Parathyroid Gland

Question 1

Describe the normal functions of calcium & phos.

Answer 1

1. Calcium
   -muscle contraction
   -nerve cell activity
   -release of hormone by exocytosis
   99% in bones
2. Phosphorus
   -bone mineralization
   -cell functions
   85% in bones, 14% intracellular

Question 2

Describe the 2 types of cells in the parathyroid.

Answer 2

1. Active cells = chief cells
   -makes PTH
2. Inactive/degen cells = oxyphil cells

Question 3

Describe chief cells.

Answer 3

-sensitive to decline in iCa
-calcium sensing receptor on membrane surface (GPCR)
-PTH secretion controlled by iCa
>decrease in iCa activates receptor
>normal Ca levels = inactive receptor

Question 4

Describe PTH synthesis.

Answer 4

-after stim, PTH made as other protein hormones
-preproPTH syn in ER & cleaved to proPTH
-‘pro’ removed in by GA & PTH is secreted by exocytosis
-metabolized by kidney/liver
-half life is 5-10 min

Question 5

Describe PTH actions.

Answer 5

-increase Ca conc in ECF
-decrease phos conc in ECF
-direct effect on bone & kidney metabolism of calcium
-indirect effect on GI metabolism of Ca

Question 6

Describe the direct effects of PTH on bone.

Answer 6

1. Bone
   -bind to receptor on osteocytes & stim osteocytic osteolysis
   >(osteocytes pump Ca from fluids within bone canaliculi into interstitial fluid & blood)
   -bind to receptor on bone osteoblast cells & stim prod of osteoclast-activating factor
   >causes activation of nearby osteoclasts
   —digest organic matrix in bone & release Ca & P

Question 7

Describe the direct effects of PTH on kidney.

Answer 7

-increase absorption of Ca in distal convoluted tubules
-decrease reabsorption of phos in prox tubules of kidneys
-activate VITD by kidneys
-binds to receptors on prox renal tubules that stim enzymes (1 alpha hydroxylase enzyme only active when PTH acted on cell)

VIT D only active when PTH action on cells

Question 8

Describe the indirect effect of PTH on GI.

Answer 8

-calcitriol stim active transport of dietary Ca across intestinal epi
-except horse & rabbit bc hindgut fermenters (absorb Ca all the time & regulate levels via urine)

Question 9

Describe calcitonin.

Answer 9

-made by Parafollicular cells / ‘C cells’
-syn as other protein hormones
-acts as counterbalance to PTH
-regulated by Ca (increased Ca = calcitonin secretion)
-GI hormones stim secretion of calcitonin

Question 10

Describe calcitonin actions.

Answer 10

-decreased movement of Ca in bone
-decrease bone reabsorption thru inhibitory effect on osteoclasts
-increase movement of phos from ECF into bone
-increase renal excretion of Ca

Question 11

Describe thyroid related Ca disorders.

Answer 11

1. Hyperparathyroidism:
   -parathyroid makes a lot of PTH = high Ca in blood
   -adenoma or parathyroid hormone secreting tumor
   -parathyroid glands may become enlarged bc hyperplasia or parathyroid or chief cells
2. Hypoparathyroidism:
   -metabolic disorder = low Ca bc low PTH
3. Thyroid parafollicular cell dysfunction:
   -C cell prod of calcitonin impaired bc high Ca

Question 12

Describe hypercalcemia.

Answer 12

increased Ca levels
CS:
-PU/PD (urinary continence)
>impaired resp of distal renal tubules to ADG
>hypercalcemia interferes w ADH = increase aquaporin expression & water reabsorption
-depression & muscle weakness
>decrease in membrane Na permeability/inhibit depol
-mild GI signs
>inappetence, weight loss, decrease GI motility bc decrease in excitability of smooth muscle cells
-Ca urolits
>forms precipitation of Ca phos
-renal failure
>mineralization of renal tissue

Question 13

Describe hypocalcemia.

Answer 13

low Ca levels
-anorexia, facial rubbing, nervousness, stiff gait
-paresthesias, hyperventilation
-tetany & seizures
>abnormally excited bc hypocalcemia increases membrane Na permeability & depol