Physiology of the Endocrine System: Mechanisms and Hormonal Actions

Comprehensive vocabulary flashcards covering the genomic and non-genomic mechanisms of hormones, specific intracellular signaling pathways, and the physiological actions and regulation of major hormones including GH, ADH, Thyroid hormones, PTH, Insulin, and Glucagon.

Genomic mechanism of action

A mechanism for lipid-soluble hormones where the hormone binds to specific receptors in the cytoplasm or nucleus to recognize target genes, leading to transcription of mRNA and the formation of new proteins such as enzymes or channels.

Non-genomic mechanism of action

A process where a hormone (1st messenger) binds to a surface membrane receptor to activate an intracellular 2nd messenger, used by protein hormones and catecholamines that cannot cross the cell membrane.

G-protein

A GTP regulatory protein composed of 3 subunits ($\alpha$, $\beta$, and $\gamma$) that mediates cellular effects when the $\alpha$ subunit dissociates, releases GDP, and binds GTP.

Adenyl cyclase

An enzyme activated by the $\alpha$ subunit of Gs protein that converts ATP into cAMP, which then activates Protein kinase A (PKA).

Phosphodiesterase (PDE)

An enzyme that terminates signaling by converting cAMP into $5'$ AMP or cGMP into inactive GMP.

IP3 (Inositol trisphosphate)

A small sugar second messenger that diffuses to the endoplasmic reticulum to open ligand-gated $Ca^{2+}$ channels, releasing $Ca^{2+}$ into the cytoplasm.

DAG (Diacylglycerol)

A second messenger that remains in the cell membrane to activate Protein kinase C (PKC), which then phosphorylates proteins.

Atrial Natriuretic Peptide (ANP)

A hormone that utilizes the cGMP second messenger pathway to produce its cellular response.

Hypothalamo-hypophyseal portal circulation

A system consisting of 2 sets of capillaries between the hypothalamus and the anterior pituitary gland used to transport releasing or inhibiting hormones.

Hypothalamo-hypophyseal tract

A bundle of neurons between the hypothalamus and the posterior pituitary gland through which ADH and Oxytocin are transported.

Let-down reflex

Also known as the milk ejection reflex, it is a neurohormonal reflex where oxytocin causes the squeezing of milk from breast alveoli into the large ducts and nipple.

Positive nitrogen balance

A result of the anabolic effect of Growth Hormone, which increases protein synthesis and decreases protein catabolism.

IGF-1 (Somatomedin-C)

A factor secreted by the liver under the effect of Growth Hormone that mediates epiphyseal growth and has insulin-like activity on carbohydrate metabolism.

V2 receptors

Receptors located on the principal cells of the late DCT and collecting ducts where ADH acts to translocate Aquaporin-2 ($AQP-2$) water channels to the apical membrane.

Iodide trapping

The second step of thyroid hormone synthesis where the $Na^+/I^-$ symporter carries iodide into the thyroid cell against a concentration gradient of $8:1$.

Organification of iodine

The process where peroxidase enzyme oxidizes iodide ($I^-$) into iodine ($I$) and binds it to tyrosine residues of thyroglobulin to form MIT and DIT.

Calorigenesis

The increase in cellular metabolism and $O_2$ consumption caused by thyroid hormones through increased mitochondrial size, number, and $Na^+/K^+$ ATPase activity.

Osteolysis

The rapid phase of bone resorption where PTH binds to osteocytes to increase membrane permeability to $Ca^{2+}$, moving it from bone fluid to the ECF via calcium pumps.

Calbindin D

A protein induced by Vitamin D ($1,25(OH)_2D_3$) that increases $Ca^{2+}$ transport from the brush border to the interstitial fluid in the intestine.

Permissive actions (Cortisol)

The requirement of cortisol for other hormones to produce effects, such as permitting glucagon to cause glycogenolysis or catecholamines to cause vasoconstriction.

Diurnal variation (Circadian rhythm)

The pattern of CRH, ACTH, and cortisol secretion which is highest in the early morning and lowest in the evening.

Renin-Angiotensin System

The main regulator of aldosterone secretion, where Angiotensin II stimulates the conversion of cholesterol to pregnenolone.

GLUT2 transporter

The transporter through which glucose enters the $\beta$-cells of the pancreas to initiate the mechanism of insulin release.

GLUT4

An insulin-regulated transporter that is inserted into the membranes of skeletal muscle and adipose tissue cells to increase glucose uptake.

Incretins

Gastrointestinal hormones such as GLP-1 and GIP that cause oral glucose to stimulate more insulin secretion than intravenous glucose.