Effect of arterial pH and PCO2on biliary HCO‐3secretion in the pig

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Abstract
The purpose of the present study was to examine the effect of changes in arterial pH and PCO2on biliary HCO‐3secretion. This was done in order to further characterize the various ion transport mechanisms considered responsible for biliary HC3secretion 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 7.40, both at PCO25.5 PCO2and PCO2, 10 kPa. In group I (n= 6), data were obtained on the effect of arterial pH and PCO2on ductular PCO2secretion in bile acid depleted (cholestyramine pretreated), secretin‐infused pigs. In group II (n= 6), the effect of pH and PCO2on canalicular HCO‐3secretion was studied in ursodeoxycholic acid (UDCA)‐infused pigs (3 μml min‐1kg‐1body wt). In group I, biliary HCO‐3secretion exhibited PCO2‐ dependent, positive straight line relationships to arterial pH. An increment in biliary HCO‐3secretion of 17 (11–24)% was seen during high PCO2at pH 7.40. In group II, biliary HCO‐3secretion exhibited PCO2‐dependent, positive curvilinear relationships to arterial pH. A median increment in HCO‐3secretion of 37 (20–62)% was seen during elevated PCO2at arterial pH 7.40. The linear dependence of ductular HCO‐3secretion on arterial pH and the effect of elevated HCO‐3on PCO2secretion 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 HCO‐3secretion does not involve the action of a Na+/H+ion exchanger—in contrast to UDCA‐dependent HCO‐3secretion. It is concluded that ductular and canalicular HCO‐3secretion could be mediated by a proton pump and a Na+/H+ion exchanger in addition to canalicular HCO‐3secretion due to solvent drag and diffusion, respectively.