Calcium Balance (Lecture 23)

Question 1

List the Effects of Hypercalcemia

Answer 1

• Excitable cells, such as neurons, are sensitive to changes in calcium ion concentrations
  
  • increases in calcium ion above normal cause progressive depression of the nervous system
    
    • symptoms begin to appear when the blood calcium level rises above 12 mg/dl
    • Reflex activities of the nervous system are sluggish
    • There is constipation and lack of appetite

Question 2

• Where is calcium stored in the body
• Describe calcium distribution and storage forms

Answer 2

• 0.1 percent of the total body calcium is in the extracellular fluid
• 1 percent of the total body calcium is in the cells and organelles
• The rest of the total body calcium is stored in the bones

Question 3

Compare the effects of changes in the levels of phosphate in the extracellular fluid to that of calcium in the extracellular fluid

Answer 3

• not nearly as well regulated as that of calcium
• Large changes in the level of phosphate in the extracellular fluid do not cause major immediate effects on the body
• but even slight changes in extraellular calcium can cause extreeme immediate physiological effects

Question 4

Explain how calcium and phosphate are absorbed and excreted

Answer 4

• usual rates of intake are about 1000 mg/day each of calcium and phosphorous
• Normally, divalent ions are poorly absorbed, but Vitamen D promotes calcium absorption by the intestines, so that 35% of ingested calcium is absorbed
• Calcium not absorbed is excreted in the feces
  
  • About 250mg/Day of the absorbed calcium enters intestines via secreted GI juices and sloughed mucosal cells
  • Thus, about 90 percent of the daily intake of calcium is excreted
• Renal excretion
  
  • about 100 mg/day (10 percent) of the ingested calcium is excreted in the urine
  • The 41 percent of the plasma calcium bound to plasma proteins is not filtered by glomerular capillaries
    
    • the rest is combined with anions or ionized and filtered through the glomeruli
  • The renal tubules reabsorb about 99 percent of the calcium in the filtrate
    
    • Renal phosphate excretion is controlled by an over-flow mechanism
    • PTH can greatly increase phosphate excretion

Question 5

Describe the relationship between calcium and phosphate and bone

Answer 5

• Calcium salts are deposied in an amorphous (noncrystalline) form
• These salts are converted into the hydroxyapatite crystals over a period of weeks or months.

Question 6

Explain why hypoxyapatite crystals do not precipitate in normal tissues

Answer 6

Inhibitors are present, such as pyrophosphate, to prevent precipitation

Question 7

What is exchangeable calcium

Answer 7

• Exchangeable calcium is found in the bone and a small portion all tissues, expeceially in highly permeable types of cells such as those of the liver and GI tract
• provides a rapid buffering mechanism
  
  • allows to compencate for rapid increase of calcium (returns to normal in 30-60 minutes)
  • Also allows for compensation if large quantities of calcium are removed

Question 8

List the three hormones that control calcium balance

Answer 8

• Vitamin D
• Parathyroid Hormone
• Calcitonin

Question 9

What is the active form of Vitamin D

Answer 9

1,25 dihydroxycholecalciferol

Question 10

Where are most Vitamin D receptors found

Answer 10

• present in most cells in the body and are located mainly in the nuclei of target cells
  *

Question 11

Diagram the synthetic pathway of Vitamin D and explain the involvement of parathyroid hormone

Question 12

Describe the mechanism by which vitamin D promotes intestinal absorption of calcium

Answer 12

• 1,25 dihydroxycholecalciferol itself promotes intestinal absorption of calcium:
  
  • By increasing, over a period of about 2 days, formation of calbindin, a calcium-binding protein, in the intestinal epithelial cells
    
    • This protein functions in the brush border of the epithelial cells to transport calcium into the cell cytoplasm
  • Then the calcium moves through the basolateral membrane of the cell by facilitated diffusion
  • Calbindin remains in the cell for weeks after the 1,25-dihydroxycholecalciferol has been removed from the body
• Also promotes calcium absorption through the formation of
  
  • A calcium-stimulated ATPase in the brush border of epithelial cells
  • An alkaline phosphatase in the epithelial cells
    *

Question 13

Describe and explain the effects of administering small quantities of vitamin D versus extreme quantities of Vitamin D

Answer 13

• Administration of extreme quantities of Vitamin D
  
  • Causes absorption of bone
  • In the absence of vitamin D, the effect of PTH in causing bone absorption is greatly reduced or even prevented
  • The mechanism of this action of vitamin D is believed to result from the effect of 1,25-dihydroxycholecalciferol to increase calcium transport through cellular membranes
• Administration of smaller quantities of Vitamin D
  
  • By increasing calcium and phosphate absorption from the intestines

Question 14

Describe and compare hyperfunction and hypofunction of parathyroid glands

Answer 14

• Hyperfunction of parathyroid glands
  
  • Results in hypercalcemia in the extracellular fluid
• Hypofunction of parathyroid glands
  
  • Causes hypocalcemia, often resulting in tetany

Question 15

List the effects of PTH on osteoclasts

Answer 15

• Osteoclasts do not have membrane receptors for PTH
  
  • believed to be activated by secondary signals from osteoblasts and osteocytes
• activation of the osteoclastic system occurs in two stages:
  
  • immediate activation of the osteoclasts that are already formed
  • formation of new osteoclasts

Question 16

What effect does PTH have on the kidneys with regard to calcium ion concentration

Answer 16

• Causes rapid loss of phosphate in the urine
  
  • due to the effect of the hormone to diminish proximal tubular reabsorption of phosphate ion
• increases renal tubular reabsorption of calcium
  
  • occurs mainly in the late distal tubules, the collecting tubules, the early collecting ducts, and possibly the ascending loop of Henle to a lesser extent

Question 17

List conditions that increase calcium ion concentration

Answer 17

• Excess quantities of calcium in the diet
• Increased Vitamin D in the diet
• Bone absorption caused by other factors such as disease

Question 18

Compare primary and secondary hyperparathyroidism

Answer 18

• Primary:
  
  • An abnormality (usually a tumor) of the parathyroid glands causes inappropriate, excess PTH secretions
  • This is more prevalent in women because pregnany and lactation stimulate the parathyroid glands and predispose to the development of such a tumor
• Secondary:
  
  • High levels of PTH occur as compensation for hypocalcemia
  • Can be caused by Vitamin D deficiency, which can lead to osteomalacia

Question 19

List causes and characteristics of rickets and osteoporosis

Answer 19

• Rickets
  
  • results from calcium or phosphate deficiency in the extracellular fluid, usually caused by lack of Vitamin D
  • characteristics
    
    • weak bones
    • tetany
• Osteoporosis
  
  • Decreased bone matrix