The Mechanism of Watery Vacuolation in the Acinar Cells of the Submandibular Gland

Watery vacuolation was induced in the acinar cells of the rat submandibular gland by allowing small pieces of tissue (2 mm3 to stand in suitable unoxygenated fluids at 20°C, and cells in various stages of vacuolation were examined by light and electron microscopy. No structural evidence for pinocytosis was found. Using basic solution (NaCl, 8.0 g/l.; CaCl2 0.2 g/l.; NaHCO3 1.0 g/l.) it was confirmed that vacuolation does not occur at 0°C, and that it is greatly reduced if Ca2+ is omitted. Vacuoles develop in pieces of tissue which have been separated from the animal for up to 2 h, provided that the pieces are then immersed in a suitable fluid. Vacuolation can occur in the absence of external Na+ or Cl-: cations substituted for Na+ allowed vacuolation in the order: Li+, full; Cs+, slight; K+ and Rb+, none. It can be prevented, however, if sufficient sucrose (> 210 m-osmoles) or glucose is added to the external fluid, but not by the same quantities of urea, glycerol, or propylene glycol. Metabolic poisons which block oxidative phosphorylation or glycolysis do not prevent vacuolation. It is concluded that vacuolation does not involve pinocytosis but represents the secondary segregation of fluid from an oedematous cytoplasm. The mechanism of secondary segregation and its possible relationship to secretion are discussed.