Question 4

Outline the role of calcium in the body (70% of marks). Outline the differences between calcium chloride and calcium gluconate solutions (30%of marks).

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College Answer

The question sought an understanding of the diverse roles of calcium. Some candidates spent considerable time in details of one or two roles. Limited marks were awarded for demonstrating knowledge of calcium distribution & homeostasis. Few candidates had a good understanding of the differences between calcium chloride and gluconate.
Syllabus: J1 and J2a
Recommended sources: Textbook of Medical Physiology, Guyton & Hall, Page 462.
Pharmacology and Physiology in Anaesthetic Practice, Stoelting, Page 612

Discussion

The "physiological role" of something is incredibly difficult to "outline" when that something is absolutely central to the processes that define organic life. They may as well ask the candidates to "outline the physiological role of the covalent bond". It is not often that an entire question is given over to the completely fictional belief that everything in the body has a job description, which the worthy candidates can easily articulate and the unworthy wouldn't be able to recognise. Oh well, at least this was transparent - usually the expectation for this sort of definition is hidden inside the college examiner comments, so we don't realise that this is what they wanted all along until they publish the paper. 

Anyway. "The diverse roles of calcium" are:

  • Structural roles
    • Component of the hydroxyapatite matrix
  • Functional roles
    • ​​​​​​​Intracellular second messenger, essential to:
      • Skeletal and smooth muscle contraction/relaxation
      • presynaptic release of neurotransmitter
      • endocrine/exocrine hormone secretion
      • apoptosis
      • mitochondrial energy production
      • cell proliferation
      • cytoskeletal changes which precede phagocytosis
    • Autocrine and paracrine extracellular messenger
    • Co-factor in extracellular enzymatic reactions:
      • Clotting cascade

The chloride and gluconate part of the question is extremely strange, and feels like a throwback to the 1980s when there was a genuine belief among physicians (even in textbooks!) that calcium chloride was somehow pharmacodynamically different to gluconate. More on this topic is presented in the calcium gluconate notes. A condensed version for  this 30% fractional answer would look like this:

  • Pharmaceutical presentation: 
    • calcium chloride is a more concentrated (6.8 mmol/L)
      • Faster to make up an infusion
      • Less instrumentation of ampoules is protective against infection and decreases the risk of drug error.
  • Chemical properties:
    • ​​​​​​​Calcium chloride: 2040 mOsm/kg, pH  5.5-7.5
    • Calcium gluconate: 680 mOsm/kg, pH 6.0-8.2
  • Irritant properties: 

References

Pozzan, Tullio, et al. "Molecular and cellular physiology of intracellular calcium stores." Physiological reviews 74.3 (1994): 595-636.

Gunter, Thomas E., et al. "Calcium and mitochondria.FEBS letters 567.1 (2004): 96-102.

Whitfield, J. F., et al. "The roles of calcium and cyclic AMP in cell proliferation." Annals of the New York Academy of Sciences 339.1 (1980): 216-240.

Ebashi, S., and Mi Endo. "Calcium and muscle contraction." Progress in biophysics and molecular biology 18 (1968): 123-183.

Nunes, Paula, and Nicolas Demaurex. "The role of calcium signaling in phagocytosis." Journal of leukocyte biology 88.1 (2010): 57-68.

Heining, M. P. D., D. M. Band, and R. A. F. Linton. "Choice of calcium salt: A comparison of the effects of calcium chloride and gluconate on plasma ionized calcium." Anaesthesia 39.11 (1984): 1079-1082.