Metabolism of Calcium, Phosphate, and Adrenal/Pancreatic Endocrine Functions

Flashcards covering the physiology and clinical disorders of calcium/phosphate metabolism, the adrenal cortex/medulla, and the endocrine pancreas based on Case 3, 4, and 5 lecture notes.

Hydroxyapatite

The crystalline form in which approximately 99% of body calcium is present in the skeleton, represented by the formula $Ca_{10}(PO_{4})_{6}(OH)_{2}$.

Hypercalcemia

An increase in extracellular fluid calcium concentration above the normal value of about $9.4\,mg/dl$, which causes progressive depression of the nervous system.

Hypocalcemia

A decrease in calcium concentration that causes the nervous system to become more excited and can lead to tetany.

Ionized Calcium

The diffusible form of calcium in plasma, constituting about 50% of the total, which is the form important for most functions including effects on the heart and nervous system.

Stable Bone Pool

The larger calcium pool containing over 99% of total body $Ca^{2+}$, consisting of mature bone that is not readily exchangeable or available for rapid mobilization.

Labile Bone Pool

The smaller calcium pool (less than 1% of total) consisting of young bone and calcium phosphate salts that is in physicochemical equilibrium with the ECF for immediate reserve.

Parathyroid Hormone (PTH)

An 84-amino acid polypeptide secreted by chief cells that acts as a hypercalcemic hormone by mobilizing $Ca^{2+}$ from bone and increasing renal reabsorption.

Calcitonin

A 32-amino acid polypeptide secreted by parafollicular (C) cells of the thyroid gland that acts as a hypocalcemic hormone by inhibiting bone resorption.

Calcitriol

The active metabolite of Vitamin D ($1,25-dihydroxycholecalciferol$) that is 100 times more potent than calcidiol and primarily increases intestinal calcium absorption.

Calbindin-D proteins

A family of calcium-binding proteins induced in the intestine by calcitriol to facilitate $Ca^{2+}$ movement across the epithelium.

Chvostek sign

A sign of tetany in humans characterized by a quick contraction of ipsilateral facial muscles elicited by tapping over the facial nerve at the angle of the jaw.

Trousseau sign

A spasm of the upper extremity muscles causing wrist/thumb flexion and finger extension, often produced by occluding circulation with a blood pressure cuff in hypocalcemic patients.

Osteitis fibrosa cystica

The cystic bone disease associated with severe primary hyperparathyroidism, characterized by extensive decalcification and large cystic areas filled with osteoclasts.

Zona Glomerulosa

The outermost layer of the adrenal cortex, located beneath the capsule, which is the site of aldosterone and corticosterone synthesis.

Zona Fasciculata

The middle and largest layer of the adrenal cortex, composed of polyhedral cells called spongiocytes that primarily synthesize cortisol.

Zona Reticularis

The innermost layer of the adrenal cortex, where cells are arranged in anastomosing cords, responsible for synthesizing cortisol and adrenal androgens like DHEA.

Chromaffin cells

Also called pheochromocytes, these neuroectodermal derivatives in the adrenal medulla synthesize epinephrine (80%) and norepinephrine (20%).

Cortisol-binding globulin (CBG)

Also known as transcortin, this $\alpha-globulin$ binds about 75% of plasma cortisol, serving as a reservoir for the free hormone.

Renin

A proteolytic enzyme synthesized, stored, and released by Juxtaglomerular (JG) cells in response to decreased renal perfusion pressure or hypovolemia.

Macula densa cells

Specialized renal tubular epithelial cells that function as chemoreceptors stimulated by a decrease in $Na^{+}$ or $Cl^{-}$ load to trigger renin release.

Angiotensin II

A potent octapeptide and vasoconstrictor that functions as the tropic hormone for the zona glomerulosa to stimulate aldosterone secretion.

Escape phenomenon

A process where an initial period of $Na^{+}$ retention by aldosterone is followed by natriuresis, reestablishing $Na^{+}$ balance and preventing edema.

Addison disease

Primary adrenal insufficiency due to destruction of the adrenal cortex, characterized by weight loss, hypotension, and hyperpigmentation of skin and gums.

Cushing’s Syndrome

A condition of glucocorticoid excess characterized by central obesity (moon face, buffalo hump), protein catabolism, hyperglycemia, and purple abdominal striae.

Conn’s Syndrome

Primary hyperaldosteronism, usually resulting from a tumor of the zona glomerulosa, characterized by hypertension, hypokalemic alkalosis, and depressed renin levels.

21\beta-hydroxylase deficiency

The most common biochemical abnormality in the adrenogenital syndrome, leading to deficient cortisol/aldosterone and excess adrenal androgens.

Islets of Langerhans

Small aggregations of endocrine tissue in the pancreas, constituting 1-2% of its volume, containing alpha, beta, delta, and F cells.

Beta cells

Cells constituting 60-70% of pancreatic islets that synthesize insulin, typically occupying the center of the islet.

Alpha cells

Islet cells (about 25%) located primarily at the periphery that secrete glucagon.

Delta cells

Islet cells (about 10%) that are the source of somatostatin, which inhibits both insulin and glucagon secretion.

C-peptide

The connecting peptide formed when proinsulin is cleaved into insulin; its circulating levels reflect endogenous beta cell activity.

GLUT-4

The insulin-sensitive glucose transporter found in muscle and adipose tissue that translocates from intracellular vesicles to the cell membrane.

Gluconeogenesis

The metabolic process of promoting the conversion of amino acids to carbohydrates, a key effect of cortisol and glucagon in the liver.

Glycogenolysis

The breakdown of stored liver glycogen into glucose, a primary effect of glucagon mediated by a cAMP-activated enzyme cascade.

HbA1c

Nonenzymatically glycated hemoglobin measured clinically as an integrated index of blood glucose control over the preceding 4 to 6 weeks.

Kussmaul breathing

Rapid, deep respiration, often described as “air hunger,” produced in response to the metabolic acidosis found in severe diabetic ketoacidosis.