Module 4: Endocrinology

Endocrine system

A hormone-based control system.

Hormones

Cell signaling molecules secreted by endocrine cells that travel via blood to act on target cells.

Specificity in endocrine signaling

Determined by receptor expression on target cells rather than hormone distribution.

Modern endocrinology

Recognizes many tissues as endocrine, such as the GIT, heart, adipose tissue, and bone.

Cell-to-cell communication

The primary concept of modern endocrinology.

Juxtracrine signaling

Cell communication through direct contact, such as adhesion molecules and gap junctions.

Autocrine signaling

Cell communication where signals act on the same cell.

Paracrine signaling

Cell communication where signals act on nearby cells.

Endocrine signaling

Hormones released into the bloodstream to act on distant target cells.

Nervous system signaling

Transmission of signals via neurotransmitters.

Neuroendocrine signaling

Neurons release hormones into the blood.

Chemical messengers

Includes peptides, steroids, amines, lipids, and metabolites.

Storage of messengers

Chemical messengers can be stored in vesicles or diffuse out immediately upon synthesis.

Peptide hormones

Hormones stored in vesicles and released by exocytosis.

Steroid hormones

Not stored; diffuse out immediately after synthesis.

Thyroid hormones

Stored as thyroglobulin and released via carrier-mediated diffusion.

Secretion regulation

Hormone release is the sum of net stimulatory vs inhibitory inputs.

Insulin secretion example

Hormone release influenced by multiple stimulatory and inhibitory factors.

Hydrophilic messengers

Travel freely in plasma and dissolve easily in aqueous solutions.

Hydrophobic messengers

Travel bound to plasma proteins and require carrier proteins.

Plasma hormone levels

Determined by secretion, activation, binding to plasma proteins, inactivation, and excretion.

Target cell sensitivity

Depends on the number of receptors, receptor affinity, and downstream signaling capacity.

GPCRs

G protein-coupled receptors that bind various hormones.

Tyrosine kinase receptors

Receptors that include those for insulin and IGF-1.

Intracellular responses

Responses that involve altered ion permeability, enzyme activation, or gene expression changes.

Signal amplification

Allows a small concentration of hormone to trigger a large cellular response.

Human Growth Hormone (GH)

A polypeptide hormone secreted by somatotrophs in the anterior pituitary.

Short-loop negative feedback

Regulatory mechanism where GH decreases GHRH secretion in the hypothalamus.

IGF-1

Insulin-like Growth Factor 1, influenced by GH and important for growth.

Hyposecretion

Condition of too little hormone release, leading to insufficient physiological responses.

Hypersecretion

Condition of too much hormone release, leading to excessive physiological responses.

Hypothalamus

Neural tissue and endocrine gland that integrates signals for hormone regulation.

Oxytocin (OT)

A neurohormone involved in uterine contractions and milk ejection.

Vasopressin (AVP/ADH)

Antidiuretic hormone released from the posterior pituitary to regulate water retention.

Growth Hormone (GH) actions

Includes increasing cell number and size as well as metabolic activities.

Prolactin (PRL)

A polypeptide hormone important for mammary gland development and lactation.

Somatomedin hypothesis

The classic view that GH stimulates growth via IGF-1 from the liver.

Dopamine's role in PRL regulation

Inhibits prolactin secretion by acting on lactotrophs in the pituitary.

Prolactinoma

A benign pituitary tumor that causes excess prolactin secretion.

GH deficiency in children

Causes pituitary dwarfism characterized by short stature.

Acromegaly

A condition caused by excess GH in adults leading to enlarged bones and tissues.

Hormonal interactions for growth

Include thyroid hormones, sex steroids, insulin, and glucocorticoids.

Feedback regulation of PRL

Short-loop negative feedback where PRL increases dopamine levels to reduce PRL secretion.

Suckling reflex

Stimulates prolactin and oxytocin release to facilitate lactation.

Growth hormone actions on muscle

Includes increased amino acid uptake and decreased glucose uptake.

Growth patterns in infancy and puberty

Two major postnatal growth spurts leading to rapid growth.