Membrane of Adrenal Chromaffin Granule : An Inside Out Membrane

It is well established that secretion of catecholamine from the adrenal medulla occurs by exocytosis. Fusion of granule with cell membrane is the critical step in this process. Understanding of the properties of granule membrane leads to the resolution of the problem of why catecholamine storage granules act as the secretory granules upon stimulation of cell membrane in the process of exocytosis. In this paper we described that 1) granule membrane maintained a higher sodium concentration than the cytoplasm and 2) sialic acid which usually exists on the outer surface of cell membrane localized on the inner surface of the chromaffin granule membrane. At the same time, it was shown that membrane of chromaffin granules shared many common compositions with cell membranes. Sodium gradient across the granule membrane, together with localization of sialic acid strongly suggests that granule membrane is inside out when compared to cell membrane. In fusion of granule with cell membrane, the first step is the adhesion of granule membrane and the inner surface of the cell membrane, and such process would be very prone to occur since these two sides of each membrane contain little or no sialic acid and they share certain common molecules as composition.