Influence of pancreatic ducts on saturation of juice with calcium carbonate in dogs

In several species, bicarbonate and calcium concentrations of pancreatic juice are know to vary during the different phases of pancreatic secretion. The effects of these variations on the saturation of juice with calcium carbonate, a critical factor for the formation of pancreatic stones, are not known. In this work, we studied the saturation degree of pancreatic juice with calcium carbonate in six unanesthetized beagle dogs equipped with Thomas cannulae during basal secretion and after bolus injections of cerulein (30 ng/kg) or secretin (0.25 units/kg). In the different samples of pure pancreatic juice, pH, $P_{CO_2 } $ , bicarbonate, and proteins were measured by standard methods. Total calcium (Ca^T) and ionized calcium (Ca²⁺) were determined using calcium-specific electrodes. Saturation with calcium carbonate was calculated by reference to the solubility product of calcite at 37° C. Almost all the samples were found to be supersaturated with calcium carbonate but large variations of the saturation index were observed. In basal samples, obtained during periods of low secretion rate, the mean saturation index (3.35±3.01) was significantly lower than under secretin (12.10±5.14) or cerulein (18.01±8.42). This low basal saturation index, in spite of a high Ca²⁺ content, was explained by a low bicarbonate concentration (37.6±18.9 mmol/liter) and a high $P_{CO_2 } $ (13.4±7.5 kPa). In contrast, in juice obtained after hormonal stimulation, $P_{CO_2 } $ (4.8±1.6 kPa) was similar to plasma $P_{CO_2 } $ (5.5±1.2 kPa). A high $P_{CO_2 } $ and low HCO₃ ⁻ concentration in basal juice are consistent with an intraductal protonation of bicarbonate. These results suggest that, when the residence time of the juice within the ducts increases, H⁺ ductal secretion, in addition to the previously established HCO₃ ⁻ resorption contributes to decrease the risk of pancreatic stone formation. In spite of this protective action of pancreatic ducts, however, the stored pancreatic juice remained thermodynamically at risk to precipitation of calcium stones.