Endocrine System Characteristics

Study Notes

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.

Description

The endocrine system operates as a distributed network of glands and messengers, often influenced by the CNS or autonomic nervous system. This system differs from other physiological systems in terms of its anatomical boundaries.