ISSN:
1432-1424
Keywords:
gallbladder
;
apical membrane vesicles
;
Na+/H+ exchange
;
Cl−/OH− exchange
;
Na+−Cl− symport
;
furosemide
;
bumetanide
;
hydrochlorothiazide
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Summary In rabbit gallbladder epithelium, a Na+/H+, Cl−/HCO 3 − double exchange and a Na+−Cl− symport are both present, but experiments on intact tissue cannot resolve whether the two transport systems operate simultaneously. Thus, isolated apical plasma membrane vesicles were prepared. After preloading with Na+, injection into a sodium-free medium caused a stable intravesicular acidification (monitored with the acridine orange fluorescence quenching method) that was reversed by Na+ addition to the external solution. Although to a lesser extent, acidification took place also in experiments with an electric potential difference (PD) equal to 0. If a preset pH difference (ΔpH) was imposed ([H+]in〉[H+]out, PD=0), the addition of Na-gluconate to the external solution caused ΔpH dissipation at a rate that followed saturation kinetics. Amiloride (10−4 m) reduced the ΔpH dissipation rate. Taken together, these data indicate the presence of Na+ and H+ conductances in addition to an amiloride-sensitive, electroneutral Na+/H+ exchange. An inwardly directed [Cl−] gradient (PD=0) did not induce intravesicular acidification. Therefore, in this preparation, there was no evidence for the presence of a Cl−/OH− exchange. When both [Na+] and [Cl−] gradients (outwardly directed, PD=0) were present, fluorescence quenching reached a maximum 20–30 sec after vesicle injection and then quickly decreased. The decrease was not observed in the presence of a [Na+] gradient alone or the same [Na+] gradient with Cl− at equal concentrations at both sides. Similarly, the decrease was abolished in the presence of both Na+ and Cl− concentration gradients and hydrochlorothiazide (5×10−4 m). The decrease was not influenced by an inhibitor of Cl−/OH− exchange (10−4 m furosemide) or of Na+−K+−2Cl− symport (10−5 m bumetanide). We conclude that a Na+/H+ exchange and a Na+−Cl− symport are present and act simultaneously. This suggests that in intact tissue the Na+−Cl− symport is also likely to work in parallel with the Na+/H+ exchange and does not represent an induced homeostatic reaction of the epithelium when Na+/H+ exchange is inhibited.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01868468
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