Study note
BiochemistryMicronutrients: Vitamins & Minerals

Calcitriol: feedback regulation and maintenance of plasma calcium

THE CONTEXT
Vitamin D controls calcium homeostasis, while its metabolism responds to factors related to plasma calcium and phosphate concentrations.

Feedback regulation

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Calcitriol limits its own synthesis by inducing 24-hydroxylase and repressing 1-hydroxylase in the kidney.

Maintenance of plasma calcium

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Calcitriol increases intestinal absorption of calcium.
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It reduces calcium excretion by stimulating reabsorption in the distal renal tubules.
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It mobilizes bone mineral.

Additional actions

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Calcitriol participates in insulin secretion and in the synthesis and secretion of parathyroid and thyroid hormones.
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It inhibits interleukin production by activated T-lymphocytes and immunoglobulin production by activated B-lymphocytes.
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It participates in differentiation of monocyte precursor cells and modulation of cell proliferation.

Cellular mechanisms

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In most actions, calcitriol behaves like a steroid hormone by binding nuclear receptors and enhancing gene expression.
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It also rapidly mobilizes calcium transporters in the intestinal mucosa.
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