Endocrinology 3

Endocrinology 3 - Endocrinology Test 3 Calcium homeostasis...

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Endocrinology Test 3 Calcium homeostasis All physiological processes in body will directly or indirectly involve calcium ions For optimum physiological response, blood calcium levels are maintained between 9mg/100cc – 11mg/100cc of blood Total calcium levels in body = 1200 grams = 1.2kg o 99% stored in bone tissue as calcium phosphate crystals called hydroxyappetites o 1% in intracellular fluid and some in extracellular fluid Ionic calcium (Ca 2+ ) is involved in physiological processes Below 9mg/100cc = hypocalcemia Above 11mg/100cc = hypercalcemia o To maintain normal levels of calcium the recommended daily calcium intake is 1000mg -1200mg Three hormones to regulate calcium levels 1. Parathyroid hormone (PTH) 2. 1,25 dihydroxyvitamin D (1,25 (OH) 2 Vit D) 3. Calcitonin PTH 84 AA Hormone Stimuli for release of PTH from parathyroid glands (located on posterior aspect of thyroid gland – embedded in thyroid gland) o Hypocalcemia – blood calcium levels below 9mg % o Hyperphosphatemia – high levels of phosphate in blood o The release of PTH occurs in presence of magnesium hence severe hypomagnesemia is associated with hypoparathyroidism Biological actions of PTH Binds to PTH-R to activate dissociation of G alpha (camp dependent signal transduction) 1. Target cells for PTH in bone tissue are osteoblasts in bone tissue, osteoblasts stimulate bone formation, osteoclasts stimulate bone resorption (breakdown) o No PTH-R on osteoclast (Weird b/c that’s where we’d expect to bind o Osteoblast stimulated by PTH will release osteoclast activating factors (OAF) (PTH has indirect effect) such as interleukin-6 (IL6) o OAF then stimulates osteoclast to causes bone resorption – calcium phosphate salt is released from bone tissue into blood 2. Stimulate phosphate excretion from blood, leaving ionic calcium to raise levels in blood back to normal 3. PTH stimulates reabsorption of calcium from tubular fluid in kidneys and put back into blood
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4. PTH does not have access to intestinal cells to cause calcium absorption from ingested food o Action on intestinal cells considered indirect b/c PTH stimulates production of active form of Vit-D called 1,25 (OH) 2 Vit D (could be 3 or 2) o 1,25 (OH) 2 Vit D is only hormone that can directly stimulate calcium absorption from small intestine Synthesis of Vitamin D Most active form is 1,25 dihydroxyvitamin D3 Synthesis starts in the epidermis or ingested in fortified foods 1,25 dihydroxy Vitamin D Synthesis in the Skin 7 dehydrocholesterol (UV light changes it to Vitamin D3) Vitamin D3 25 hydroxyvitamin D3 (by 25 hydroxylase in the liver) 1,25 dihydroxyvitamin D3 (by 1 alpha hydroxylase) 1 alpha hydroxylase enzyme is activated by PTH, increased GH (acromegaly) Synthesis of 1,25 dihydroxyvitamin D2 Starts in the ingested fortified foods Vitamin D2 25 hydrovitamin D2 1,25 hydroxyvitamin D2 (SAME ENZYMES) 1,25 (OH 2 ) VitaminD2 or D3 is a steroid hormone and it binds to Vitamin D receptors=
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This document was uploaded on 10/27/2011 for the course BSCI 447 at Maryland.

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Endocrinology 3 - Endocrinology Test 3 Calcium homeostasis...

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