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Department of Physiological Sciences, University of Manchester.

1. Intracellular pH (pHi) was measured using the fluorescent pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein in acini isolated from the rabbit mandibular salivary gland. 2. Stimulation of the acinar cells with acetylcholine (ACh) evoked an intracellular acidosis, the size of which was dependent on the HCO3-concentration in the bathing medium. A half-maximal acidosis was observed at approximately 10 mM-HCO3-. ACh also evoked an acidosis in HCO3(-)-free solutions containing acetate; a half-maximal acidosis was observed at about 10 mM-acetate. 3. Propionate, lactate and butyrate were also able to support the ACh-evoked acidosis to varying extents. In contrast, formate, pyruvate and salicylate did not support the ACh-induced acidosis to any great extent. 4. Acetazolamide greatly reduced the size of the acidosis in HCO3(-)-buffered medium, but had no effect in acetate-buffered medium, suggesting that the inhibitory effect of acetazolamide was due to a specific inhibition of carbonic anhydrase activity. 5. The Cl- channel blockers diphenylamine-2-carboxylic acid (DPC, 1 mM) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (0.5 mM) abolished the ACh-evoked acidosis in both HCO3(-) -and acetate-buffered media. 6. The data are consistent with the presence in the acinar cell of relatively non-specific anion channels sensitive to DPC and its derivatives. Such channels, activated on stimulation with ACh, would allow HCO3- and other weak acid ions to leave the cell, leading to the observed acidosis. The existence of such channels, located in the apical membrane, could explain why HCO3- or acetate can sustain fluid secretion in the intact perfused rabbit mandibular gland in the absence of Cl-.