Targeted disruption of the Nhe1 gene prevents muscarinic agonist-induced up-regulation of Na(+)/H(+) exchange in mouse parotid acinar cells

Na(+)-dependent transporters mediate HCO(3)(-) salvage across the luminal membrane of the main pancreatic duct

To study the roles of Na(+)-dependent H(+) transporters, we characterized H(+) efflux mechanisms in the pancreatic duct in wild-type, NHE2(-/-), and NHE3(-/-) mice. The pancreatic duct expresses NHE1 in the basolateral membrane, and NHE2 and NHE3 in the luminal membrane, but does not contain NHE4 or NHE5. Basolateral Na(+)-dependent H(+) efflux in the microperfused duct was inhibited by 1.5 microM of the amiloride analogue HOE 694, consistent with expression of NHE1, whereas the luminal activity required 50 microM HOE 694 for effective inhibition, suggesting that the efflux might be mediated by NHE2. However, disruption of NHE2 had no effect on luminal transport, while disruption of the NHE3 gene reduced luminal Na(+)-dependent H(+) efflux by approximately 45%. Notably, the remaining luminal Na(+)-dependent H(+) efflux in ducts from NHE3(-/-) mice was inhibited by 50 microM HOE 694. Hence, approximately 55% of luminal H(+) efflux (or HCO(3)(-) influx) in the pancreatic duct is mediated by a novel, HOE 694-sensitive, Na(+)-dependent mechanism. H(+) transport by NHE3 and the novel transporter is inhibited by cAMP, albeit to different extents. We propose that multiple Na(+)-dependent mechanisms in the luminal membrane of the pancreatic duct absorb Na(+) and HCO(3)(-) to produce a pancreatic juice that is poor in HCO(3)(-) and rich in Cl(-) during basal secretion. Inhibition of the transporters during stimulated secretion aids in producing the HCO(3)(-)-rich pancreatic juice.

Muscarinic receptor-induced acidification in sublingual mucous acinar cells: loss of pH recovery in Na+-H+ exchanger-1 deficient mice

1. Intracellular pH (pHi) plays an important role in regulating fluid and electrolyte secretion by salivary gland acinar cells. The pH-sensitive, fluorescent dye 2', 7'-bis(carboxyethyl)-5(6)-carboxylfluorescein (BCECF) was used to characterize the mechanisms involved in regulating pHi during muscarinic stimulation in mouse sublingual mucous acinar cells. 2. In the presence of HCO3-, muscarinic stimulation caused a rapid decrease in pHi (0.24 +/- 0.02 pH units) followed by a slow recovery rate (0.042 +/- 0.002 pH units min-1) to the initial resting pHi in sublingual acinar cells. The muscarinic receptor-induced acidification in parotid acinar cells was of a similar magnitude (0. 25 +/- 0.02 pH units), but in contrast, the recovery rate was approximately 4-fold faster (0.181 +/- 0.005 pH units min-1). 3. The agonist-induced intracellular acidification was inhibited by the anion channel blocker niflumate, and was prevented in the absence of HCO3- by treatment with the carbonic anhydrase inhibitor methazolamide. These results indicate that the muscarinic-induced acidification is due to HCO3- loss, probably mediated by an anion conductive pathway. 4. The Na+-H+ exchange inhibitor 5-(N-ethyl-N-isopropyl)amiloride (EIPA) amplified the magnitude of the agonist-induced acidification and completely blocked the Na+-dependent pHi recovery. 5. To examine the molecular nature of the Na+-H+ exchange mechanism in sublingual acinar cells, pH regulation was investigated in mice lacking Na+-H+ exchanger isoforms 1 and 2 (NHE1 and NHE2, respectively). The magnitude and the rate of pHi recovery in response to an acid load in acinar cells isolated from mice lacking NHE2 were comparable to that observed in cells from wild-type animals. In contrast, targeted disruption of the Nhe1 gene completely abolished pHi recovery from an acid load. These results demonstrate that NHE1 is critical for regulating pHi during a muscarinic agonist-stimulated acid challenge and probably plays an important role in regulating fluid secretion in the sublingual exocrine gland. 6. In NHE1-deficient mice, sublingual acinar cells failed to recover from an acid load in the presence of bicarbonate. These results confirm that the major regulatory mechanism involved in pHi recovery from an acid load is not Na+-HCO3- cotransport, but amiloride-sensitive Na+-H+ exchange via isoform 1.