Endocrinology: Principles of Hormone Action and Regulation

Comprehensive vocabulary flashcards covering the introductory endocrinology lecture, including hormonal classification, mechanisms of action, signaling pathways, and feedback regulations.

Endocrine Glands

Ductless glands that secrete their hormonal products directly into the bloodstream rather than through a duct system.

Hormone

A chemical messenger released from endocrine glands directly into the bloodstream that travels to and acts on specific target organs; derived from a Greek term meaning 'I excite' or 'arouse.'

Hormone Specificity

The principle that a circulating hormone affects only cells displaying specific receptors for that hormone (for example, ACTH acts on the adrenal cortex, whereas LH acts on gonadal tissue).

Neural Control

A control system characterized by a short latent period, rapid action, and the regulation of specific localized groups of cells over a brief duration.

Endocrine Control

A control system characterized by a long latent period, slower onset of action, and systemic effects across numerous target cells sustained over longer periods.

Major Endocrine Organs

The primary specialized ductless organs of the endocrine system: the hypothalamus, pineal gland, pituitary gland, thyroid gland, parathyroid glands, thymus, adrenal glands, pancreas, ovaries, and testes.

Neuroendocrine Secretion

A process in which neurons synthesize hormones that are transported along a neural tract to be released into circulation (for example, hypothalamic synthesis of ADH and oxytocin transported along the hypophyseal tract to the posterior pituitary).

Radioimmunoassay (RIA)

A quantitative laboratory technique in which an unknown serum hormone sample competes with a known amount of radioactively labeled hormone for a fixed number of antibody binding sites.

Steroid Hormones

Lipid-soluble chemical messengers derived from cholesterol that act on intracellular receptors and are synthesized on demand without vesicle storage; includes sex hormones, adrenocortical hormones, and active vitamin D ($1,25\text{-DHCC}$).

Amine Hormones

Hormones derived from the amino acid tyrosine, including catecholamines (epinephrine, norepinephrine) and iodinated thyroid hormones ($T_3$ and $T_4$).

Peptide Hormone Synthesis

The formation of protein hormones through sequential cleavage: initial synthesis of a pre-pro-hormone in the rough endoplasmic reticulum, conversion to a prohormone in the Golgi apparatus, and final packaging into secretory vesicles as the active hormone.

Intracellular Receptors

Receptors located in the cytoplasm or nucleus that bind lipid-soluble hormones (steroids and thyroid hormones), forming a complex that binds DNA to alter transcription and translation into functional proteins.

cAMP / Adenylate Cyclase Mechanism

A second messenger system where hormone binding to a membrane receptor activates a $G_s$ protein; its $\alpha_s$ subunit binds GTP, stimulating adenylyl cyclase to convert ATP into cyclic AMP ($\text{cAMP}$), which then activates protein kinase A.

Phospholipase C Mechanism

A membrane-bound signaling pathway where hormone binding activates a $G_q$ protein, stimulating phospholipase C to cleave $\text{PIP}_2$ into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate ($\text{IP}_3$), prompting intracellular $Ca^{2+}$ release and activating protein kinase C.

Negative Feedback Regulation

The primary endocrine control mechanism in which an increase in circulating hormone concentration or its biological effect directly or indirectly inhibits further secretion of that hormone to maintain homeostasis.

Positive Feedback Regulation

A rare regulatory cycle in which the biological action of a hormone induces additional secretion of that same hormone (for example, the preovulatory estrogen surge driving an LH/FSH surge).

Downregulation of Receptors

A reduction in the total number of functional receptors on target cells caused by chronic, sustained high circulating hormone levels, resulting in tissue resistance (as seen with insulin receptors in Type II NIDDM).

Upregulation of Receptors

An increase in the density of receptors on target cells that increases target tissue sensitivity to a hormone (such as estrogen increasing uterine LH and oxytocin receptors).

Free Form vs. Bound Form Hormones

The physiological division between unbound circulating hormone, which is biologically active and available to target tissues, and hormone bound to hepatic transport globulins (e.g., CBG, TBG), which acts as a circulating reservoir.

Binding Globulin Alteration in Pregnancy

The phenomenon where elevated pregnancy estrogen increases hepatic production of hormone-binding globulins, elevating total plasma bound hormone while biologically active free hormone levels remain normal, preventing signs of hormone hyperfunction.