimulating Hormone), ACTH (Adrenocorticotropic Hormone).

    • Nontropic effects: Prolactin, MSH (Melanocyte-Stimulating Hormone).

    • Both: GH (Growth Hormone).

Stress Response

  • Short-term stress response:

    • Adrenal medulla releases epinephrine and norepinephrine.

    • Effects:

    • Glycogen breaks down to glucose, blood glucose levels increase.

    • Increased blood pressure, heart rate, and metabolic rates; changes in blood flow.

  • Long-term stress response:

    • Adrenal cortex secretes mineralocorticoids and glucocorticoids.

    • Effects:

    • Retention of sodium and water by kidneys; increased blood volume and pressure.

    • Breakdown of proteins and fats for glucose conversion; suppression of immune system activity.