ISOSMOTIC MODULATION OF CELL-VOLUME AND INTRACELLULAR ION ACTIVITIES DURING STIMULATION OF SINGLE EXOCRINE CELLS

Stimulation of salivary secretion is associated with a rise of [Ca2+]( i) in acinar cells. We examined the osmotic and ionic consequences of activation of Ca2+-dependent K+ and Cl- channels, by simultaneous opti cal determinations of cell volume and [Ca2+](i), [Cl-](i) or [Na+](i) during muscarinic stimulation of single salivary acinar cells, using a differential interference contrast (DIC)-fluorescence microscope. Car bachol caused a rapid rise of [Ca2+](i), as well as a substantial cell shrinkage. Despite variability in the level and kinetics of the subse quent sustained phase of the [Ca2+](i) response, cell volume was corre lated with [Ca2+](i) in all cases. Elevated [Ca2+](i) was both necessa ry and sufficient to cause these changes in cell volume. The propositi on that changes in cell volume reflected changes in cell solute conten t was confirmed by simultaneously measuring [Cl-](i) and cell volume. Simultaneous determinations of cell volume and [Na+](i) indicated that the initial cell shrinkage was due entirely to K+ and Cl- efflux. Sub sequent to the initial shrinkage, [Na+](i) rose to high levels, primar ily due to activation of Na+/H+ exchange. Thus, modulation of ion tran sport activities under isosmotic conditions results in substantial cha nges in cell solute content and cell volume. Subsequent to the early C a2+-induced changes in these parameters, other transporters become act ive, but it is unclear what signals their activation. Cell swelling by osmotic dilution of the bath resulted in compensatory cell shrinkage (RVD) which was sensitive to K+ and Cl- gradients. Nevertheless, a ris e of [Ca2+](i) was not necessary for RVD. Osmotic shrinkage and/or cel l acidification were insufficient to activate Na+ influx. Our data sug gest that the intracellular signalling pathways used by cells to activ ate ion transport under anisosmotic conditions may not be the same as those employed under isosmotic conditions. (C) 1994 Wiley-Liss, Inc.