Evidence for involvement of a zymogen granule Na+/H+ exchanger in enzyme secretion from rat pancreatic acinar cells

We have characterized a Na+/H+ exchanger in the membrane of isolated zymogen granules (ZG) from rat exocrine pancreas and investigated its role in secretagogue-induced enzyme secretion. ZG Na+/H+ exchanger activity was estimated by measuring Naf or Li+ influx and consequent osmotic swelling and lysis of ZG incubated in Na- or Li-acetate. Alternatively, intra-granule pH was investigated by measuring absorbance changes in ZG which had been preloaded with the weak base acridine orange. Na+- or Li+-dependent ZG lysis was enhanced by increasing inward to outward directed H+ gradients. Na+-dependent ZG lysis was not prevented by an inside-positive K+ diffusion potential generated by valinomycin which argues against parallel operation of separate electrogenic Na+ and H+ permeabilities and for coupled Na+/H+ exchange through an electroneutral carrier. Na+- and Li+-dependent ZG lysis was inhibited by EIPA (EC(50) similar to 25 mu M) and benzamil (EC(50) similar to 100 mu M), but only weakly by amiloride. Similarly, absorbance changes due to release of acridine orange from acidic granules into the medium were obtained with Na+ and Li+ salts only, and were inhibited by EIPA, suggesting the presence of a Na+/H+ exchanger in the membrane. Na+ dependent lysis of ZG was inhibited by 0.5 mM MgATP and MgATP-gamma-S by about 60% and 35%, respectively. Inhibition by MgATP was prevented by incubation of ZG with alkaline phosphatase (100 U/ml), or by the calmodulin antagonists calmidazolium (0.75 mu M), trifluoperazine (100 mu M) and W-7 (500 mu M), suggesting that the ZG Na+/H+ exchanger is regulated by a ZG membrane-bound calmodulin-dependent protein kinase. Na+ dependence of secretagogue (CCK-OP)-stimulated amylase secretion was investigated in digitonin permeabilized rat pancreatic acini and was higher in acini incubated in Na+ containing buffer (30 mM NaCl/105 mM KCl buffer; 6.4 +/- 0.4% of total amylase above basal) compared to buffer without Na+ (0 mM NaCl/135 mM KCl buffer; 4.7 +/- 0.4% of total amylase above basal, P < 0.03). EIPA (50 mu M) reduced CCK-OP-induced amylase secretion in Na+ containing buffer from 7.5 +/- 0.6% to 4.1 +/- 0.8% (P < 0.02). In the absence of Na+ in the buffer, CCK-OP-stimulated amylase release was not inhibited by 50 mu M EIPA. The data suggest that an amiloride insensitive, EIPA inhibitable Na+/H+ exchanger is present in ZG membranes, which is stimulated by calmodulin antagonists and could be involved in secretagogue-induced enzyme secretion from rat pancreatic acini.