Calcitonin is the hypocalcaemic hormone discovered by Copp in 1962. It is a 32 amino acid single chain polypeptide (MW 3600) produced by parafollicular ‘C’ cells of thyroid. Parathyroids, thymus and cells of medullary carcinoma of thyroid also contain calcitonin.
CALCITONIN
Calcitonin is the hypocalcaemic hormone discovered by Copp in
1962. It is a 32 amino acid single chain polypeptide (MW 3600) produced by parafollicular
‘C’ cells of thyroid. Parathyroids, thymus and cells of medullary carcinoma of
thyroid also contain calcitonin.
Synthesis and
secretion of calcitonin is regulated by plasma Ca2+ concentration itself: rise
in plasma Ca2+ increases, while fall in plasma Ca2+ decreases calcitonin
release. However, the physiological role of calcitonin in regulating plasma
Ca2+ appears to be minor. The plasma t½ of calcitonin is 10 min, but its action
lasts for several hours.
Actions
The actions of calcitonin are generally opposite to that of PTH.
It inhibits bone resorption by direct action on osteoclasts—decreasing their
ruffled surface which forms contact with the resorptive pit. Whether it also promotes
calcium deposition by osteoblasts is not certain. The hypocalcaemic action of
calcitonin lasts ~8 hours.
Calcitonin inhibits proximal tubular calcium and phosphate
reabsorption by direct action on kidney. However, hypocalcaemia overrides the
direct action by decreasing the total calcium filtered at the
glomerulus—urinary Ca2+ is actually reduced.
The actions of calcitonin are mediated through a Gprotein
coupled receptor and increase in cAMP formation, but its target cells are
different from that of PTH.
Preparation And Unitage
Synthetic salmon calcitonin is used clinically,
because it is more potent due to slower metabolism. Human calcitonin has also
been produced. 1 IU = 4 μg of standard preparation.
CALSYNAR, ZYCALCIT: Synthetic salmon calcitonin 100 IU/ml amp.
for i.m. or s.c. injection.
Adverse effects experienced are nausea, flushing, tingling of
fingers, bad taste and allergic reaction. By lowering plasma Ca2+ calcitonin
may interfere with the action of digoxin.
Uses
1. Hypercalcaemic states
Hyperparathyroidism, hypervitaminosis D, osteolytic bony metastasis and hypercalcaemia
of malignancy; 4–8 IU/kg i.m. 6–12 hourly only for 2 days. It acts rapidly
within 4 hours, the response peaks at 48 hours and then refractoriness
develops. Calcitonin is a relatively weak hypocalcaemic drug. Therefore, used
only to supplement BPNs initially, because they take 24–48 hours to act.
2. Postmenopausal osteoporosis: 100 IU s.c. or i.m.
daily along with calcium and vit D supplements.
A nasal spray formulation
delivering 200 IU per actuation has become available (MIACALCIN NASAL SPRAY 2200 IU in 2 ml). One spray in one
nostril daily has been shown to increase bone mineral density in menopausal
women. It is less effective than HRT/BPNs. Calcitonin is indicated only when
other drugs cannot be given and in women who are menopausal for at least 5
years with definite evidence of osteoporosis. Rhinitis, epistaxis, nasal
ulceration and headache are the side effects.
3. Paget’s disease 100 U daily or on alternate
days produces improvement for few months. Later, resistance usually develops
due to production of antibodies. Bisphosphonates are preferred; calcitonin may
be used as adjuvant or 2nd line drug.
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