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Physiology I
Section 6
Calcium-Regulating Hormones

Key Words

Osteoclasts are polynucleated macrophage like cells that digest bone matrix by secreting protease enzymes. They remodel and repair bone and they are target cells for PTH that promotes bone resorption and calcium mobilization.

Osteoblasts are young bone forming cells, secreting collagen into the bone matrix. Once surrounded by bone matrix, their activity diminishes and they are considered to be mature, and are called osteocytes. Osteo blasts are usually found near bone surfaces.

Osteocyte are found in or near the lacunae. They develop extensive processes ( filopodia), that run thru the canulicula, connecting with other osteocytes. These membraneous processes facilitate the exchange of nutrients, especially Ca between bone and blood.

PTH a polypeptide hormone which is secreted by the chief cells of the parathyroid gland. PTH acts on bone, kidney, and intestine; to increase Ca levels in plasma.

Calcitinin a large peptide hormone secreted by the parafollicular C cells of the thyroid. It is secreted in response to an increase in the plasma Ca level and it works to decrease Ca in plasma with in minutes. Calcitonin is very necessary in children but not so necessary in adults.

Vitiman D a steroid hormone (fat soluble) which undergoes changes in 2 places 1st being in the liver to 25 hydroxycholecalciferol (calcidiol), and 2nd in the kidney from 25 to 1,25 dihydroxycholecalciferol

(calcitriol), and this 2nd reaction is stimulated by and tightly controlled by PTH. Absorption of 1,25 di occurs in the small intestine. This absorption occurs in response to the 1,25 di stimulating the formation of 1) Ca binding proteins, 2) of Ca ATP ase, and 3) of alkaline phosphatase; all of which promote absorption of Ca & PO4 ions out of the lumen of the intestine into the plasma. The Ca is divalent and therefore cant cross the cell membrane of intestinal epithelium with out the above 3 mechanisms, which are activated by the 1,25 di Therefore Ca absorption will occur at a rate determined specifically by the 1,25 di described above. Problems in synthesis of 25, or 1,25 di or inability to make 1 3, can cause pathology.

Calcium a cation, an alkaline earth metal ( reactive) in group 2a ( looses its 2 electrons easily to go to Ar) . oxidation state is +2 , ionic radius is @ 100 pm, atomic radius is 197 pm, 1st ionization level is 6.113 eV, electronegativity is 1.0, BP is 1494 C, MP is 842 C, Density is 1.55g/cc

Phosphorous is an anion with an electronegativity of 2.1( wants to hold onto electrons more than Ca does), and its a non metal so it wants to combine with calcium, is part of the bone salts, and is regulated by the kidney and PTH ( and vit D) . Is present in tooth enamel. Exists as PO4.

Fast exchange PTH activates membrane bound Ca pump in the intestine, which pumps out Ca with out PO4.

Slow exchange - is the actual dissolving of bone via osteoclasts action.

Resorption - is where PTH causes demineralization ( release of bone salts) via osteoclasts which in turn allows tranpsort of Ca from bone into blood stream via the ostercytes. This is slow exchange.

Reabsorption occurs when ECF Ca is dec. The Ca is reabsorbed into renal circulation from the tubules so that it can be distributed to the blood circulation. Ca absorbed from the collecting duct increases and excretion of Ca decreased. PO4 is reabsorbed from the proximal tubule and PO4 excretion is increased.

Absorption - occurs when ECF Ca is low. PTH level increases Inc rate of formation 1,25 di and increase the concentration of 1,25 di stimulates the formation of Ca binding proteins and other factors in the epithelium of the small intestine which increases the rate of absorption of Ca from the lumen of the gut.

Rickets is caused by inadequate absorption of Ca from the GI tract due to

  1. decreased PO intake of it and
  2. failure to from adequate amounts of 1,25 di which cauld be from renal damage: Would see:
    1. increased PTH Increased osteoclastic resorption increased Ca in ECF &
    2. renal retention of Ca and increased excretion of PO4, with both situations weak bones over time. Tetany not seen until bone demineralization occurs to the point where a normal level of Ca can no longer be maintained.

Effect on bones PTH extreme osteoclastic activity weaker bone increases physical stress on bone rapid osteoblastic activity, but osteoid cant become calcified because of insufficient Ca and PO4 ions. Over time the newly formed uncalcified and weak osteoid takes place of older bone that is being resorbed Treat this by giving adequate amounts of Ca & PO4.

