Endocrinology lectures 22

Endocrinology Overview

• Endocrinology studies the hormonal systems that regulate metabolic processes.

• It emphasizes homeostatic control mechanisms central to physiological integration.

  • Key aspects include metabolism, salt and water balance, temperature regulation, reproduction, and growth.

Hormonal Characteristics

• Hormones exhibit diverse characteristics:

  1. Multiple hormones can be produced by a single endocrine gland.

  2. A single hormone can be secreted by multiple tissues.

  3. Target cell types for one hormone can vary.

  4. Hormone secretion is time-variable and responsive to environmental changes.

  5. Single target cells can respond to multiple hormones.

  6. Hormones can originate from blood or neuronally derived sources.

  7. Some hormones have additional non-endocrine functions.

Hormonal Functionality

• Hormones operate in a feedback system:

  • Cell A secretes a hormone, and Cell B responds to it.

• Feedback loop mechanisms are predominantly negative to maintain stability; output counters input.

Hormone Classification

• Hormones can be categorized chemically:

  • Peptides: Made of amino acids (3 to 500+ in structure). Secreted via exocytosis.

  • Amino Acid Derivatives: Such as catecholamines, thyroid hormones, and melatonin, which exhibit varied solubility.

  • Steroids: Derived from cholesterol, secreted via diffusion due to hydrophobic nature.

Hormone Processing and Activity

• Hormonal processing goes through several stages:

  1. Secretion (regulated or constitutive).

  2. Transport via carrier proteins to target sites.

  3. Activation and metabolism of free hormones at target cells.

  4. Inactivation and eventual excretion from the body.

  • Hormone half-lives vary by type:

    • Amine derivatives: minutes.

    • Peptide hormones: minutes to hours.

    • Steroid hormones: hours.

Hormonal Control Mechanisms

• Feedback Mechanisms: Ensure hormonal regulation is responsive to physiological needs, commonly through:

  • Trophic hormones regulating target gland activity.

  • Temporal Rhythms: Hormone release cycles are affected by environmental factors, such as day-night cycles.

  • Example: Melatonin peaks at night; cortisol levels show pronounced peaks during the day.

Target Cell Response Mechanisms

• Target cells respond variably based on:

  1. Up and Down Regulation: Receptor abundance adjusts per hormone level.

  2. Permissiveness: One hormone’s effect may require another hormone’s presence.

  3. Synergism: Combining hormones effects exceed the sum.

  4. Antagonism: One hormone opposes another's action.

Hypothalamic-Pituitary Axis

• A vital component for endocrine control:

  • Anterior pituitary secretes trophic hormones under hypothalamic control.

  • Posterior pituitary hormones synthesized in the hypothalamus, emphasizing neuroendocrine links.

Endocrine Dysfunction

• Conditions arise from hormonal secretion imbalances:

  • Hyposecretion: Due to gland atrophy (treated via replacement).

  • Hypersecretion: Typically from tumor presence (managed through inhibition).

  • Target cell responsiveness issues: Receptor deficiencies can hinder actions like insulin effectiveness.

Growth Regulation

• Growth is influenced by hormones with a focus on:

  • Nutritional state, genetic factors, and multiple hormonal actions.

  • Neonatal growth hinges on placental hormones and varies throughout life stages.

  • Growth hormones (GH) exhibit peaks typically connected to sleep cycles.

Physiological Actions of GH and IGFs

• GH indirectly stimulates growth through Insulin-like Growth Factors (IGFs).

• Actions on tissues: cell division, protein synthesis, and metabolic adjustments.

Hormonal Influence on Calcium Regulation

• Calcium plays an integral role in various bodily functions.

  • Hormones manage blood calcium levels: Parathyroid hormone (PTH) regulates calcium release from bones and kidney handling.

  • Vitamin D enhances intestinal calcium absorption, illustrating inter-hormonal cooperation.

  • Calcitonin promotes lower blood calcium levels, especially following calcium-rich meals.

Thyroid Hormones

• The thyroid gland produces T3 (triiodothyronine) and T4 (thyroxine), crucial in energy metabolism.

• Iodine is vital for hormone synthesis; deficiencies lead to metabolic dysfunction.

Stress Hormones

• The adrenal cortex produces glucocorticoids (e.g., cortisol) that mobilize energy during stress and affect metabolism.

• Catecholamines (e.g., epinephrine) from the adrenal medulla enhance immediate physiological responses during stress.

Insulin and Glucagon Regulation

• Insulin lowers blood glucose by facilitating cellular uptake and storage; critical during the fed state.

• Glucagon opposes insulin, raising blood glucose through liver glycogenolysis during fasting.

• Balance between these hormones is essential for maintaining homeostasis.