H+ transporters in the main excretory duct of the mouse mandibular salivary gland

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Abstract
1 We used microspectrofluorimetry with the pH‐sensitive fluoroprobe 2′,7′‐bis(2‐carboxy‐ethyl)‐5(and‐6)‐carboxyfluorescein (BCECF) to study the regulation of cytosolic pH (pHi) in the isolated, perfused main excretory duct of the mouse mandibular gland. 2 In nominally HCO3−free solutions, removal of Na+ from the lumen alone caused pHi to decline whereas removing it from the bath alone did not. 3 Readmission of Na+ to the lumen of ducts studied under zero‐Na+ conditions caused pHi to recover fully. This recovery was blocked by 5‐(N‐ethyl‐N‐isopropyl)‐amiloride (EIPA) with a half‐maximum concentration of 0.5 μmol l−1, indicating the presence of an apical Na+–H+ exchanger. 4 Readmission of Na+ to the bath of ducts studied under zero‐Na+ conditions also caused pHi to recover. This recovery was blocked by 100 μmol l−1 EIPA, indicating the presence of a basolateral Na+‐H+ exchanger. 5 Measurements of H+ fluxes indicated that the apical Na+–H+ exchanger was approximately four times more active than the basolateral Na+‐H+ exchanger. 6 In three sets of experiments (in the absence of Na+, in the presence of Na+, and in the presence of Na+ plus 100 μmol l−1 EIPA), the effects of changing luminal K+ concentration on pHi were examined. We found no evidence for the presence of K+–H+ exchange or Na+‐coupled K+–H+ exchange in the apical membranes of duct cells. 7 pHi recovery under nominally HCO3−‐free conditions following acidification with an NH4Cl pulse was abolished by removal of Na+ from the bath and luminal solutions, indicating that no Na+‐independent systems such as H+‐ATPases were present. 8 A repeat of the above experiments in the presence of 25 mmol l−1 HCO3− plus 5% CO2 did not reveal any additional H+ transport systems. The removal of luminal Cl−, however, caused a small rise in pHi. This latter effect was blocked by 500 μmoll−1 4,4′‐diisothio‐cyanatodihydrostilbene‐2,2′‐disulphonic acid (H2‐DIDS), suggesting that a Cl−–HCO3− exchanger in the apical membrane might contribute in a minor way to pHi regulation. 9 We conclude that the predominant H+ transport systems in the mouse mandibular main excretory duct are Na+‐H+ exchangers in the apical and the basolateral membranes. The model we postulate to account for electrolyte transport across the main duct in the mouse mandibular gland is quite different from that previously developed for the rat duct but is similar to that developed for the rabbit duct. The difference is in concordance with the known ability of the mandibular gland of the rat, but not the rabbit or the mouse, to secrete a HCO3−‐rich final saliva.