Diet intestine Stool Calcium pool Kidney Urine Bone mg 825 mg Absorbed 500 mg Secreted 325 mg Formation 280 mg reabsorption 280 mg Reabsorbed mg Filtered mg 175 mg Calcium balance

hormones regulating Ca 2+ balance Parathyroid hormone ( = PTH ) produced in parathyroid gland) increases Ca 2+ reabsorption in the kidney Vitamin D ( = 1,25 - Dihydroxycholecalciferol ) generated from Cholecalciferol in the kidney ● increases Ca 2+ - reabsorption in kidney and intestinum ● deposition of Ca 2+ in the newly formed bone Calcitonin ( = Thyreocalcitonin ) from C cells of thyroid gland ● stimulate Ca 2+ excretion in the kidney

Genesis of D -hormones 25-OH-Cholecalciferol Dehydro -Cholesterin Liver: 1,25-(OH)2-Cholecalciferol = Vitamin D Kidney: Cholecalciferol Skin (+ UV)

Influential factors on Ca ++ handling along a Nephron A: Ca ++ Reabsorption 8% 70% 20% 1% B: Regulation of Ca ++ -Reabsorption I. hormone: Parathormone: increases overall the Ca ++ reabsorption, but strongly in the Henle`s loop and in distal tubule. PTH lowers the reabsorption of NaCl and Water and thus of Ca ++ in proximal tubule. The overall result is a net Increase in Ca++ reabsorption Calcitonin: inhibits Ca ++ reabsorption in Henle`s loop and in distal tubule Calcitriol: increases Ca ++ reabsorption in distal tubule II. other influential factors: Alkalosis: increases the reabsorption in distal tubule and acidosis does the opposite increased Plasma phosphate: induces the release of PTH and thus elevates Ca ++ reabsorption ECV reduction: increases Ca ++ reabsorption and ECV entlargement reduces Ca ++ reabsorption

Ca 2+ -sensing receptor lowers levels of cAMP, which otherwise stimulates Na + /K + /Cl - Cotransporter. Tubule lumen Interstitial space Ca 2+ 2 Ca 2+ Ca 2+ Receptor Paracellular Diffusion Na + 3 Na + K+K+ 2 Arachidonic acid P HETE K+K+ 2 Cl -

Chief cell of parathyroid PTH _ + Plasma Ca 2+ 8,5-10,6 mg/dl Bone Ca 2+ reabsorptionCa 2+ absorption Intestine Tubule Phosphate reabsorption Kidney Conversion of 25-OH-Vitamin D Cu 1,25-(OH) 2 -Vitamin D Tubule Ca 2+ reabsorption _ + + _

Chief cell of parathyroid PTH Plasma Ca 2+ 8,5-10,6 mg/dl Plasma Phosphate Bone Ca 2+ reabsorption Phosphate Intestine Ca 2+ absorption Phosphate kidney Tubule Ca 2+ reabesorption Tubule Phosphate reabsorption Conversion of 25-OH-Vitamin D to 1,25-(OH) 2 -Vitamin D _ _ _