In the Golgi region of cultured rat atrial myocytes, condensed secretory protein was seen in Golgi-associated tubules or cisternae which lay beyond, and often separated from, the remainder of the Golgi stacks. These structures appeared to be involved in packaging of condensed secretory protein into atrial granules. Binding sites of HRP-conjugated wheat-germ agglutinin (WGA) in saponin-treated cultured atrial myocytes were examined by electron microscopy with special reference to atrial granules and the tubular structures associated with the Golgi stacks. HRP reaction products were observed in both trans-cisternae of the Golgi stacks and the associated tubular structures. While the majority of atrial granules were devoid of reaction products, some granules, which were connected to the WGA-positive tubular structures in the vicinity of the Golgi trans-cisternae, showed HRP reaction products at their connected necks. Similar results were obtained when sections of the cells embedded in Lowicryl K4M were labeled with WGA coupled to colloidal gold (G-WGA); the Golgi complex was G-WGA positive, whereas no specific binding of G-WGA to atrial granules was observed. These results suggest that glycoproteins and/or glycolipids with oligosaccharides recognized by WGA in the Golgi transcisternae, may be separated from atrial natriuretic peptides which are packaged into atrial granules.
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