Endocrine System and Hormone Function

Vocabulary-style flashcards based on lecture notes covering the endocrine system, hormone classifications, stimuli, and cellular response mechanisms.

Antagonist

A synthetic drug that works by blocking receptors for a naturally occurring hormone to prevent its action.

Autocrine

A local chemical messenger that exerts its effects on the same cell that secreted it.

Endocrine system

A system that releases hormones into the blood to reach diffuse locations, resulting in a slower response than the nervous system.

Up-regulation

A process where a deficiency of a hormone leads to an increase in the number of receptors, making the target tissue more sensitive.

RU486 (mifepristone)

A synthetic drug that blocks progesterone receptors, preventing the hormone from sustaining a pregnancy.

Lipid-soluble transport

Hormones like steroids and thyroid hormones must attach to transport proteins synthesized by the liver to move through the blood.

Down-regulation

A process where a long-term excess of circulating hormone leads to a decrease in the number of receptors and cell sensitivity.

Permissiveness

A relationship where one hormone cannot exert its full effects without another hormone being present.

Water-soluble hormone mechanism

Hormones that bind to receptors on the plasma membrane and use a second-messenger cascade to trigger cellular responses.

Neural stimulus

When nerve fibers, such as sympathetic fibers, stimulate an endocrine gland to release hormones.

Humoral stimulus

Stimulation of hormone secretion by changing blood levels of certain ions or nutrients, such as calcium.

Half-life

The time required for a hormone's concentration in the blood to decrease by half.

Amplification

A phenomenon where a single hormone molecule binding to a receptor triggers a cascade resulting in a massive physiological response.

Phosphodiesterase

The enzyme responsible for inactivating cAMP and turning off the cellular response to water-soluble hormones.

Synergism

Occurs when more than one hormone produces the same effects on a target cell, resulting in a combined effect greater than the sum of their individual effects.

Paracrine

A local hormone that acts on neighboring cells within the same tissue without entering the bloodstream.

Anti-insulin effect

A hormone effect that decreases the rate of cellular glucose uptake to save blood glucose for use by the brain.

Nervous system modulation

When the nervous system overrides normal endocrine controls, such as allowing blood glucose to rise during severe stress.

Circulating hormone

Hormones produced by endocrine glands that travel in the blood to act on distant target cells.

Exocrine gland

Glands that produce nonhormonal substances and possess ducts to carry secretions to a membrane surface.

Steroid

A class of lipid-soluble hormones derived from cholesterol.

Plasma membrane receptors

Integral proteins that serve as binding sites for water-soluble, amino-acid based hormones.

G protein

A component in the cAMP second-messenger system that is activated by a receptor to then activate adenylate cyclase.

Transport proteins

Proteins synthesized by the liver required to move lipid-soluble hormones through the blood.

Antagonism

Occurs when one or more hormones oppose the action of another hormone, such as hormones that increase vs. decrease heart rate.

Affinity

The strength of binding between a hormone and its specific receptor.

Negative feedback

The primary control mechanism where the response to a hormone eventually inhibits further release of that hormone.

Second messenger

Intracellular substances such as cAMP, calcium ions, or cGMP that relay signals from membrane-bound receptors.

Lipid-soluble mechanism

The hormone enters the cell to activate genes, resulting in the synthesis of new proteins via mRNA translation.

Free fraction

The active portion of a lipid-soluble hormone ($0.1\% \text{ to } 10\%$) that is not bound to a transport protein.

Hormonal stimulus

Occurs when one hormone released by an organ triggers another endocrine organ to release its hormones.

Water-soluble half-life

Generally short because these hormones circulate free in the plasma and are easily removed by the kidneys.

Growth hormone inhibiting hormone (GHIH)

An amino acid-based hormone that can exert its influence by decreasing the level of cAMP in target cells.

Eicosanoids

Local hormones derived from arachidonic acid, including prostaglandins and leukotrienes.

Nitric oxide ($NO$)

A gas that acts as a local lipid-soluble hormone in several tissues.

Adenylate cyclase

The enzyme that converts ATP to the second messenger cAMP.

Protein kinases

Enzymes activated by cAMP that phosphorylate other proteins to produce a final cellular response.

Hormone storage

Water-soluble hormones are stored in secretory vesicles, while lipid-soluble hormones are synthesized on demand.

Half-life calculation

After three half-lives (e.g., $60 \text{ minutes}$ with a $20 \text{ minute}$ half-life), an initial $100 \text{ unit}$ concentration drops to $12.5 \text{ units}$.

Atrial Natriuretic Peptide (ANP)

A hormone released by the heart in response to high blood sodium levels via humoral stimulus.

Thyroid hormone

An amine-based hormone that is an exception because it is lipid-soluble and can be taken orally.

Major endocrine processes

The system primarily controls reproduction, growth and development, and the mobilization of body defenses.

First messenger

In a second-messenger system, this refers to the hormone itself that binds to the membrane receptor.

Response duration

The duration of a hormone's effect is limited, typically ranging from $10 \text{ seconds}$ to several hours.

Membrane permeability

A hormonal action that alters cell potential by opening or closing ion channels.

Prostaglandins

A type of eicosanoid hormone derived from the fatty acid arachidonic acid.

Activation factors

Target cell activation depends on blood levels, receptor number, and the affinity of binding between receptor and hormone.

G protein reset

Once the hormone leaves the receptor in the cAMP system, the G protein returns to its inactive state.

Liver disease impact

Disease that prevents the synthesis of plasma proteins will hinder the transport of lipid-soluble hormones.

Steroid response time

Typically takes hours to days because it involves gene transcription and protein synthesis.

Permissiveness example

The requirement of thyroid hormone presence for reproductive hormones to exert their full effect.

cAMP inactivation

The cellular response is turned off by the enzyme phosphodiesterase, which degrades cAMP.

Catecholamines

Hormones like epinephrine and norepinephrine released by the adrenal medulla during a neural stimulus.

Fight or flight physical symptoms

Nervous stimulation causes rapid heart rate and increased blood glucose through the release of epinephrine.

Parathyroid hormone (PTH)

Hormone released in response to low blood calcium levels, representing a humoral stimulus.

Secretory vesicles

Storage structures within glands specifically for water-soluble hormones.

Thyroid-stimulating hormone (TSH)

A pituitary hormone that triggers the thyroid gland, illustrating a hormonal stimulus.

Arachidonic acid

The fatty acid from which lipid-soluble eicosanoid local hormones are derived.

Hormone absorption

Lipid-soluble hormones like thyroid hormone can be absorbed across the gut wall when taken orally.

Reflex arc

A rapid-response mechanism of the nervous system that is not a function of the endocrine system.