Plasticity of functional epithelial polarity

Proton pump activity and Mg-ATPase activity in rat kidney cortex brushborder membranes: effect of 'proton ATPase' inhibitors

In order to further characterize the ATP driven proton pump present in the luminal membrane of the renal proximal tubule, brushborder membranes were isolated from rat kidney cortex and the effect of various proton ATPase inhibitors on intravesicular ATP hydrolysis in sealed brushborder membrane vesicles and on Mg-ATPase activity in permeabilized brushborder membranes was investigated. The protonophor induced intravesicular ATP hydrolysis (ATP driven proton pump) was inhibited by DCCD and filipin but not by diethylstilbestrol and duramycin. All four compounds decreased Mg-ATPase activity, the two former inhibited the ATPase activity with a lower potency than the proton pump. NEM--up to 10 mM--and orthovanadate did not affect intravesicular ATP hydrolysis nor Mg-ATPase activity. From the relative sensitivity of the proton pump and the Mg-ATPase activity to the inhibitors it is concluded that about 35% of the Mg-ATPase activity found in the brushborder membrane can be attributed to the ATP-driven proton pump. Furthermore, the results obtained with NEM and duramycin suggest that the brushborder membrane proton pump has different properties than the proton pump in clathrin-coated vesicles or endosomes. The results presented above raise the possibility that the brushborder membrane proton pump is predominantly involved in acid secretion by the proximal convoluted tubule whereas the proton pump in clathrin-coated vesicles may be predominantly involved in the endocytosis of larger peptides and proteins.