AJP - Heart and Circulatory Physiology, Vol 257, Issue 5 1587-H1598, Copyright © 1989 by American Physiological Society
Membrane cytochemistry of the atrial peptide secretory pathway
N. J. Severs
Department of Cardiac Medicine, National Heart and Lung Institute, London, United Kingdom.
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.
