CATECHOLAMINES ARE PRESENT IN A SYNAPTIC-LIKE MICROVESICLE-ENRICHED FRACTION FROM BOVINE ADRENAL-MEDULLA

''Synaptic-like microvesicles'' are present in all neuroendocrine cell s and cell lines. Despite their resemblance to small synaptic vesicles of the CNS, a thorough biochemical characterization is lacking. Moreo ver, the subcellular distribution of synaptophysin, the most abundant integral membrane protein of small synaptic vesicles, in adrenal medul la is still controversial. Using gradient centrifugation, we were able to compare the distribution of several markers for small synaptic ves icles and chromaffin granules. Synaptophysin was found at a high densi ty (1.16 g/ml), purifying away from dopamine beta-hydroxylase and cyto chrome b561. Both noradrenaline and adrenaline showed a parallel distr ibution with synaptophysin, suggesting their presence in synaptic-like microvesicles. Experiments in the presence of tetrabenazine did not i nfluence the catecholamine content. Additionally, tetrabenazine bindin g showed a consistent shoulder in the region of synaptophysin. [H-3]No radrenaline uptake was blocked by tetrabenazine, but not by desipramin e. Also chromogranin A parallels the distribution of synaptophysin; ho wever, a localization in the Golgi cannot be ruled out. Synaptophysin was shown to undergo very fast phosphorylation, together with another triplet protein of approximately 18 kDa. In contrast, the latter showe d a rather bimodal distribution coinciding with synaptophysin and dopa mine beta-hydroxylase. Immunoelectron microscopy of synaptic-like micr ovesicle fractions showed an intense labeling for synaptophysin on 60- 90-nm organelles. Whereas abundant gold labeling for cytochrome b561 W as found over the entire surface of chromaffin granules, synaptophysin labeling was encountered mostly on vesicles adsorbed to granules. We conclude that catecholamines might be stored in synaptic-like microves icles of the chromaffin cell.