Hormones: Types and Functions

• Hormones are chemical messengers secreted into the blood by specialized cells.
• Hormones control growth and development, metabolism, regulation of the internal environment (temperature, water balance, ions), and reproduction.
• Hormones act on target cells by:
• Controlling the rates of enzymatic reactions.
• Controlling the transport of ions or molecules across cell membranes.
• Controlling gene expression and protein synthesis.

Hormone Locations, Classes, and Effects:

• Pineal gland:
• Produces melatonin (amine).
• Affects circadian rhythms.
• Hypothalamus:
• Produces trophic hormones (peptides).
• Affects the anterior pituitary, releasing or inhibiting pituitary hormones.
• Posterior pituitary:
• Produces oxytocin (OT) and vasopressin (ADH) (peptides).
• Oxytocin affects milk ejection, labor, delivery, and behavior.
• Vasopressin affects water reabsorption in the kidney.
• Anterior pituitary:
• Produces prolactin (PRL), growth hormone (GH), corticotropin (ACTH), thyrotropin (TSH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH) (peptides).
• Thyroid:
• Produces triiodothyronine and thyroxine (iodinated amines/peptide).
• Calcitonin (peptide).
• Affects metabolism, growth, and development.
• Parathyroid:
• Produces parathyroid hormone (PTH) (peptide).
• Regulates plasma calcium and phosphate levels.
• Thymus:
• Produces thymosin and thymopoietin (peptides).
• Affects lymphocyte development.
• Heart:
• Produces atrial natriuretic peptide (ANP) (peptide).
• Increases sodium excretion.
• Liver:
• Produces angiotensinogen and insulin-like growth factors (IGF) (peptides).
  • Angiotensinogen affects aldosterone secretion and increases blood pressure.
  • Insulin-like growth factors affect growth.
• Stomach and small intestine produce peptides.
• Pancreas produces peptides.
• Adrenal cortex produces steroids.
• Adrenal medulla produces amines.
• Kidney produces peptides and steroids.
• Skin produces steroids.
• Testes (male) produce steroids and peptides.
• Ovaries (female) produce steroids and peptides.
• Adipose tissue produces peptides.
• Placenta (pregnant females only) produces steroids and peptides.

General Information

• Hormones depend on cell-to-cell communication molecules, made in glands or cells, transported by blood, and binds to distant target tissue receptors activating physiological responses.
• Pheromones are used for organism-to-organism communication.
• Hormones control enzymatic reactions, ion/molecule transport across cell membranes, gene expression, and protein synthesis. They exert effects at low concentrations, bind to target cell receptors, and their half-life indicates their activity length.

Hormone Classification

• Peptide or protein hormones:
• Molecular structure includes amino acid chains.
• Water-soluble and lipophobic, so they bind to membrane cell receptors.
• Short life-time but triggers rapid responses.
• Steroid hormones:
• Molecular structure includes a cholesterol molecule.
• Made only in few organs (adrenal gland, gonads, placenta)
• Not water-soluble, lipophobic
• Enters the nucleus, affects transcription= genomic effect
• Amine hormones:
• Small hormone derived from tryptophan and tyrosine
• Catecholamines (epinephrine, norepinephrine, dopamine) are neurohormones and are lipophobic.
• Thyroid hormones (T3, T4, Thyroxine) are lipophilic and bind intracellular receptors.

Peptide/Protein Hormone Synthesis

• Preprohormone:
  • Large, inactive, and has multiple copies of a peptide hormone.
• Prohormone:
  • Post-translational modification: inactive prehormones are cleaved by proteolytic enzymes.
• Peptide hormone-receptor complex:
  • Signal transduction system.

Steroid Hormones

• Cholesterol-derived hormone.
• Synthesized as needed, not stored.
• Lipophilic and can enter target cell
• Binds to cytoplasmic or nuclear receptors, which activates DNA for protein synthesis.
• Has slower activation and longer half-life, so it does not mediate reflex pathways

Amine Hormones

• Derived from one of two amino acids (tryptophan or tyrosine).
• Thyroid hormones bind intracellular receptors.
• Catecholamines are neurohormones that bind cell membrane receptors.
• Examples include epinephrine, norepinephrine and dopamine.

Endocrine Reflex Pathways

• Reflex pathways are important for maintaining the internal environment.
• Endocrine/neuroendocrine reflexes output signals as hormones/neurohormones.
• Components: stimulus, sensor, input signal, integration, output signal, target(s), and response.
• Hormones may have multiple stimuli for release.
• Endocrine cells can act as receptors.
• Cells use various signals to decide how much hormone to produce. The output signal is a hormone that leads to a physiological action and negative feedback.

Endocrine Reflexes

• The nervous and endocrine systems overlap structurally and functionally.
• The central nervous system regulates hormone release using efferent neurons.
• Some neuron groups secrete neurohormones.
• Pineal and pituitary glands are incorporated into the brain.

Pituitary Gland

• The pituitary gland sits in a protected bone pocket and connects to the brain by a stalk.
• Anterior pituitary is a true endocrine gland secreting six classic hormones.
• Hypothalamic trophic neurohormones control the release of anterior pituitary hormones through a portal system.
• The posterior pituitary produces vasopressin (ADH) and oxytocin.
• Hormones of the hypothalamic-anterior pituitary pathway control vital functions. The pituitary is called the master gland.

Feedback Loops

• Most complex endocrine reflexes involve three integrating centers: hypothalamus, anterior pituitary, endocrine target of the pituitary hormone.
• Hormones themselves, not the response, act as the feedback signal.

Endocrine Control

• Three Levels
• Hypothalamic stimulation from the CNS
• Pituitary stimulation via hypothalamic trophic hormones
• Endocrine gland stimulation from pituitary trophic hormones
• Long-loop feedback and short-loop feedback

Hormone Interactions:

• Synergism: combined effect is greater than the sum of individual effects.
• Permissiveness: one hormone needs another to exert its full effect.
• Antagonism: one hormone opposes the action of another.

Endocrine Pathologies:

• Caused by hormone imbalance (excess, deficiency, or abnormal responsiveness).
• Hypersecretion: exaggerated effects caused by tumors or exogenous sources.
• Hyposecretion: deficient hormone levels, often due to decreased trophic hormone levels.
• Goiter (low secretion of thyroxin)
• Diabetes (low secretion of insulin).
• Abnormal hormone response: target tissues don't respond correctly due to downregulation or receptor abnormalities.
• Diagnosis can be simple or complicated. If the problem arises in the last endocrine gland it is a primary pathology. If a problem occurs in one of the tissues producing trophic hormones, it is a secondary pathology.