REVIEW - Calcium homeostasis: reassessment of the actions of parathyroid hormone

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Gen Comp Endocrinol. 2007 Nov 12 [Epub ahead of print]

Calcium homeostasis: Reassessment of the actions of parathyroid hormone.

Talmage RV, Mobley HT.

Department of Orthopaedics, UNC School of Medicine, Chapel Hill, NC, USA.

The purpose of this report is to adjust our interpretation of the

actions of parathyroid hormone (PTH) to include its action on the bone

mineral-noncollagenous protein interactions at all bone surfaces. The

three primary areas that respond to PTH are: (1) all bone surface

areas in contact with the extracellular fluid (ECF), (2) the kidney,

and indirectly the intestinal tract, and (3) the bone remodeling

sequence. The primary rapid action of the hormone is to set and

maintain the free calcium concentration of the ECF. This it does by

raising the equilibrium level at bone mineral surfaces. It affects the

noncollagenous protein bone mineral process to raise the free calcium

level in the ECF from the base level of 3.5mg/100ml to the

physiological level of 5.0mg/100ml. Maintaining the higher level

requires continuous secretion of parathyroid hormone. The action of

PTH at bone surfaces tends to be catabolic in nature in regard to bone

loss. The hormone also acts on the kidney to raise the threshold for

calcium reabsorption and by stimulation of renal hydroxylation of

vitamin D to increase intestinal absorption of calcium. Its action

here is to supply the ECF with calcium derived from food intake. This

is the extent of PTH action on renal processes. PTH acts on all three

steps in the bone remodeling process. While its total function here is

not clear, the result is a net increase in the synthesis of collagen.

The report concludes by comparing the actions of PTH as proposed here

to the functions of PTH that have been proposed in the past.

PMID: 18160068

http://www.ncbi.nlm.nih.gov/pubmed/18160068

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