Endocrine System Characteristics

Endocrine System Overview

• Differs from other physiological systems as it operates as a network of glands and circulating messengers influenced by the CNS and autonomic nervous system
• Comprises hormones such as steroids, amines, and peptides
• Hormones are grouped into families based on structural similarities and receptors they activate
• Increased complexity of homeostasis with the rise in life form complexity leads to more hormones and diversity in function

Hormone Diversity

• Peptides are the most numerous type of hormones
• Example: Several peptide hormones form heterodimers with a common α chain and distinct β-chain, like LH, TSH, and FSH
• TSH, FSH, and LH have the same α chain with different β-chains originating from a common ancestral gene
• Evolution in hormone structure requires evolution in receptors for actions and specificity spread, like GPCR for peptide and amine hormones

Hormone Synthesis and Regulation

• Synthesis of peptides involves gene transcription, while amine and steroids regulate enzyme and substrates involved in synthesis
• Peptide hormones are synthesized initially as pre-pro-hormones, then processed into pre-hormones, and finally into the active hormone
• Factors influencing hormone availability include rate of secretion, nature of secretion, degradation rate, receptor affinity, and carrier plasma proteins

Hormone Secretion and Function

• Hormones are secreted in a pulsatile manner, influenced by circadian rhythms and other regulators
• Adrenaline and most peptides are soluble in plasma and don't require transportation
• Steroid hormones are released by diffusion and synthesized from cholesterol in mitochondria
• Plasma carrier proteins modulate the level of free hormones in the blood

Hormone Availability

• Free hormones are biologically active and can mediate feedback regulation
• Plasma carriers like SBG and SBP regulate the equilibrium between bound and free hormones
• Equilibrium shifts determine the availability of hormones for actions
• Regulation of carrier protein expression influences hormone availability

Nuclear Receptors and Hormone Action

• Two classes of nuclear receptors important in endocrine physiology:
  1. Directly stimulate transcription by inducing binding of transcriptional co-activator
  2. Indirectly stimulate transcription by dislodging a co-repressor and recruiting a co-activator, allowing for dynamic regulation

Hormone Diversity in Action

• Peptides and catecholamines exert acute effects by binding to specific receptors and initiating various physiological responses.### G-Protein Coupled Receptors (GPCR)

• Most cell surface receptors belong to the GPCR family.

• Hormones that interact with GPCR include catecholamines, peptides, and prostaglandins.

Nuclear Receptors

• Steroids and thyroxine (hydrophobic hormones) exert their actions via nuclear receptors.
• These receptors are predominantly intracellular.

Feedback Loops

• Feedback loops can be either negative or positive.
• Positive feedback loop: enhances or continues to stimulate the original release mechanism/stimulus (e.g., parturition).
• Negative feedback loop: inhibits or dampens the initial hormone release mechanism/stimulus (more common control mechanism).
• Feedback control loops are important for diagnostic strategies in evaluating patients with suspected endocrine disorders.

Diagnostic Strategies

• Evaluating patients with suspected endocrine disorders, such as hypothyroidism, can be done using feedback control loops.
• For example, normal levels of TSH rule out a primary defect at the level of the thyroid gland itself, but suggest a defect at the level of the anterior pituitary.
• High levels of TSH suggest a defect in the thyroid gland itself to synthesize enough thyroid hormone to suppress TSH synthesis.