Normal calcium levels must be analyzed in relation to:
A. Sodium
B. Glucose
C. Protein
D. Fats
Correct Answer: C. Protein
Some calcium is bound to protein, so abnormal calcium levels are analyzed in relation to proteins. The ability of the protein to bind calcium acts as a buffer that alters the effect of an acute load of calcium on the concentration of ionized calcium by about 50%. Still another consequence of the large number of unfilled binding sites for calcium is that competition by magnesium does not have a significant effect on ionized calcium concentration.
Option A: PTH also acts to maintain the steady-state level of serum calcium by its action on the kidney. It increases the tubular reabsorption of calcium and magnesium and decreases the tubular reabsorption of phosphate, sodium, bicarbonate, potassium, and amino acids. PTH activates the adenylate cyclase system by binding with receptor sites in the renal cortex. It thus leads to an increase in cyclic adenosine monophosphate.
Option B: Abnormal calcium regulation may contribute to reduced ?-cells function, thereby promoting altered glucose homeostasis. In vitro studies have also found that high cytosolic calcium may contribute to insulin resistance within adipocytes and skeletal muscle.
Option D: The serum phosphorus level also plays a role in the maintenance of a steady-state concentration of serum calcium. While there is no exact solubility product for calcium and phosphorus, a rise in serum phosphate usually leads to a fall in serum calcium. Some of this decrement may be caused by the enhanced formation of CaHPO4 complexes in the serum.
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