Endocrinology and Hormone Mechanisms

Comprehensive vocabulary flashcards covering general endocrinology principles, hormone classifications, glands and secretions, receptor regulation, intracellular signaling mechanisms, hormonal interactions, and laboratory measurement assays based on lecture notes.

Endocrinology

The branch of medicine and biology concerned with the study of endocrine glands and hormones.

Exocrine Gland

A gland that possesses ducts to transport its secretions.

Endocrine Gland

A ductless gland that secretes chemical messengers directly into extracellular fluid and the bloodstream.

Hormone

A chemical substance secreted by one cell or group of cells into the extracellular fluid that exerts physiological control effects on other cells of the body.

Paracrine Messenger

A chemical messenger released from one cell that acts locally on nearby target cells, such as nitric oxide.

Autocrine Messenger

A chemical messenger released by a cell that acts upon the same cell that secreted it, such as interleukin-1 in monocytes.

Neuroendocrine Messenger

A chemical messenger synthesized by neurons that travels via the bloodstream to target tissues, such as ADH and oxytocin.

Cytokines

Signaling peptides such as interleukins and lymphokines that can function through endocrine, paracrine, or autocrine mechanisms.

Supraoptic and Paraventricular Nuclei

Hypothalamic nuclei that synthesize ADH (vasopressin) and oxytocin, which are transported down axons and stored in the posterior pituitary gland.

Hypothalamohypophyseal Portal Vessel

The vascular system connecting the hypothalamus to the anterior pituitary gland through which hypothalamic releasing and inhibitory hormones travel.

Hypothalamohypophyseal Nerve Tract

The neural tract along which hormones synthesized in the hypothalamus travel down axons to reach the posterior pituitary gland.

Adrenal Cortex

The peripheral outer region of the adrenal gland that secretes steroid hormones including cortisol and aldosterone.

Adrenal Medulla

The central inner core of the adrenal gland that secretes catecholamines, specifically norepinephrine and epinephrine.

Islets of Langerhans

Endocrine clusters in the pancreas comprising $\alpha$ cells (glucagon), $\beta$ cells (insulin and amylin), $\delta$ cells (somatostatin), $\gamma$ cells (pancreatic polypeptide), and epsilon cells (ghrelin).

Hormone Receptors

Protein structures on or within target cells numbering $2{,}000$ to $100{,}000$ per cell that are highly specific for binding a single hormone.

Up-Regulation

An increase in the total number of target cell receptors that enhances cellular sensitivity and responsiveness to a hormone.

Down-Regulation

A decrease in receptor number or activity caused by receptor destruction, reduced production, or molecular inactivation, decreasing cellular sensitivity and activity in response to elevated hormone levels.

Local Hormones

Hormones that act near their site of secretion, such as the gastrointestinal tract hormones gastrin and secretin.

General Hormones

Hormones that travel systemically to affect tissues throughout the body, including growth hormone, insulin, and thyroid hormone.

Tropic Hormones

Hormones whose primary function is to stimulate and control the secretion of other endocrine glands, such as TSH, ACTH, and FSH.

First Messenger

An extracellular chemical substance, such as a hormone or neurotransmitter, that binds directly to cell surface receptors to initiate a response.

Second Messenger

Intracellular signaling molecules formed after the first messenger binds its receptor, including $\text{cAMP}$, $\text{IP}_3$, $\text{Ca}^{2+}$, and calcium-calmodulin.

Adenylyl Cyclase - cAMP System

A second messenger pathway where hormone binding activates a G protein to stimulate adenylyl cyclase, converting $\text{ATP}$ to $\text{cAMP}$, which then activates $\text{cAMP}$-dependent protein kinase to phosphorylate target proteins.

Phospholipid Second Messenger System

A pathway initiated by phospholipase C activation that breaks down $\text{PIP}_2$ into $\text{IP}_3$ (which mobilizes $\text{Ca}^{2+}$ from mitochondria and ER) and DAG (which activates protein kinase C).

Calcium-Calmodulin Second Messenger System

A mechanism in which hormone receptor binding opens membrane calcium channels, allowing $\text{Ca}^{2+}$ to enter and bind calmodulin, altering its shape to activate or inhibit specific protein kinases.

Steroid Hormone Mechanism of Action

A process where lipid-soluble steroid molecules enter the target cell, bind cytoplasmic receptors, translocate into the nucleus, and bind chromosomal DNA to drive transcription of $\text{mRNA}$ and translation of new proteins.

Permissive Effect

A hormonal interaction where the presence of one hormone amplifies or enables the action of another, such as thyroid hormone greatly increasing epinephrine-induced fatty acid release.

Synergistic Effect

A hormonal interaction where the combined effect of two hormones is greater than the additive sum of their individual effects, such as thyroid hormone potentiating growth hormone action.

Antagonistic Effect

A hormonal interaction where one hormone opposes or counteracts the physiological action of another, exemplified by insulin lowering blood glucose while glucagon increases it.

Radioimmunoassay (RIA)

A sensitive quantitative hormone measurement technique based on antigen-antibody reactions that utilizes hormone tagged with a radioactive isotope, labeling the antigen.

Enzyme-Linked Immunosorbent Assay (ELISA)

A widely used diagnostic assay based on antigen-antibody reactions that measures peptide and steroid hormones by labeling the antibody and using a suitable dye to detect the hormone-antibody complex.