Definition of the Extracellular Space in Secreting and Nonsecreting Oxyntic Cells

Oxyntic, or acid-secreting, cells of the gastric epithelium contain a system of abundant smoothsurfaced tubular membranes within the cell apices . It has been proposed that this tubular membranous system may communicate with the apical plasma membrane, and thereby contribute to the elaborate ultrastructural changes which are associated with secretory activity (1-3) . Direct evidence that such intercommunications can occur has recently been provided by Sedar, who applied horseradish peroxidase to the mucosal solution of bullfrog stomachs and traced its incorporation into oxyntic cells after exposures of 0 .5-1 .5 hr (4) . He found that the large molecular weight tracer substance (40,000 mol wt) penetrated the tubular membrane system even to the level of the nucleus, and that all peroxidase activity was confined to the inner aspects of the tubular membranes. Sedar's experiment shows that the luminal extracellular space can be continuous with the smooth-surfaced endoplasmic tubules of oxyntic cells . However, as he pointed out, it leaves open one important aspect for the mode of penetration, that is to say, whether penetration of tracer occurred by simple diffusion into pre-existing and permanent channels, or whether the route of access was directly dependent upon secretory activity, e .g . via active membrane transformations which occur with HCl secretion . Our experiments were designed to distinguish between the above possibilities . We exposed glutaraldehyde-fixed gastric tissue to lanthanum nitrate, which is often used for the determination of extracellular space at the ultrastructural level (5, 6), and determined its distribution in and around the oxyntic cells . Moreover, we assessed the distribution of the extracellular tracer in resting gastric tissue and in counterparts stimulated to maximum HCl secretion .