The effect of the bicarbonate anion on serum ionized calcium concentration in vitro

The effect of changes in bicarbonate ion concentration on calcium ion concentration was examined in vitro in serum and protein-free solution. The findings in this study support the formation of a calcium-bicarbonate complex (CaHCO3+) that has a KA of 5.20 in protein-free solution. [Ca++] varied inversely with [HCO3-] in both serum and protein-free solution. This variation was independent of the known variation of [Ca++] with pH. In serum [Ca++] varied 0.0036 mM Ca++ per 1 mM change in [HCO3-] compared with a variation of 0.0060 mM Ca++ per 0.01 unit change in pH. Addition of bicarbonate to serum (Pco2 constant) produced a 50% greater decrease in [Ca++] than that produced by a reduction in Pco2 which gave the same pH change. These findings indicate that abnormal bicarbonate concentrations should be considered when ionized calcium concentrations are estimated from total plasma calcium values in acid-base disorders. In metabolic acid-base disorders, the bicarbonate effect adds to the pH effect on calcium ion concentration. In compensated respiratory acid-base disorders, the bicarbonate effect subtracts from the pH effect on calcium ion concentration.