Parathyroid hormone and Vitamin D

How does PTH regulate gut calcium/phosphate reabsorption? (vitamin D)

Vitamin D helps the intestines to absorb calcium, but it must be activated by PTH before it can do so. So PTH levels regulate the amount of active vitamin D in the body, therefore affecting Ca reabsorption at the gut.

How are calcium levels regulated within a tight reference range?

An increase in serum Ca (in response to PTH secretion) is picked up by the Ca sensing receptors in parathyroid glands. This stops PTH secretion and therefore serum Ca levels within a tight reference range. Negative feedback loop.

How are serum calcium levels monitored by the parathyroid glands?

Calcium sensing receptors in parathyroid glands pick up falling/rising levels of calcium in serum.

Food sources of vitamin D

Fish, milk, eggs.

Why is PTH released?

In response to falling calcium levels within reference range.

Describe role of PTH in calcium metabolism

Regulates serum calcium through its effects on bone, kidneys and gut. Overall acts to increase calcium levels. Stimulates bone resorption by osteoclasts to mobilise bone calcium. Doesn't act upon osteoclasts directly - instead activates osteoblasts which in turn stimulate osteoclasts. Increases renal reabsorption of calcium from distal convoluted tubules and collecting ducts. Increases intestinal reabsorption of calcium by activating vitamin D, which increases Ca+ absorption from the gut.

Describe role of PTH in phosphate metabolism

Regulates serum phosphate through its effects on bone, kidneys and gut. Overall acts to decrease phosphate levels. Increases bone resorption to mobilise bone phosphate. But less phosphate released than calcium. Reduces renal reabsorption of phosphate from the proximal convoluted tubule, so more is excreted in the urine. Increases intestinal reabsorption of phosphate by activation of vitamin D.

Conversion of biologically inactive vitamin D to its active form

Skin = sunlight photoisomerises 7-dehydroxycholesterol found in the skin. Gut = biologically inactive vitamin D3 from food. Liver = inactive D3 (cholecalciferol) converted to calcidiol 25(OH)D, mediated by 25-hydroxylase. Kidney = calcidiol converted to biologically active calitriol 1,25(OH)2D, rate limited by 1 a-hydroxylase.

Effects of calcitriol production at kidney - what does calcitriol do?

1,25(OH)2D: Increases bone resorption releasing bone calcium. Increases absorption of dietary calcium (INTESTINES) and phosphorus.

What is the enzyme limiting the rate of conversion to active calcitriol?

1-a-hydroxylase

What are the 3 main effects of a rise in PTH?

1. Vitamin D converted to active Calitriol (vit D3). This then allows Ca reabsorption from intestines. 2. More Ca is reabsorbed at the renal tubules. 3. More Ca is released from bone because PTH stimulates osteoblasts which in turn activates osteoclasts.