Osteomalacia adult rickets, which is usually caused by steatorrhea ( fatty stool) and vit D is fat soluble so out it goes with the stool. Ca forms insoluble soap and fats in the presence of steatorrhea so both are excreted in the stool. Rarely will adults progress to tetany, but osteomaoicia can cause severe bone disability. VIT D resistant rickets or Renal rickets are caused by 1) failure to convert 25 to 1,25 di and 2) congential hypophosphatemia which decreases reabsorption of PO4 by renal tubules.

Rickets and osteomalacia have impaired mineralization of bone as opposed to osteoarthritis which has reduction in bone mass rather than select reduction of mineral content.

Osteoporosis occurs secondary to diminished organic bone matrix ( a reduction in bone mass). Osteoblastic activity is < than normal depressed bone osteoid deposition, also hyper parathyroidism inc osteoclastic activity can cause osteoporosis and the following things are seen:

  1. dec stress on bones ,
  2. malnutrition prevents protein matrix,
  3. lack of vitiman C inhibits intercellular secretion of substances including formation of osteiod by osteoblasts,
  4. after menopause, lack of estrogen secretion occurs decreasing the estrogen stimulating effect of osteoblastic activity,
  5. old age dec GH and poor anabolic function so the bone matrix unsatisfactorily is deposited,
  6. Cushing's ds glucocorticoids dec protein deposotion and inc catabolism of protein depressed osteoblastic activity,
  7. diseases of protein deficiency cause deficient protein metabolism altered bone regeneration.


Regulation of Ca level by

  1. PTH synthesis and release of PTH is regulated by the conc of ionized Ca in the plasma; 1)in bone osteoclastic activity, 2)in the intestine Ca uptake is indirectly inc due to the stimulating effect of PTH on the formation of vit D in the kidneys; 3)in the kidneys the reabsorption of of Ca is increased, which is important on account of the rising Ca conc in serum, and also PO4 reabsorption is dec hypophosphatemia which stimulates Ca release from the bone.
  2. Calcitonin In hyper Ca, calcitonin lowers serum conc of Ca by inhibiting osteoclastic activity which is stimulated by PTH.
  3. Vit D where once it is converted into active form in the kidney, it boosts intestinal absroption of Ca into plasma, by mechanisms mentioned previously.
Identify the target tissues of each calcium regulating hormones
  1. PTH bone activates osteoclasts > inc Ca and PO4 resorption intestine - increases Ca abs from food kidney promotes activation of vit D and inc Ca reabsorption, inc PO4 exc
  2. Vit D intestine , boosting Ca & PO4 absorption into plasma, Bone causing mineralization of skeleton, maintains Ca transport system Kidney, placenta, and mammary glands- calcitriol seems to inc transport of Ca and PO4
  3. Calcitonin bone inhibits osteoclastic activity dec Ca & PO4 resorption Kidney dec Ca & PO4 reabsorption
Effects of excess and deficient amounts of
  1. PTH excess - overtaxes ca regulation hyper Ca from excessive osteolysis calcification of kidneys, hypercalciuria, poly uria, nocturia, polysypsia, tiredness, lethargy, with muscle weakness, N/V, constipation. Hypophosphatemia.
    Deficit tetnany (Chvostec & Trosseaus) due to increased sensitivity of Na gates in the presence of low Ca., prolonged QT interval due to prolonged refractory period in hypocalcemia, inability to convert vit D into active form dec Ca & PO4 abs from intestine hypo Ca & hyper PO4, rickets or osteomalacia
  2. Vitamin D excess decalcifies bone, incr GI abs of Ca into plasma supresses PTH inc Ca excretion hypercalsuria. An increase in (vitiman D stores) 25 may displace 1,25 di from its carrier proteins and lead to inc levels of 1,25 di which boosts GI abs hyper Ca.
    Deficit rickets, osteomalacia
  3. Calcitonin excess stimulates PTH secretion, over excretion of Ca over a short peroid of time.
    Deficit ? abnormal bone growth in children, if osteoclastic activity isn't regulated.
  4. Vitamin D deficit - occurs secondary to diet, no sun ( UV lite), malabsorbtion of vit D, or age and these lead to decreased Ca abs to ECF inc PTH (to compensate for dec Ca) inc osteoclastic activity bones demineralize bones soften and break over time rickets & osteomalacia.


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