The relationship between cytosolic concentrations of Ca2+ (Caz) and Na' (Na:) were studied in preparations of rat submandibular and pancreatic acini loaded with the Ca2+-sensitive dye Fura-2 or the Na+-sensitive dye SBFI. Pancreatic acini showed no changes in Na: during either transient or persistent changes in CaZ. Increases in Ca2+, produced by exposure of submandibular gland acini to carbachol, a muscarinic cholinergic agonist, were followed by an increase in Na+i after a delay of 5-10 s. When Ca2+ stores were mobilized without Ca2+ influx Na: also increased, but in acini loaded with BAPTA, a nonfluorescent Ca2+ chelator, the transient increase in Ca2+ caused by mobilization of stored Ca2+ was virtually abolished, as was the increase in Na:. In the presence of ionomycin, increases in Ca" were followed by increases in Na:. Ca2+-dependent increases in Na: were abolished in Na+-free buffer and by the presence of furosemide, a blocker of Na'-K+-2CI-cotransport. In other studies, extracellular ATP (ATP,) produced an increase in Ca:' and Na:. The steady-state increase in Ca:+ was reduced by increasing extracellular Na+ concentrations (Na:) in dosedependent fashion (IC50 = 16.4 ? 4.7 mM Na'). Likewise, increasing Na,' reduced ATP,-stimulated 45Ca2' uptake at steady state (lC50 = 15.8 ? 9.2 mM Na+). Changing Na,' had no effect on carbachol-stimulated increases in Ca". We conclude that, in rat submandibular gland acini, ATP, promotes an increase in Ca?' and Na: via a common influx pathway and that, under physiologic conditions, Na' significantly limits the ATP,-stimulated increase in Ca:' . In the presence of carbachol, however, Na: rises in Caz+-dependent fashion in submandibular gland acini via stimulation of Na' -K' -2CI-cotransport. o 1996 WiIey-Liss, Inc.
Evidence has accumulated suggesting that Ca?' and Na: are functionally related in salivary gland cells and that the well-established role of Ca2+ as a regulator of salivary cell function may involve changes in the activities of Na' handling systems as well. Martinez and coworkers (1990) as well as Landis and Putney (1979) have shown that Na' influx during secretagogue exposure in salivary gland cells re uires the presence of Ca2+. They also showed that Ca%' mobilization increases Na' influx. Gallacher and Moms (1987) have suggested, based on patch clamp studies, that Na' is involved in Ca2+ gating in submandibular gland acini. Na+ efflux is mediated primarily by the Na'-K' AT-Pase. Na' influx may be mediated by at least three systems, a Naf-K+-2C1-cotransporter and by Na+-Ca2+ and Na'-H' exchangers or antiports. All these Na' handling systems are components of the plasma membrane of cells in which they operate, and their presence in cells of the major salivary glands has been studied in several laboratories (Turner et al.,