Membrane cytochemistry of the atrial peptide secretory pathway.

The characteristics of the principal membrane systems involved in atrial peptide secretion are explored in rabbit and rat atrial muscle cells using freeze-fracture electron microscopy combined with cytochemistry. Structural features of the Golgi apparatus and secretory granule membrane are described, and two features of the membranes' composition, cholesterol content and terminal glycocomponents, are probed in situ in the cell. Both the secretory granule membrane and the plasma membrane contain morphologically detectable cholesterol, and both are rich in the terminal sugar residues (sialic acid and N-acetyl glucosamine) of membrane glycocomponents. Some cisternal membranes of the Golgi apparatus reveal detectable cholesterol but others do not, and only focal labeling of terminal glycocomponents in Golgi membranes is observed, usually adjacent to the accumulation of secretory product. Nuclear membranes, endoplasmic reticulum, and mitochondria contain virtually no detectable cholesterol nor terminally glycosylated components. The Golgi apparatus, which is exclusively equipped with the enzymes responsible for terminal glycosylation of membrane glycoproteins, appears actively to modify the chemical composition of the membrane segments in which it packages atrial peptide. This ensures that the composition of the atrial granule membrane resembles that of the plasma membrane into which it is ultimately incorporated.