Secretin empties bile duct cell cytoplasm of vesicles when it initiates ductular HCO3- secretion in the pig.

To determine whether secretin has any effect on bile duct cell ultrastructure, bile duct cells from liver biopsy specimens of pigs were analyzed morphometrically. During secretory rest, bile duct cell cytoplasmic vesicles totaled 96 (84-103) arbitrary units per cell volume (U). Secretin increased bile HCO3- secretion from 9 mumol/min (range 6-15) to 131 mumol/min (range 118-200) and lowered the bile duct cell vesicles to 5 U (range 3-9). Acute elevation of arterial PCO2 to 10.9 kPa (range 10.2-11.1) doubled vesicle number in resting duct cells and augmented the secretory response to secretin. At high arterial PCO2, secretin cleared the duct cell cytoplasm of vesicles and more than doubled the basolateral plasma membrane surface area. Taurocholate-induced canalicular choleresis, in contrast, did not alter duct cell morphology. It is concluded that secretin clears the bile duct cell cytoplasm of vesicles as it initiates ductular HCO3- secretion, possibly through causing exocytotic insertion of vesicle material into the basolateral plasma membrane.