A Textbook of Clinical Pharmacology and Therapeutics

298 CALCIUM METABOLISM

1. Calcium and ergocalciferoltablets provide a
   physiological dose of vitamin D. They are used in the
   prophylaxis of rickets and osteomalacia. The small dose of
   calcium is unnecessary, but a preparation of vitamin D
   alone is not available.
   2.Calciferoltablets provide a pharmacological dose of
   vitamin D and are used for treatment of hypo-
   parathyroidism and in cases of vitamin D-resistant
   rickets due to intestinal malabsorption or chronic liver
   disease.
   3.α-Calcidol rapidly undergoes hydroxylation to 1,25-
   DHCC. It is used in:
   
   
   • renal rickets, together with a phosphate-binding
     agent;
   
   
   • hypoparathyroidism and (paradoxically) secondary
     hyperparathyroidism;
   
   
   • vitamin D-resistant rickets;
   
   
   • nutritional and malabsorptive rickets can be treated
     with small doses of α-calcidol instead of conventional
     vitamin D.
     4.Calcitriol(1,25-DHCC) is also available for the treatment
     of vitamin D-resistant rickets and is the treatment of
     choice for pseudohypoparathyroidism (an uncommon
     metabolic disorder where low plasma calcium is caused
     by resistance to the biochemical action of PTH).
   
   
   
   

Adverse effects

Hypercalcaemia, which can accelerate renal dysfunction, is the
main problem. Regular plasma calcium and creatinine meas-
urements (weekly initially) are essential.

CALCIUM

Calcium salts (lactate or gluconate) are used in conjunction

withcalciferolin the treatment of rickets and osteomalacia, and

in hypocalcaemic tetany. Calcium chloride (i.v.) is uniquely

Diet Ultraviolet light on skin

Vitamins D 2 and D 3

(inactive)

Vitamins D 3

(inactive)

7-dehydrocholesterol

Liver

25-hydroxycholecalciferol

(major circulating metabolite but inactive)

Kidney

Low plasma
calcium and
high phosphate

Normal plasma calcium

and phosphate

1,25-dihydroxycholecalciferol

(1,25-DHCC) – powerfully

active hormone

24,25-dihydroxycholecalciferol –

weak activity

PTH

()

Figure 39.1:Metabolic pathway of vitamin D.

PTH, parathormone.

Key points

Vitamin D and calcium metabolism

• Plasma calcium concentrations are tightly controlled by
  the balance of hypocalcaemic effects of calcitonin and
  hypercalcaemic effects of PTH and vitamin D and Ca^2 
  intake.


• Vitamin D is available in a number of forms, many of
  which are derived from each other by sequential
  metabolism in the skin, liver and kidney, and each of
  which has specific indications.


• The most potent and rapid-acting orally available
  vitamin D preparations are 1,25 dihydroxy-
  cholecalciferol, and 1-α-hydroxycholecalciferol. They
  are used in renal rickets or vitamin D-resistant rickets.


• When patients are hypocalcaemic, calcium can be
  supplemented orally as calcium carbonate with or
  without various preparations of vitamin D. If urgent
  calcium replacement is required, a 10% solution of
  calcium lactate or gluconate (the former yielding
  more calcium) may be administered intravenously.


• Patients who are receiving vitamin D plus calcium
  should have periodic checks of their serum Ca^2 and
  creatinine concentrations, as the major adverse effect is
  hypercalcaemia.