Pancreatic calcification: formation constants of CaHCO3+ and CaC03(0) complexes determined with Ca2+ electrode

Pancreatic calcification is a frequent complication of chronic pancreatitis, and pancreatic stones (95% CaCO3 as calcite) are observed in both humans and cattle, but little is known about the complex equilibriums governing calcium solubility. Using the Ca2+ electrode and equilibrium dialysis of NaHCO3-CaCl2-NaCl solutions (24 +/- 2 degrees C; total ionic strength = 0.153-0.161), studies were made at variable pH and total calcium and carbonate concentrations to determine the formation constants of the three possible calcium complexes: CaHCO3+, CaC03(0), and Ca(HCO3)20. If the first two complexes were present, a plot of ([Ca]/[Ca2+]-1)/[HCO3-] against the ionic ratio [C03(2)-]/[HCO3-] should be linear with intercept of the CaHCO3+ formation constant (K'aB) and slope of the CaC03(0) formation constant (K'aC). This was found to be the case in both dialysands and dialysates, using two different methods ("pH" and "K'sp") for estimation of [CO32-] values. Two other methods of data analysis were also used: simultaneous equations and multiple linear regression by matrix analysis. For all three methods, mean K'aB = 12.48 and mean K'aC = 1,870. There was no evidence for a Ca(HCO3)2(0) complex. We conclude that CaHCO3+ and CaC03(0) complexes may account for a substantial fraction of total soluble calcium is pancreatic juice. These studies provide a necessary step toward the construction of a quantitative physicochemical model of pancreatic calcium solubility.