Hormone Classification and Signaling

Hormone Classification and Types

• Peptide hormones consist of chains of amino acids and are characterized by <4 amino acids.
• Lipid-derived hormones can diffuse through plasma membranes due to their hydrophobic nature, primarily derived from cholesterol.
• Amino acid-derived hormones originate from individual amino acids, playing essential roles in signaling.
• Hormones that primarily act as peptide hormones are composed of polypeptide chains.

Signaling Mechanisms

• First messengers are hormones that bind to plasma membrane receptors, initiating signal transduction pathways.
• Adenylate cyclase converts ATP to cyclic AMP, a crucial step in many hormone signaling pathways.
• G-proteins are activated by first messengers and facilitate the production of cyclic AMP.
• cAMP inhibitors block the activation of protein kinases, halting the signaling pathway initiated by peptide hormones.

Hormone Receptors and Regulation

• Hormone receptors can be classified as plasma membrane receptors (on the cell surface) or intracellular receptors (in the cytoplasm or nucleus).
• Down-regulation occurs when increased hormone levels lead to a decrease in receptor numbers, while up-regulation is the opposite, increasing receptor numbers in response to elevated hormone levels.

Specific Hormones and Their Functions

• Alcohol consumption inhibits the release of antidiuretic hormone (ADH), increasing urine output.
• Gonadotropin-releasing hormone (GnRH) stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary.
• Insulin, produced by beta cells in the pancreas, lowers blood glucose levels, while glucagon, from alpha cells, raises it.

Calcium and Bone Metabolism

• Parathyroid hormone (PTH) is secreted when blood calcium levels are low and stimulates osteoclast activity to increase blood calcium.
• Calcitonin reduces blood calcium levels and promotes bone growth, secreted by parafollicular cells in the thyroid gland.

Disorders Related to Hormones

• Acromegaly results from GH overproduction in adults, while gigantism occurs in children due to excessive GH.
• Addison's disease is caused by hyposecretion of corticosteroids, while Cushing's disease results from excessive glucocorticoids.
• Diabetes insipidus occurs because of underproduction of ADH, and diabetes mellitus arises from low insulin activity.

Hormone Response and Production Mechanisms

• Control of hormone release can occur through:
  • Humoral stimuli: changes in blood composition (e.g., glucose levels).
  • Hormonal stimuli: release triggered by other hormones.
  • Neural stimuli: direct nervous system stimulation of endocrine glands.

Endocrine Glands Overview

• The adrenal cortex produces corticosteroids; the adrenal medulla produces epinephrine and norepinephrine.
• The anterior pituitary secretes six hormones, including FSH, LH, and adrenocorticotropic hormone (ACTH), which stimulates corticosteroid release during stress.
• The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), which control metabolic rates.

Additional Hormonal Actions and Functions

• Aldosterone, a mineralocorticoid, promotes sodium reabsorption and potassium secretion.
• Leptin, produced by adipose tissue, helps regulate appetite by promoting satiety.
• Erythropoietin (EPO) stimulates red blood cell production in response to low oxygen levels, produced by the kidneys.

Important Concepts in Endocrinology

• Hormones are signaling molecules released into the bloodstream to regulate various physiological processes.
• Understanding hormone pathways and their effects is crucial for diagnosing and treating endocrine disorders.
• Hormonal interactions can involve complex feedback loops, illustrating the intricacies of the endocrine system.