Effect of arterial pH and PCO2 on biliary HCO3- secretion in the pig

The purpose of the present study was to examine the effect of changes in arterial pH and PCO2 on biliary HCO3- secretion. This was done in order to further characterize the various ion transport mechanisms considered responsible for biliary HCO3- secretion in the pig. Experiments were performed on two groups of six pigs. In both groups arterial pH was varied in steps from pH 7.40 to 7.00, both at PCO2 5.5 kPa and PCO2 10 kPa. In group I (n = 6), data were obtained on the effect of arterial pH and PCO2 on ductular HCO3- secretion in bile acid depleted (cholestyramine pretreated), secretin-infused pigs. In group II (n = 6), the effect of pH and PCO2 on canalicular HCO3- secretion was studied in ursodeoxycholic acid (UDCA)-infused pigs (3 mumol min-1 kg-1 body wt). In group I, biliary HCO3- secretion exhibited PCO2-dependent, positive straight line relationships to arterial pH. An increment in biliary HCO3- secretion of 17 (11-24)% was seen during high PCO2 at pH 7.40. In group II, biliary HCO3- secretion exhibited PCO2-dependent, positive curvilinear relationships to arterial pH. A median increment in HCO3- secretion of 37 (20-62)% was seen during elevated PCO2 at arterial pH 7.40. The linear dependence of ductular HCO3- secretion on arterial pH and the effect of elevated PCO2 on HCO3- secretion fit well with findings in other epithelia, where proton transport is thought be driven by a proton pump. A computer simulation provided evidence suggesting that secretin-dependent HCO3- secretion does not involve the action of a Na+/H+ ion exchanger--in contrast to UDCA-dependent HCO3- secretion. It is concluded that ductular and canalicular HCO3- secretion could be mediated by a proton pump and a Na+/H+ ion exchanger in addition to canalicular HCO3- secretion due to solvent drag and diffusion, respectively.