Endocrinology: Parathyroid hormones in bone metabolism
Parathyroid glands
-4 small glands paired -ind may have up to 8 glands -close contact with the posterior face of the thyroid gland -produce & secrete parathyroid hormone (PTH)
PTH signaling
- Bones: stimulates Ca2+ release from bone - Kidneys: stimulates the conversion of vitamin D into its active metabolite, which inc renal absorption as well as phosphate excretion -Intestine: through activated vitamin D, PTH increases the intestinal absorption of Ca2+
Vitamin D
-act in synergy with PTH: similar role in Ca2+ homeostasis & bone metabolism, intermediates PTH signaling on intestinal Ca2+ absorption -fat soluble secosteroid hormone promotes osteoclastic bone resorption -osteoid matrix mineralization -intestinal calcium absorption *chronic kidney disease impairs the kidney's ability to activate vitamin D
Ca2+ homeostasis & bone metabolism
-ca2+ is stored in bone tissue as hydroxyapatite -involved in: coagulation, muscle contraction, nervous impulses -blood levels are regulated by: hormone signaling, digestive absorption, bone resorption, kidney resorption
Bone remodeling
-continuously active process -main cell types involved: *osteoblasts (produce bone matrix) *osteoclasts (resorb bone matrix) -intricate equilibrium btwn them -essential in Ca2+ storage & release
Bone metabolism
-lifelong physiological process -dynamic process btwn bone formation & resorption -involved in Ca2+ & PO4 homeostasis
PTH & bone metabolism
-net effect: bone resorption, elevating blood Ca2+ levels -PTH target G-coupled protein receptors on osteoblasts surface -stimulated osteoblastic production of IL-1, IL-6 and M-CSF -indirectly activates and inc # of osteoclasts -note: osteoclasts do not have specific PTH receptors
Regulators of bone metabolism
-parathyroid hormone -calcitonin -vitamin D -others (TSH, GH, sex hormones, cytokines)
Calcitonin
-polypeptide hormone -opposite effect to PTH: inhibits osteoclastic activity -produced by "C" cells of the thyroid gland -secretion regulated by calcium serum levels
Parathyroid hormone (PTH)
-stored in secretory granules -release controlled by Ca2+ serum concentration -crucial for bone metabolism *PTH is the single most important hormone in calcium levels control
What is the immediate source of calcium when parathyroid hormone is released in response to hypocalcemia?
Bone
What hormone has activity that directly opposes the action of parathyroid hormone on bony tissue?
Calcitonin
Endocrine disturbances:
PTH deficiency: -hypocalcemia -tetany -convulsions PTH excess: -hypercalcemia -bone friability and cysts -tissue calcium deposition
Which hormone affects bone metabolism by directly stimulating osteoclast activity?
Parathyroid hormone
A single-chain polypeptide hormone stimulates conversion of vitamin D into its active metabolite. Under normal conditions, what triggers secretion of this chemical?
decrease in serum calcium
What is the major stimulant for the release of parathyroid hormone in the health human body?
hypocalcemia
Parathyroid hormone is stimulating calcium release from bone storage. What type of cell contains the receptor for this chemical?
osteoblast Osteoblasts contain receptors for parathyroid hormone. Osteoblasts activate osteoclastactivity to increase serum calcium levels.
What is the main activity of parathyroid hormone on bone cells?
stimulates osteoclast activity The main goal of parathyroid hormone is to increase calcium, so it stimulates osteoclast activity, which will release this mineral.