Local dynamic changes in confined extracellular environments within organs.

1. Herein we review past work that has studied the composition of luminal fluid in organs, with a focus on measures of calcium and pH in the exocrine glands. This luminal environment is 'external' to the mammalian body and is not subject to the usual mechanisms of homeostatic control. Instead, it is controlled by the behaviour of the cells that line the lumen. 2. We discuss the likely possibility that rapid and local changes in calcium and pH occur within microdomains in the lumen. Further, we present preliminary evidence, using live cell imaging of intact pancreatic fragments, that supports the idea that pH changes do occur. Our evidence indicates that exocytosis of secretory granules in pancreatic acinar cells leads to a loss of protons from the granule and a subsequent local acidification of the lumen. 3. These changes in luminal composition are placed in the context of diseases of the pancreas, such as cystic fibrosis and pancreatitis, both of which are known to result in perturbations of luminal fluid composition.