Concepts in Endocrinology

Concepts in Endocrinology

• Define hormone, target cell, and receptor
• Contrast endocrine, paracrine, autocrine, and neuroendocrine based on site of release and pathway to target tissue
• Contrast peptide, steroid, and amine hormones in terms of receptor location, signal transduction, solubility in blood, and transport in blood

Homeostatic Control

• Homeostatic control mechanisms include reflex loops where the response decision is made at a distance from the target cell
• Two general classes: endocrine and neuroendocrine
• Endocrine: hormone reaches its site of action via blood circulation
• Neuroendocrine: hormone is secreted from neurons into the blood to act at a distance

Receptors Determine Specificity

• Hormones are widely distributed throughout the body and have access to all cells but only target cells respond
• Target cells possess a receptor for the hormone
• Receptor contains a recognition site that binds its hormone with high affinity and selectivity
• Lipid solubility of the hormone dictates the cellular location of its receptor
• Plasma insoluble hormones bind to intracellular receptors; plasma soluble hormones bind to cell surface receptors

Hormone Signaling

• Peptide hormones bind to cell surface receptors, which activate a second messenger
• Second messengers provide amplification, memory, and complex regulation
• Permissiveness (or tropism) occurs when the actions of a tropic hormone sensitize the target tissue to a second hormone

Inactivation of Hormone Signaling

• Inactivation of hormone signaling occurs at two levels: cellular inactivation and systemic inactivation
• Cellular inactivation involves the termination of the receptor's response
• Systemic inactivation involves homeostatic control mechanisms such as negative feedback

Negative Feedback Loops

• Negative feedback loops eliminate the initiating stimulus
• Example: pituitary and adrenal glands are in a negative feedback loop
• The stimulus, hypoglycemia, leads to the secretion of cortisol from the adrenal gland
• Cortisol acts on liver, muscle, and fat to raise blood glucose levels

Importance of Hormone Levels

• Disorders of the endocrine system result from too little or too much hormone or target cell resistance
• Hypo-secretion: results in an insufficiency of hormone
• Hyper-secretion: results in an excess of hormone
• Resistance: is an abnormal target organ response due to receptor and/or second messenger dysfunction

Hormone Measurement

• Hormones are present at very low concentrations (nM) in body fluids and many have related structures
• Measurement requires very high sensitivity and high specificity
• Two common types of assays: radio-immune assay (RIA) and bioassay (suppression and stimulation)
• RIA measures total hormone (carrier-bound hormone plus free) but only free is active
• Bioassay measures biological activity to a challenge dose of hormone or substrate

General Concepts

• Peptide hormones are soluble in plasma, act via surface receptors, are fast-acting, and short-lived
• Thyroid hormones and steroid hormones are insoluble in plasma, act via intracellular receptors, are slow-acting, and long-lived
• Thyroid and steroid hormones can be converted to more active (or less active) hormones within target tissues

Learn about key concepts in endocrinology, including hormone definition, target cells, and receptors, as well as differences between endocrine, paracrine, autocrine, and neuroendocrine systems.