Datensatz

Zusammenfassung

Isolated acinar cells from the rat exocrine pancreas were loaded with 6-carboxyfluorescein diacetate (CFDA), and the intracellular pH (pH_i) was estimated from the pH-dependent fluorescence intensity of trapped 6-carboxyfluorescein liberated from CFDA by intracellular esterases. The intracellular fluorescence intensity was calibrated by equilibrating the internal and external pH with nigericin in K⁺ buffers. In the absence of Na⁺ (130 mmol/l K⁺) a pH_i of 6.86 ± 0.04 was found; in its presence (130 mmol/l Na⁺) a pH_i of 7.17. Acute addition of Na⁺ increased intracellular pH with increasing Na⁺ concentrations, reaching a maximum at 150 mmol/l with an apparent K_m of approximately 40 mmol/l. Of the different cations tested on pH_i, such as Li⁺, K⁺, Rb⁺, and Cs⁺, only Li⁺ showed an effect on pHi similar to that of Na⁺. Amiloride dose dependently inhibited both Na⁺- and Li⁺-induced alkalinization (apparent K_m approximately 10^-5 mol/l). In the presence of ouabain pH_i was decreased by 0.2 pH units. Intracellular acidification induced by permeable buffers such as acetic acid-acetate or CO_2-HCO₃^- was dissipated more rapidly in the presence of Na⁺ compared with K⁺ or with Na⁺ and amiloride in the medium. In Li⁺-preincubated cells intracellular acidification was higher in the absence of Li⁺ in the extracellular medium than in its presence. This Li⁺ gradient-induced acidification was dependent on the extracellular pH, was highest at an extracellular pH of 7.05, and decreased with increasing pH to 7.5. The results allow the conclusion that a coupled Na⁺-H⁺ exchange is present in pancreatic acinar cells and that the intracellular pH rather than the extracellular Na⁺ concentration regulates this transport mechanism.