Untitled Quiz

Endocrine Physiology Study Notes

• Endocrine system is the internal chemical communication system involving hormones.
• Hormones are chemical signals secreted into body fluids (usually blood) and are effective in minute amounts.
• Hormones act on cells, tissues, or organs at a location other than where they were produced.
• They are not catalysts but rather influence the rate of pre-existing metabolic functions, either positively or negatively.
• Hormones are effective at minute concentrations (10⁻¹² to 10⁻⁸ M).
• Hormones have a short half-life in circulation (minutes to hours).
• Hormones are inactivated or degraded at a constant rate to an inactive form (H*).
• Hormone concentration in circulation depends on the secretion rate.
• Physiological effects of hormones are proportional to hormone concentration, determined by hormone concentration at the target cell surface, rate of production, rate of delivery, rate of metabolism, degradation, and elimination, sensitivity of target cell, number of functional target cells, and availability of cell membrane receptors on target cells.

Types of Glands

• Two different types of glands
  • Exocrine - ducted, secrete into ducts
  • Endocrine - ductless, secrete into interstitial fluid, diffuse into blood
• Endocrine glands include: pituitary, thyroid, parathyroid, adrenal and pineal glands, hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, and placenta.

Endocrine Glands and Hormone Function

• Hypothalamus: releasing peptide hormone acting on the anterior pituitary (GHRH, CRH, TRH, GnRH)
• Anterior Pituitary: Growth Hormone (GH), Corticotrophin (ACTH), Thyroid Stimulating hormone (TSH), Luteinizing hormone (LH), Follicle stimulating hormone (FSH)
• Posterior Pituitary: oxytocin, vasopressin (ADH)
• Pancreatic Islets of Langerhans: insulin and glucagon
• Adrenal cortex: aldosterone and cortisol
• Adrenal Medulla: Adrenaline and Noradrenaline
• Thyroid Gland: thyroxine and Tri-iodothyronine
• Testis: testosterone (androgen)
• Ovary and placenta: oestradiol (oestrogens), progesterone

Hormone Classification

• Site of Production
• Type of action
  • Primary hormone of reproduction (FSH, LH, estradiol, progesterone)
  • Metabolic hormone (thyroxin, insulin, TSH)
• Chemical Structure
  • Proteins and polypeptides
  • Steroids
  • Fatty acids
  • Modified amino acid
• Size

Hormone Synthesis & Storage

• Most endocrine glands produce their hormones continually at levels determined by:
  • Body requirements
  • Rate of hormone inactivation
  • Rate of hormone clearance from the body

Hormone Transport in the Blood

• Protein hormones are hydrophilic and carried in the plasma in dissolved form.
• Steroids and thyroid hormones are lipophilic and carried in plasma in association with both specific and non-specific binding proteins.
• The amount of unbound, active hormone is relatively small.

Hormone-Cell Interaction

• Protein hormones have specific receptors on target tissue plasma membranes.
• Steroid hormones have specific receptors within the cytoplasm or nucleus.

Post-Receptor Cell Responses

• Steroids interact directly with the cell nucleus through complex formation with its cytosolic receptor.
• Protein hormones need a messenger because they cannot enter the cell.

Feedback Control Mechanisms

• The most important feedback control for hormones is the negative feedback system, where increased hormone concentrations result in decreased production, usually through interaction with the hypothalamus or pituitary gland.
• Endocrine secretory patterns influenced by factors like sleep or light, producing circadian rhythms.

Hormone Activity

• Hormones affect only specific target tissues with specific receptors.
• Receptors are constantly synthesized and broken down.
  • Up-regulation: stimulation of receptor synthesis
  • Down-regulation: internalization of receptors by endocytosis, receptor synthesis suppression
• Circulating hormones circulate in blood throughout the body.
• Local hormones act locally:
  • Paracrine - act on neighboring cells
  • Autocrine - act on the same cell that secreted them

Physiological Effects of Hormones

• Action on target cells includes alteration in intracellular protein synthesis, enzyme activity, plasma membrane transport, and secretory activity.
• Receptor activation leads to related molecular events inside the cell, including second messenger generation, changes in ion fluxes, activation/inhibition of protein kinases, and activation/inhibition of transcription factors.
• These events eventually regulate key metabolic enzyme activity and change the level of key protein gene transcription.

