Ca+2

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Introduction

The average adult body contains 1 kg of Ca+2, 99% of which is deposited in bone in the form of hydroxyapatite. The remainder is found in soft tissues & the extracellular space.

extracellular Ca+2
- normal serum concentration of Ca+2 is 9.0-10.5 mg/dL (2.2-2.6 mmol/L) with 50-60% bound to serum protein or complexed with citrate or phosphate
- free or ionized Ca+2 is biologically active
- plasma ionized Ca+2 (~ 1 mmol/L) is regulated under tight control by PTH, 1,25-(OH)2 vit D3, & to a lesser extent calcitonin, varying < 0.1 mg/dL under normal conditions
- a Ca+2 sensing receptor is expressed on parathyroid cells, renal tubular cells & others & serves to monitor ionized Ca+2 concentrations
intracellular Ca+2
- intracellular Ca+2 is present in much lower concentrations than extracellular Ca+2 (2-4 orders of magnitude lower), except for intracellular compartmentalized stores of Ca+2 in the endoplasmic reticulum & mitochondria
- intraneuronal Ca+2 is typically 100 nM (0.1 mM)
dietary Ca+2 & Ca+2 balance:
- normal dietary Ca+2 varies between 400-1500 mg/day
- 150-200 mg is excreted in the urine
- ~500 mg is deposited in & released from bone each day from bone remodeling
- bone serves as a reservoir for extracellular Ca+2 in the face of diminished dietary supply
- diminished intake of Ca+2 stimulates production of PTH by the parathyroid thus enhancing bone resorption, renal tubular reabsorption of Ca+2, & 1,25-(OH)2 vit D3 production by the kidney which in turn stimulates Ca+2 absorption from the intestine
- excess absorbed Ca+2 is excreted in the urine as a result of diminished levels of PTH
- see calcium carbonate for calcium supplementation

↑ Medical Knowledge Self Assessment Program (MKSAP) 11, American College of Physicians, Philadelphia 1998
↑ Prescriber's Letter 14(3): 2007 Calcium and Vitamin D Supplementation: Who Needs It? Detail-Document#: http://prescribersletter.com/(5bhgn1a4ni4cyp2tvybwfh55)/pl/ArticleDD.aspx?li=1&st=1&cs=&s=PRL&pt=3&fpt=25&dd=230304&pb=PRL (subscription needed) http://www.prescribersletter.com
↑ National Cancer Institute: Calcium and Cancer Prevention https://www.cancer.gov/about-cancer/causes-prevention/risk/diet/calcium-fact-sheet
↑ National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) Eating, Diet, & Nutrition for Kidney Stones https://www.niddk.nih.gov/health-information/urologic-diseases/kidney-stones/eating-diet-nutrition