The stoichiometry of the electrogenic sodium bicarbonate cotransporter pNBC1 in mouse pancreatic duct cells is 2 HCO3-: 1 Na+

1. electrogenic sodium bicarbonate cotransporter pNBC1. is believed to play a major role in the secretion of bicarbonate by pancreatic duct cells, by transporting bicarbonate into the cell across the basolateral membrane. The rmodynamics predict that this function can be achieved only if the reversal potential of the cotransporter is negative to the cell's membrane potentia l, or equivalently that the HCO3-:Na+ stoichiometry is not larger then 2:1. However, there are no data available on either the reversal potential or t he HCO3-:Na+ stoichiometry of pNBC1 in pancreatic cells. 2. We studied pNBC1 function in mouse pancreatic duct cells. RT-PCR analysi s of total RNA revealed that these cells contain the message for pNBC1, but not for kNBC1, NBC2 or NBC3. 3. To measure cotransporter activity, mouse pancreatic duct cells were grow n to confluence on a porous substrate, mounted in an Ussing chamber, and th e apical plasma membrane permeabilized with amphotericin B. Ion flux throug h pNBC1 was achieved by applying Na+ concentration gradients across the bas olateral plasma membrane. The current through the cotransporter was isolate d as the difference current due to the reversible inhibitor dinitrostilbene disulfonate (DNDS). 4. Current-voltage relationships for the cotransporter, measured at three d ifferent Na+ concentration gradients, were linear over a range of about 100 mV. The reversal potential data, obtained from these current-voltage relat ionships, all corresponded to a 2 HCO3-:1 Na+ stoichiometry. 5. The data indicate that pNBC1 is functionally expressed in mouse pancreat ic duct cells. The cotransporter operates with a 2 HCO3-:1 Na+ stoichiometr y in these cells, and mediates the transport of bicarbonate into the cell a cross the basolateral membrane.