Endocrinology Hormones Overview

Study Notes

Hormones regulate energy storage and mobilization in response to food availability and physiological stress.
Insulin, produced by beta cells in the pancreas, is a peptide hormone that facilitates glucose uptake and promotes anabolic processes.
Blood glucose levels stimulate insulin secretion; stress hormones (catecholamines) inhibit it.
Key actions of insulin include:
- Increasing glucose uptake by muscle and adipose tissues
- Promoting glycogen, protein, and triacylglycerol synthesis
- Mediated through binding to tyrosine kinase receptors, activating insulin receptor substrate proteins.

Glucagon, produced by alpha cells in the pancreas, opposes insulin's actions and raises blood glucose levels.
Secretion of glucagon is triggered by:
- Low blood glucose
- Certain amino acids
- Catecholamines
Elevated glucose and insulin levels inhibit glucagon release.
Glucagon activates adenylyl cyclase through high-affinity receptors in hepatocytes, producing cAMP and regulating metabolic enzymes.

During fasting, a catabolic state occurs due to decreased insulin and increased counterregulatory hormones.
Priorities during fasting:
- Maintain blood glucose for brain energy needs
- Mobilize fatty acids and produce ketone bodies for other tissues
Liver converts substrates for gluconeogenesis and ketogenesis, while adipose tissue provides fatty acids and glycerol.

Muscle can utilize fatty acids and ketone bodies, and muscle protein may degrade to supply amino acids for gluconeogenesis, which decreases with rising ketone body levels.
The brain can utilize both glucose and ketone bodies for energy.
Kidneys synthesize glucose and handle ammonium excretion in prolonged fasting and starvation.

Male puberty is regulated by increased steroid hormones from gonads and adrenal glands, affecting genitalia development, body hair, and voice changes.
Key features of the male genital system include the urethral routes through the prostate, pelvic muscles, and penis.

Hypoglycemia presents with low blood glucose and symptoms resolved quickly by glucose administration.
Insulin-induced and fasting hypoglycemia lead to glucagon and adrenaline release.
The feed-fast cycle regulates metabolic flows through:
- Substrate availability
- Enzyme activation/inhibition
- Covalent enzyme modification
- Induction/repression of enzyme synthesis
In the absorptive state (2-4 hours post-meal):
- Nutrients are stored as glycogen, TAG, and protein.
- Elevated plasma glucose and TAG levels trigger insulin release and inhibit glucagon.
- This anabolic state allows tissues to use glucose and replenish liver glycogen, proteins, and TAGs.

In the fed state, the brain relies exclusively on glucose for fuel.
Absence of food leads to declining plasma levels of glucose, amino acids, and TAGs, prompting decreased insulin and increased glucagon and epinephrine secretion.