Other Endocrine Glands and Hormone Physiology: Study Notes

Other Endocrine Glands

Pineal Gland

The pineal gland is a small endocrine organ located in the brain, involved in the regulation of circadian rhythms and reproductive timing.

• Peak secretion of melatonin occurs between ages 1-5; secretion decreases by 75% by puberty.

• Produces serotonin during the day, which is converted to melatonin at night.

• Regulates circadian rhythm (sleep-wake cycle).

• May influence the timing of puberty in humans.

• Melatonin levels are increased in Seasonal Affective Disorder (SAD) and PMS; phototherapy can decrease melatonin and alleviate symptoms such as depression, sleepiness, irritability, and carbohydrate craving.

Thymus

The thymus is an immune organ with endocrine functions, especially important in early life.

• Located in the mediastinum, superior to the heart.

• Shrinks after puberty (involution).

• Secretes hormones that regulate the development and maturation of T-cells (critical for adaptive immunity).

Parathyroid Glands

The parathyroid glands are small glands located on the posterior aspect of the thyroid gland, essential for calcium homeostasis.

• Secrete parathyroid hormone (PTH).

• PTH is released in response to low blood calcium (Ca2+) levels.

• PTH increases Ca2+ in the blood by stimulating bone resorption, increasing intestinal absorption, and promoting kidney reabsorption of calcium.

Pancreas

The pancreas has both endocrine and exocrine functions, playing a central role in glucose metabolism and digestion.

• Retroperitoneal, located inferior and dorsal to the stomach.

• Islets of Langerhans (2% of pancreatic tissue) produce hormones.

• 98% of the pancreas produces digestive enzymes (exocrine function).

Pancreatic Hormones

• Insulin (from β cells):

  • Secreted after meals with carbohydrate and/or protein intake.

  • Stimulates glucose and amino acid uptake by cells.

  • Antagonizes glucagon.

• Glucagon (from α cells):

  • Secreted during low carbohydrate diets or fasting.

  • Stimulates breakdown of glycogen and fat catabolism.

  • Antagonizes insulin.

Diabetes Mellitus

Diabetes mellitus is a group of metabolic disorders characterized by chronic hyperglycemia due to defects in insulin secretion, insulin action, or both.

• Signs and symptoms of insulin hyposecretion:

  • Polyuria (excessive urination), polydipsia (excessive thirst), polyphagia (excessive hunger)

  • Hyperglycemia, glycosuria (glucose in urine), ketonuria (ketones in urine)

  • Osmotic diuresis: increased glucose in urine draws water into urine by osmosis

Types of Diabetes Mellitus

Type	Features
Type I (IDDM)	Autoimmune destruction of β cells; 10% of cases; onset ~age 12; requires insulin therapy
Type II (NIDDM)	Insulin resistance; 90% of cases; risk factors: heredity, age, obesity; managed with diet, exercise, oral medications
Gestational	Occurs during pregnancy; placental hormones deactivate insulin receptors; risk for mother developing Type II later

Other Pancreatic Disorders

• Hyperinsulinism: Excess insulin (injection or tumor) causes hypoglycemia, weakness, hunger, anxiety, sweating, increased heart rate; can lead to insulin shock (disorientation, convulsions, coma).

Gonads

The gonads (ovaries and testes) are reproductive organs with major endocrine functions.

• Ovaries: Secrete estrogens and progesterone; regulate female reproductive system, menstrual cycle, pregnancy, and mammary gland preparation.

• Testes: Produce androgens; regulate male reproductive system, physique, sperm production, and sex drive.

Endocrine Functions of Other Organs

• Heart: Releases atrial natriuretic peptide (ANP) in response to increased blood pressure; decreases blood volume and blood pressure.

• Stomach and Small Intestines: Secrete about 10 enteric hormones; coordinate digestive motility and secretion.

• Kidneys: Produce 85% of erythropoietin, stimulating red blood cell production in bone marrow.

• Liver: Continues synthesis of calcitriol (active vitamin D); produces some erythropoietin.

• Placenta: Secretes estrogen, progesterone, and other hormones to regulate pregnancy and stimulate fetal and mammary gland development.

Hormone Physiology

Chemical Nature of Hormones

• Most hormones are protein-based (water-soluble) (e.g., histamine, epinephrine, insulin).

• Steroid hormones are lipid-soluble.

• Hormones circulate throughout the body but only affect target cells with specific receptors.

• There are two main mechanisms of hormone action: water-soluble and lipid-soluble.

Hormone Mode of Action

• Lipid-soluble hormones: Penetrate the plasma membrane and enter the nucleus to affect gene expression.

• Water-soluble hormones: Bind to cell-surface receptors and activate second messenger systems.

Lipid-Soluble Hormone Mechanism

• Steroid hormone diffuses through the plasma membrane and binds to an intracellular receptor.

• The receptor-hormone complex enters the nucleus and binds to DNA, initiating transcription to mRNA and protein synthesis.

Water-Soluble Hormone Mechanism

• Hormone binds to a membrane receptor, activating a G protein.

• G protein activates adenylate cyclase, which converts ATP to cAMP (second messenger).

• cAMP activates protein kinases, leading to cellular responses (e.g., enzyme activation, secretion).

Enzyme Amplification

Hormones can trigger a cascade of enzymatic reactions, amplifying a small stimulus into a large cellular response.

• One hormone molecule can activate many enzymes via second messengers, resulting in a significant metabolic effect.

Hormone Clearance

• Hormone signals must be terminated after their action.

• Hormones are degraded by the liver and kidneys, then excreted in bile or urine.

• Metabolic clearance rate (MCR): Rate of hormone removal from the blood.

• Half-life: Time required to clear 50% of a hormone from circulation.

Modulation of Target Cell Sensitivity

• Cells can adjust their sensitivity to hormones by changing receptor density.

• Upregulation: Increased receptor density, stronger response.

• Downregulation: Decreased receptor density, diminished response.

Control of Pituitary: Feedback from Target Organs

• Hormone secretion is often regulated by negative feedback from target organ hormones to the pituitary and hypothalamus.

• Example: Thyroid hormones inhibit TRH and TSH release.

Hormone Interactions

• Synergistic effects: Two hormones amplify each other's effects (e.g., glucagon and epinephrine).

• Permissive effects: One hormone enhances the response to another (e.g., estrogen and prolactin).

• Antagonistic effects: One hormone opposes the action of another (e.g., insulin and glucagon).

Stress and Adaptation

• Stress: Any situation that disrupts homeostasis and threatens well-being (physical or emotional).

• Examples: exercise, pregnancy, illness, starvation, sleep deprivation, emotional trauma.

General Adaptation Syndrome

• The body's response to stress occurs in three stages:

  1. Alarm reaction (mobilize resources)

  2. Resistance (cope with stressor)

  3. Exhaustion (reserves depleted)

Paracrine Secretions

Paracrine secretions are local chemical messengers that affect nearby cells, distinct from neurotransmitters and hormones.

• Histamine: Involved in allergies; causes vasodilation.

• Nitric oxide: Released from blood vessels; causes vasodilation.

• Eicosanoids: Diverse functions; involved in inflammation, pain, fever.

Eicosanoids: Paracrine Secretions

• Mediate allergic and inflammatory reactions (e.g., leukotrienes, prostaglandins, thromboxanes).

• Stimulate vasoconstriction, clotting, smooth muscle contraction/relaxation, and sensitize neurons to pain.

• Targeted by anti-inflammatory drugs (e.g., steroids, NSAIDs).