A sodium/proton antiporter in chromaffin-granule membranes.

Chromaffin granules, the secretory vesicles of the adrenal medulla, have a Na+/H+ exchange activity in their membranes which brings their proton gradient into equilibrium with a Na+ gradient. This explains why Na+ is mildly inhibitory to amine transport (which is driven by the H+ gradient) The activity can be demonstrated by using accumulation of 22Na+ in response to a pH gradient that is either imposed by diluting membrane 'ghosts' into alkaline media, or generated by ATP hydrolysis. It can also be monitored indirectly by fluorescence measurements in which the pH inside 'ghost' is monitored by quenching of a fluorescent weak base. This method has been used to monitor Na+ entry into acid-loaded 'ghosts' of H+ entry into methylamine accumulation. The exchanger appears to be reversible and non-electrogenic, with a stoichiometry of 1:1. Using an indirect assay we measured an apparent Km for Na+ of 4.7 mM, and a Ki for amiloride, a competitive inhibitor, of 0.26 mM. Direct assays using 22Na+ suggested a higher Km. Ethylisopropylamiloride was not inhibitory.