PROXIMAL DUODENAL ENTEROCYTE TRANSPORT - EVIDENCE FOR NA+-H+ AND CL--HCO3- EXCHANGE AND NAHCO3 COTRANSPORT

The duodenum, in contrast to the jejunum, actively secretes HCO3- at a high rate, a process that protects the mucosa from acid/peptic injury. Our purpose was to define the mechanisms involved in HCO3- transport by studying the acid-base transport processes in isolated duodenal enterocytes. Individual rat duodenocytes, isolated by a combination of Ca2+ chelation and collagenase, attached to a collagen matrix were loaded with the pH-sensitive fluoroprobe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM), and intracellular pH was monitored by microfluorospectrophotometry. To identify Na+-H+ transport, cells in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid 1) were pulsed with NH4Cl (40 mM) in the absence and presence of amiloride and 2) were removed of Na+. To examine Cl-, HCO3- exchange, Cl- was removed from Ringer-HCO superfusate in the presence and absence of dihydro-4,4'-diisothio-cyanostilbene-2,2'-disulfonic acid (H-2DIDS). The NaHCO3, cotransporter was studied by addition and subtraction of Na+ to amiloride-treated and Cl--depleted enterocytes perfused with Na+- and Cl--free Ringer-HCO- buffer with and without H2DIDS. Mammalian duodenocytes contain at least three acidbase transporters: an amiloride-sensitive Na+-H+ exchanger that extrudes acid, a DIDS-sensitive Cl--HCO3- exchanger that extrudes base, and a NaHCO:, cotransporter. also DIDS sensitive, that functions as a base loader. These acid-base transporters likely play a key role in duodenal mucosal HCO3- secretion.