Multiple Effects of NA+ Removal on Pancreatic Secretion in Vitro

The effects of removing Na+ from the incubation medium on basal and secretagogue induced zymogen release by pancreatic fragments and isolated pancreatic acini were studied by both morphological evaluation and measurement of amylase release. In both fragments and isolated acini, removal of Na+ led to an increased basal secretion of zymogen granule contents from acinar cells via exocytosis; secretory material, however, accumulated in acinar and ductular lumina as a result of the lack of fluid secretion necessary to wash out the enzymes. In studies with fragments, after Na+ removal there was no significant increase in amylase release into the medium; isolated acini, in contrast, showed an increased amylase release consistent with the shorter distance from the acinar lumen to the bathing medium. Stimulation with either bethanechol or caerulein led to a further delpletion of zymogen granules in both preparations; in the absence of Na+ secretory product accumulated in intracellular lakes as well as in duct lumens. The hypothesis that Na+ influx is important in stimulus-secretion coupling to release intracellular Ca2+ was directly tested by measuring 45Ca2+ efflux. No effect of removing Na+ on 45Ca2+ efflux was seen. It was concluded therefore, that while Na+ is essential for pancreatic fluid secretion, it is not necessary for the secretion of zymogen granule contents into acinar lumina.