Activation of salivary secretion: coupling of cell volume and [Ca2+]i in single cells

High-resolution differential interference contrast microscopy and digital imaging of the fluorescent calcium indicator dye fura-2 were performed simultaneously in single rat salivary gland acinar cells to examine the effects of muscarinic stimulation on cell volume and cytoplasmic calcium concentration (Ca2+]i). Agonist stimulation of fluid secretion is initially associated with a rapid tenfold increase in Ca2+]i as well as a substantial cell shrinkage. Subsequent changes of cell volume in the continued presence of agonist are tightly coupled to dynamic levels of Ca2+]i, even during Ca2+]i oscillations. Experiments with Ca2+ chelators and ionophores showed that physiological elevations of Ca2+]i are necessary and sufficient to cause changes in cell volume. The relation between Ca2+]i and cell volume suggests that the latter reflects the secretory state of the acinar cell. Agonist-induced changes in Ca2+]i, by modulating specific ion permeabilities, result in solute movement into or out of the cell. The resultant cell volume changes may be important in modulating salivary secretion.