Expression and distribution of adenosine diphosphate-ribosylation factors in the rat kidney

Adenosine diphosphate (ADP)-ribosylation factors (ARFs) are small guanosine triphosphatases involved in membrane traffic regulation. Aiming to explore the possible involvement of ARF1 and ARF6 in the reabsorptive properties of the nephron, we evaluated their distribution along the different renal epithelial segments. ARFs were detected by immunofluorescence and immunogold cytochemistry on renal sections, using specific anti-ARF antibodies. ARF1 was detected in proximal and distal tubules, thick ascending limbs of Henle's loops, and cortical and medullary collecting ducts. By immunofluorescence, labeling was mostly localized to the cell cytoplasm, particularly in Golgi areas. By electron microscopy, the Golgi apparatus and the endosomal compartment of proximal and distal tubular cells were labeled. ARF6 immunofluorescence was observed in brush border membranes and the cytoplasm of proximal convoluted tubular cells, whereas it was restricted to the apical border of proximal straight tubules. ARF6 immunogold labeling was detected over microvilli and endocytic compartments of proximal tubular cells. This study demonstrates the following: (a) the heterogeneous distributions of ARF1 and ARF6 along the nephron, (b) the existence of cytosolic and membrane-bound forms for both ARFs, and (c) their association with microvilli and endocytic compartments, suggesting an active participation in renal reabsorption.