Parathyroid Hormone (PTH) and calcitriol (the active form of vitamin D) help to
ID: 3523118 • Letter: P
Question
Parathyroid Hormone (PTH) and calcitriol (the active form of vitamin D) help to ensure enough calcium in the blood.
Calcitriol does so by making sure enough Ca2+ is absorbed from the diet. In order for Ca2+ to be absorbed, cells lining the small intestine need Ca2+ transporters. Calcitriol is a steroid hormone.
1. Propose a mechanism for how calcitriol works to increase Ca2+ absorption from the diet.
One way that PTH works is by ensuring kidneys don't allow Ca2+ to leave via urine. In order to do this, kidney cells need Ca2+ transporters. PTH is a peptide.
2. Propose a mechanism for how PTH works at kidney cells. (hint: PTH uses the cAMP system)
3. When you take vitamin D as a supplement, it is in the form of inactive vitamin D3. Given all of this, does it matter if you take V-D and calcium at the same time? Explain.
Explanation / Answer
1. Calcium is absorbed from the food in the intestine employing both active as well as passive transport mechanisms. The active sytsem is particularly regulated by the biologically active form of Vitamin D, i.e. Calcitriol. The mechanism of action of calcitriol involves activation of vitamin D receptors (VDRs) by it, followed by VDR transcriptional activity by binding to corresponding DNA binding sites called Vitamin D response elements (VDREs).
2. PTH stimulates the following changes in the renal cells: a) activation of membrane bound adenylate cyclase enzyme so as to result in an increase in cyclic AMP levels in the cells, b) variation in the distribution of Calcium ions in the cells, c) alteration in levels of certain intermediary metabolites and stimulation of renal gluconeogenesis. Thus, PTH increases Calcium ions entry into the renal cells by its direct action on plasma membrane and the cAMP thus produced affects the transfer of Calcium ions from mitochondria to cell cytosol.
3. It is generally not advisable to take both Vitamin D and Calcium at the same time in high doses. The amounts of the two have to be regulated depending upon the bone density and other factors. Excess doses may not solve the underlying problem, albeit it would result in other problems like kidney stones etc.
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