T3 Calcium Flashcards
What occurs with an increase in calcium?
causes progressive depression of the nervous system
What occurs with a decrease in calcium?
causes the nervous system to become more excited.
What types of cells are sensitive to changes in calcium ion concentrations?
excitable cells, like neurons
Where is 85% of the body’s phosphate stored?
bones
What are the major changes when the level of phosphate in the EC fluid changes?
none
Why does the membrane of neurons become more excitable during times of hypocalcemia?
increased neuronal membrane permeability to sodium ions
Hypocalcemia may cause ___________, which ordinarily occurs when the blood concentration of Ca2+ falls from 9.5 to 6 mg/dl (35% lower than normal).
tetany
How much of the daily percentage of calcium is excreted?
90%, or 250 mg/day
Renal phosphate excretion is controlled by what mechanism?
over-flow mechanism
What hormone can greatly increase phosphate excretion?
PTH
What is the normal level of calcium?*
9.5 mg/dl
What is the lethal level of calcium?*
4 mg/dl
What are the intake rates of calcium and phosphorus daily?
1000 mg/day
What cofactor promotes calcium absorption in the intestines?
Vitamin D
What is an inhibitor that does not allow hydroxyapatite precipitation?
pyrophosphate
Where are hydroxyapatite inhibitors?
all tissues and plasma, to prevent precipitation
The precipitation of __________ along the collagen fibers eventually forms hydroxyapatite crystals.
calcium
Under abnormal conditions, calcium salts do not precipitate. When does this occur?
when the inhibitor factors that normally prevent deposition of calcium salts disappear from the tissues
What form of calcium can be absorbed rapidly when there is a need for extra calcium in the extracellular fluid?
amorphous state allows for rapid reabsorption
If you inject calcium salts intravenously, what is the effect?
calcium ion concentration may increase immediately to high levels, but within 30-60 minutes the concentration returns to normal. same if you remove large quantities of calcium
What is the function of vitamin D?
causes intestines, kidneys, and bones to increase absorption of calcium and phosphate into the extracellular fluid and contribute to feedback regulation of these substances
Where are vitamin D receptors?
in the nucleus of target cells; has a DNA and hormone binding domain
What is calbindin? Where is it found?
A calcium binding protein; found in the intestinal epithelia
What is the function of calbindin?*
functions in the brush border of the epithelial cells to transport calcium into the cell cytoplasm
What two things are formed that promote calcium absorption by vitamin D?
a calcium stimulated ATPase in the brush border and an alkaline phosphatase in the epithelial cells
High levels of vitamin D cause what effect?
absorption of bone
Low levels of vitamin D cause what effect?
the effect of PTH in the absence of vit D causing bone absorption is greatly reduced or even prevented
Small quantities of vit D cause bone ________. How?
calcification; by increasing calcium and phosphate absorption from the intestines.
What is the result of excess PTH?
absorption of calcium salts from the bone, and causes hypercalcemia in the EC fluid.
What is the result of hypofunction of PTH?
hypocalcemia, which also results in tetany
PTH is first synthesized as what?
preprohormone
Once PTH is cleaved to an active form, what happens?
the hormone is packaged in granules
Are parathyroid glands vascular or avascular?
vascular
Where is the synthesis of PTH?
ER and Golgi
What is the effect of PTH on bone causing absorption of calcium and phosphate?
initially its a rapid absorption of calcium and phosphate, then a slower phase that promotes the proliferation of osteoclasts, increasing the reabsorption of the bone itself, not just the CaP crystals
The cell membranes of both the osteoblasts and osteocytes have receptor proteins for binding _____.
PTH
Do osteoclasts have receptors for PTH?
No; osteocytes have receptors for PTH and signal the osteoclasts. the secondary signal is osteoprotegerin ligand.
What are the two stages of activation of the osteoclastic system?
- immediate activation of the osteoclasts that are already formed
- formation of new osteoclasts
Administration of PTH has what effect on phosphate?
rapid loss of phosphate in the urine
Administration of PTH has what effect on calcium?
increases renal tubular reabsorption of calcium, in the late distal tubules, collecting ducts, and maybe ascending loop of henle. this diminishes phosphate reabsorption and increases the rate of magnesium reabsorption, and decreases reabsorption of NA, K, and aa ions
Without the effect of PTH on the kidneys to increase calcium, what would happen?
urine would eventually deplete both EC fluid and bones of calcium
What conditions decrease calcium ion concentration?
rickets, pregnancy, and lactation
What are conditions that increase calcium concentration above normal?
- excess ca in diet
- increased vit D in diet
- bone absorption caused by other factors such as disease
What kind of hormone is calcitonin? from where is it secreted?
peptide hormone; thyroid gland
What is the effect of calcitonin?
decrease plasma calcium contration.
What synthesizes calcitonin?
parafollicular/C cells
What occurs in Paget’s disease?
osteoclastic activity is greatly accelerated, and calcium has a more potent effect of reducing calcium reabsorption
An initial reduction if the calcium ion concentration caused by __________ leads within hours to a powerful stimulation of ______ secretion.
calcitonin; PTH
What are two effects of hypoparathyroidism?
- osteoclasts become totally inactive
2. ca reabsorption from the bone is so depressed that blood ca levels decrease.
What is primary hyperparathyroidism?
usually from a tumor, causes inappropriate excess PTH secretions. more prevalent in women because pregnancy and lactation stimulate the parathyroid glands, and predispose to the development of such a tumor
What is secondary hyperparathyroidism?
- high levels of PTH occur as a compensation for hypocalcemia
- can be caused by a vit D deficiency, which can lead to osteomalacia
