LONG-TERM STIMULATION CHANGES THE VESICULAR MONOAMINE TRANSPORTER CONTENT OF CHROMAFFIN GRANULES

Bovine chromaffin cells cultured for 5 days in the presence of depolarizing concentrations of K+ ions show a decreased number of secretory (chromaffin) granules per cell. These cells were still capable of exocytosis, Their contents in catecholamine and chromogranin A, components of the granule matrix, and cytochrome b(561), a major protein of the granule membrane, were decreased to 35, 30, and 50% of control cells, respectively, However, in the same cells, the number of [H-3]dihydrotetrabenazine binding sites, a specific ligand of the vesicular monoamine transporter, was increased to 180% of controls. In situ uptake of noradrenaline in permeabilized cells indicated that [H-3]dihydrotetrabenazinc binding sites were associated with a functional vesicular monoamine transporter, When analyzed by isopycnic centrifugation, these sites cosedimented with catecholamine, chromogranin A, and cytochrome b(561), in a peak with a density lighter than that from controls, The composition of this peak suggests that it contains incompletely matured secretory granules, with a 3-5-fold increase in the vesicular monoamine transporter content of this membrane, This increase might indicate that an adaptative process occurs which allows a faster filling of the granules in continuously secreting cells.