Intracellular Signaling Cascades

• Hormone initiates a cascade of reactions involving the signal transfer, signal transformed and relayed, signal amplification, signal diverges, and modulated effects, finally leading to regulation of metabolic pathway and gene expression.

Mechanisms of Hormone Action

• Response depends on both hormone and target cell. Lipid-soluble hormones bind to receptors inside target cells, while water-soluble hormones bind to receptors on the plasma membrane, activating a second messenger system.
• Responsiveness depends on hormone's concentration, abundance of target cell receptors, and influence exerted by other hormones. Permissive, synergistic, and antagonistic effects may occur.

Receptors

• Receptors can be on surface or internal to the target cell.
  • Surface receptors are for amino acid-derived hormones
  • Internal receptors are for steroid hormones

Feedback Loops (e.g., Hypothalamus-Pituitary-Target Organ)

• Some hormones are part of a feedback loop where the increase in a hormone can lead to a decreases or negative feedback.
• The response in negative feedback is usually caused by the hormone acting on the hypothalamus or the pituitary gland.
• Some processes like ovulation or milk ejection are part of positive feedback, where an increase in a hormone stimulates an increase in another hormone.

Examples of Endocrine Glands and Diseases

• Hormone deficiency can result from destruction of the gland due to things like infarction, infection, neoplasms or autoimmune processes, genetic defects in hormone production, or inactivating mutations of hormone receptors.
• Hormone excess is due to exogenous intake, overproduction by an endocrine gland, or activating mutations of cell surface receptors.
• Gland enlargement can lead to space-occupying lesions, producing pressure effects.

Pituitary Gland and Hormones

• The anterior pituitary receives signals from the hypothalamus and releases the appropriate hormones to other glands.
• The posterior pituitary stores and releases hormones made by the hypothalamus (oxytocin and ADH)

Hormone Control Mechanisms

• Hormone secretion can be regulated by signals from the nervous system, changes in blood chemistry, or other hormones.

Summary of Hormone Effects

• All hormones act by regulating the metabolic activities of specific cells.
• The magnitude of their effects is proportional to their concentration.

Other Hormone-Producing Structures

• Heart
  • secretes atrial natriuretic peptide (ANP), which decreases blood Na⁺ concentration, reducing blood pressure and blood volume.
• Kidneys
  • secrete erythropoietin, stimulating red blood cell production
  • secrete renin, initiating the renin-angiotensin-aldosterone mechanism.
• Thymus
  • secretes thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin, involved in T cell maturation
• Pineal gland
  • secretes melatonin, a hormone affecting timing of sexual maturation, puberty, and circadian rhythms.
• Pancreas -islets of Langerhans are both exocrine and endocrine structures, producing digestive enzymes and regulating blood sugar with the hormones insulin and glucagon.
• Gonads -Ovaries secrete estrogens and progesterone; Testes secrete testosterone. They have important roles in reproduction, growth, and development.
• Adipose tissue (Adipokines) -Secretes hormones like leptin, adiponectin, and others. These impact various metabolic and physiological functions like regulating energy balance, insulin sensitivity, inflammation and more.

Local Hormones (Autocoids)

• These are short-lived hormones produced by almost all body cells, except RBCs, that are often involved in paracrine signaling.
• They include various types of chemicals relevant to body functions, including amines (histamine, serotonin), lipid-derived molecules (eicosanoids), and proteins/peptides. Each type is characterized by the particular function.

Specific Examples of Hormones (Action and Effects)

Hormones such as cortisol, ACTH, ghrelin, leptin, PTH etc., have specific actions in their respective systems.

Factors Affecting Hormone Release

• Elevated blood glucose, amino acids and fatty acids, parasympathetic nervous system signals, various other hormone levels, and sympathetic nervous system signals. These are factors that influence hormone release in specific glands, like the pancreas or the adrenal glands.

Negative Feedback Regulation

• Negative feedback system regulates hormone release, inhibiting further release of the hormone when the concentration reaches a normal level.

Summary Discussion

• These are some important details for studying Endocrine Physiology, and are specific examples. The more general overview of the topic has been covered. Further details must be obtained from other sources such as textbooks, research, and relevant literature.