Calcium carbonate saturation in human pancreatic juice: possible role of ductal H+ secretion.

Saturation with calcium carbonate was measured in human pancreatic juice anaerobically collected for diagnostic purposes in 15 patients who were ultimately found not to have pancreatic disease. Bicarbonate, PCO2, proteins, and total and ionized calcium were measured in samples collected every minute during a 20-min period after intravenous administration of secretin (1 U/kg) and, 10 min later, caerulein (75 ng/kg). All samples were supersaturated with calcium carbonate. The first sample (washing-out sample) contained the highest concentrations of ionized calcium (Ca2+) and proteins but was least supersaturated with calcium carbonate. Washing-out samples also differed significantly from samples under caerulein by having a lower pH (7.52 +/- 0.30) and higher PCO2 (103.1 +/- 32.20 mmHg) versus 8.22 +/- 0.15 and 38.4 +/- 4.5 mmHg, respectively. Values of PCO2 increased and bicarbonate concentration decreased linearly with protein concentration in washing-out samples but not under hormonal stimulation. This suggests that a concentration of pancreatic juice occurs in pancreatic ducts in association with luminal H+ secretion and CO2 formation, which results from bicarbonate neutralization. During stimulation, ionized calcium concentration increased with protein concentration without any change of PCO2, so that supersaturation was more pronounced under caerulein than under secretin stimulation. Disturbances of the ductal concentration of pancreatic juice during interdigestive periods could be important for pancreatic stone formation in humans.