Calcium Metabolism

OVERVIEW

• highly regulated cation
• involved in: cell death, duration and strength of cardiac muscle contraction, muscle contraction in blood vessels, airways and uterus, coagulation, bone metabolism, neurotransmitter and hormone release…
• Ca²⁺ exists in the extracellular plasma two states:
  1. free ionized state and
  2. bound to other molecules (mostly albumin, rest – beta-globulins, phosphate, citrate)
• ionized Ca²⁺ concentration is inversely related to pH -> an increase in pH results in a decrease in ionized Ca²⁺

Calcium levels

• total Ca²⁺ range = 2.2-2.5mmol/L (55% bound, 45% ionized)
• ionized Ca²⁺ range (50%) = 1.1-1.3mmol/L
• protein bound Ca²⁺ range (40%) = 0.95-1.2mmol/L
• complex Ca²⁺ (10% – calcium phosphate, salts) = 0.05mmol/L

CALCIUM METABOLISM

Vitamin D

• group of related sterols
  cholecalciferol is formed in the skin -> in liver to 25-hyrdroxcholecalciferol -> in kidney proximal tubules to 1,25-dihydroxycholecalciferol -> this then helps calcium absorption in the intestine.
• controlled by parathyroid hormone

-> increases intestinal absorption of Ca²⁺
-> increases renal Ca²⁺ reabsorption
-> mobilises bone Ca²⁺ and PO₄^3-

Parathyroid hormone

• secretion increased by hypocalaemia & hypomagnesaemia
• secretion decreased by hypercalcaemia & hypermagnesaemia

-> mobilses Ca²⁺ from bone
-> increases renal Ca²⁺ reabsorption
-> increases renal PO₄^3- excretion
-> increases formation of 1, 25-dihyroxycholecalciferol.

Calcitonin

• antagonist of parathyroid hormone
• secreted by the parafollicular cells of the thyroid gland in response to:
  – hypercalcaemia
  – catecholamines
  – gastrin

-> inhibits the mobilisation of bone Ca²⁺
-> increases renal Ca²⁺ and PO₄^3- excretion

References and Links

• CCC – Calcium metabolism
• CCC – Calcium basics
• CCC – Hypercalcaemia Overview
• CCC – Hypercalcaemia DDx
• CCC – Hypocalcaemia
• CCC – Hypocalcaemia DDx
• CCC – Calcium replacement (advantages/disadvantages)
• CCC – Calcium as an inopressor