Anion and proton transport in chromaffin granules

Both intact chromaffin granules and ghosts behave as osmometers in response to the relative internal and external concentrations of solutes. The permeability properties of the granule membrane can therefore be studied by measuring the osmotic consequences of ion permeation. These include lysis of intact granules and shrinking or swelling of ghosts. This approach has been used to identify at least two types of selective anion transport sites in the membrane. One is observed in the presence of K+ ionophores and appears to be electrogenic in nature. It is insensitive to a group of compounds known to inhibit anion transport in erythrocytes. The other site is sensitive to those inhibitors and is involved with electroneutral cotransport with protons. This proton entry can be driven by either the proton-pumping ATPase of the granule membrane or by a simple inward concentration gradient of protons. The proton entry-coupled anion transport site may be structurally related to the ATPase itself based on parallel anion and inhibitor sensitivities of the site and the enzyme. One or both of the anion transport sites described here may play a role in regulating exocytosis from cells based on a chemiosmotic mechanism that has been postulated to be responsible for this process.