Calcium and phosphate homeostasis: renal contribution Flashcards
What 3 organ systems are involved with calcium and phosphate reabsorption/secretion?
Intestine
Bone
Kidneys
What 3 things regulate calcium and phosphate movement through these organ systems?
Parathyroid hormone
Calcitriol
Calcitonin
Where is most of the calcium stored in in the body?
Bone
What is bone resorption mediated by?
Osteoclasts
What is bone formation mediated by?
Osteoblasts
How is calcium found in the ICF?
Sequestered in organelles eg smooth ER
Complexed with macromolecules
What is 40% calcium bound to in plasma?
Albumin
Globulin
What happens in acidosis?
Increased H+ displaces Ca2+ from proteins -> increasing plasma conc. of ionized Ca2+
What happens in hypo/hyperalbuminemia?
Hypo - increases ionized Ca2+
Hyper - decrease ionized Ca2+
What two factors does Ca2+ homeostasis depend on?
Total amount of Ca2+ in the body
Distribution between bone and ECF
What cell types are found in the thyroid gland?
Follicular cells and C cells
What do c cells have?
Ca2+ sensing receptors
What do C cells secrete and why?
Calcitonin - reduces Ca2+ release when plasma calcium is high
What is the predominant cell type in parathyroid glands?
Principal (chief) cells
What do principal cells secrete and why?
PTH - promotes Ca2+ release from bone when plasma calcium is low
How does Vitamin D produce calcitriol?
PCT contains alpha 1 hydroxylase which converts 25-Hydroxycholecalciferol (from Vit D) into calcitriol
What does calcitriol cause?
Increased Ca2+ and Pi absorption from small intestine
Increased Ca2+ and Pi release from bone
Decreased Ca2+ and Pi excretion
What are other actions of increased PTH?
Increases calcitriol levels
Acts mainly on bone
How is Ca2+ reabsorbed in the PT?
Paracellular - by solvent drag and due to shift in luminal voltage to +ve
Transcellular - Chaperoned binding to Calbindin
What transporter is expressed on basolateral side of PT?
Ca2+ATPase transporter (PMCa1b)
How does the TAL absorb Ca2+?
Transcellular
Paracellular - no solvent drag
CORTICAL only
How is Ca2+ reabsorbed in DCT and CD?
Hormonally
PTH stimulates Ca2+ opening channels on apical membrane
Where is most of the phosphate stored in the body?
Bone
What 4 forms is phosphate present in the ICF?
Acid (H2PO4-)
Alkaline (HPO4^2-)
Inorganic
Organic - ATP, ADP and cAMP
What 2 ways is phosphate homeostasis dependent on?
Total amount in body
Distribution between ICF and ECF
What are the 3 main regulators of phosphate?
PTH
Calcitriol
Calcitonin
How is phosphate reabsorbed in PT?
Transcellular
Where in the kidneys is negligible amounts of phosphate reabsorbed?
LoH
CD
What transporter is expressed on the apical membrane for phosphate reabsorption in PT?
NPT2
What hormone is Pi reabsorption decreased by?
PTH
