Abstract
CO2 directly stimulates H+ secretion, possibly by causing an increase in the number of H+ pumps in the luminal membrane. In order to test this hypothesis we perfused CD and PST on the stage of an inverted epifluorescence microscope with fluorescein isothiocyanate dextran (mw 70,000) in CO2-free medium. Uptake of this substance can only occur by endocytosis. After wash out we noted punctate fluorescence in endocytic vesicles in some CD and in all PST cells. More cells took up fluorescent dextran in medullary than in cortical CD, consistent with the greater rate of H+ secretion in medullary CD. Using the pH dependence of the excitation spectrum of the fluorescence we found the pH of the vesicles to be acid (c. pH 6). Addition of NH3 or nigericin increased vesicular pH by 0.6 ± 0.2 units, suggesting that the acidity of the vesicles was due to H+ pumps. CO2 added isohydrically to the medium reduced fluorescence intensity by 25±7% in CD and 29 ± 5% in PST. Since this effect was prevented by colchicine added to the bath, we believe that CO2 causes a decrease in cytoplasmic fluorescence by stimulating exocytic fusion of the vesicles. We conclude that some cells in CD and all cells in PST incorporate fluorescent dextran into the apical cytoplasmic vesicles and acidify them with H+ pumps. CO2 causes fusion of these vesicles with the luminal membrane and thus stimulates H+ secretion, at least in part, by increasing the number of functioning H+ pumps.
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Schwartz, G., Al-Awqati, Q. CO2 CAUSES EXOCYTOSIS OF VESICLES CONTAINING H* PUMPS IN ISOLATED SEGMENTS OF COLLECTING DUCT (CD) AND PROXIMAL STRAIGHT TUBULE (PST). Pediatr Res 18 (Suppl 4), 369 (1984). https://doi.org/10.1203/00006450-198404001-01657
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DOI: https://doi.org/10.1203/00006450-198404001-01657