Lecture 18: Calcium and Phosphorus Metabolism 2 - Parathyroid Hormone

Cells of the Parathyroid

chief cells and oxyphil cells

which of the two parathyroid cells is the predominant one?

chief cells

what do chief cells do?

Synthesize and secrete PTH but also cause the degradation of PTH when blood calcium levels are too high.

what do oxyphil cells do?

unknown

what is the function of PTH?

Increases calcium concentration in the blood

what type of hormone is PTH

peptide hormone

when is PTH secreted?

in response to low blood calcium levels

explain the PTH negative feedback loop

Calcium-sensing receptors on parathyroid cells are activated when [Ca2+] is elevated and then cause activation of G-protein coupled cascade. overall you get a net effect of high intracellular Ca2+ --> inhibits the secretion of preformed PTH

is the PTH receptor located on osteoblasts or osteoclasts?

Only present on osteoblasts, NOT osteoclasts

how does PTH stimulate osteoblasts indirectly via osteoclasts?

Because there is no PTH receptor on the osteoclast, but there is on the osteoblast, PTH binds to osteoblast receptors which then communicates to the osteoclasts to break down the bone.

Is PTH interaction with bones direct or indirect?

indirect because there is no PTH receptor on the osteoclast, but there is on the osteoblast

Is PTH interaction with kidney direct or indirect? Why?

diret; PTH binds directly to PTH1 receptors --> increase in cAMP, IP3 and DAG --> increase Calcium renal reabsorption and Phosphate renal excretion

Is PTH interaction with intestines direct or indirect? Why?

indirect; PTH stimulates the renal enzyme that converts vitamin D to its active form but has no direct effects on intestinal transport of either calcium or phosphate.

what is Hypoparathyroidism

parathyroid does not produce enough PTH --> low plasma Ca++ levels

what happens in patients with Hypoparathyroidism

they are unresponsive to PTH

What does calcitonin cause?

inhibits osteoclast function and calcium reabsorption from the kidneys --> decrease in blood Ca2+ levels

How could calcitonin's effects lead to disease when unregulated?

paget's disease: Osteoclast activity is too high and osteoblasts try to build bones to overcompensate but they are not good quality bones ⇒ larger and more fragile bones

What could prevent Paget's Disease?

Inhibit the digestion of bone by encouraging osteoclasts to undergo apoptosis(cell death) thereby slowing bone loss