The subcellular distribution of p65 (synaptotagmin), of the endoproteases PC1 and PC2 and of secretoneurin was studied in bovine posterior pituitary by differential and density gradient centrifugation. All these peptides were found to be present in the neurosecretory granules (large dense core vesicles). p65 can therefore participate not only in exocytosis from small synaptic vesicles but also from large dense core vesicles. Secretoneurin is a peptide derived from secretogranin II. Processing of the propeptide apparently occurs during axonal transport of the large dense core vesicles and is complete in the posterior pituitary. Thus, stimulation of the hypothalamic magnocellular neurons can lead to the release of this newly characterized, functional neuropeptide.

Specific antisera were raised against the A and B chains of glycoprotein III. Immunoblotting revealed that in adrenal medulla both chains migrate very closely together in two-dimensional electrophoresis. Both chains with slightly differing molecular sizes are found in several endocrine tissues and in brain, kidney, liver, and serum. The mRNA has an analogous widespread distribution. In primary cultures of chromaffin cells the level of message becomes significantly increased by treatment with histamine or 12-O-tetradecanoylphorbol 13-acetate/forskolin. However, the increase is small when compared with that of secretogranin II. The subcellular localization of glycoprotein III in endocrine organs and in the posterior pituitary was investigated by subcellular fractionation and immunoelectron microscopy. Glycoprotein III was found to be confined to the large dense-core vesicles of these organs. For a discussion of the function of glycoprotein III, its localization in these organelles has to be taken into account.

No abstract