The Ca2+ dependence of surface membrane retrieval (i.e., the process by which the excess surface membrane resulting from exocytosis is recycled to the cytoplasm of secretory cells) has been investigated in rat parotid tissue lobules first incubated for 40 min in the presence of a secretagogue drug (the β-adrenergic agonist isoprenaline) and then in the presence of the β-blocker, 1-propranolol, up to 4 h. The dynamics of the luminal surface membrane was monitored by measuring, in ultrathin sections, the length of the luminal profile of all examined acinar cells abutting to a lumen before and immediately at the end of the stimulation, as well as at various times thereafter. Such a profile doubled during isoprenaline stimulation, concomitantly with the discharge of most secretion granules. After the stimulation was blocked, the luminal profile decreased to reach values even lower than those observed in unstimulated cells. The kinetics of this reduction was apparently first-order, both in the presence and in the absence of extracellular Ca2+. However, its rate differed appreciably in these two situations: it was relatively fast (apparent t 1 2~18.5 min) in lobules incubated in complete medium (Ca2+ concentration, 2 mM), and much slower (apparent t 1 2~84.5 min) in lobules incubated in a Ca2+-free medium containing 1 mM EGTA. The slowing down of the membrane retrieval occurring in Ca2+-free conditions was rapidly reversed by reintroduction of Ca2+ into the medium. These findings indicate that the retrieval of the luminal surface membrane in parotid acinar cells is Ca2+-dependent.
ASJC Scopus subject areas
- Cell Biology