Tubular GFP uptake pattern in the rat and frog kidneys

The renal tubular uptake of green fluorescent protein (GFP) after its bolus intravenous injection was studied in both frogs and rats. GFP fluorescence in the proximal tubule (PT) was revealed by fluorescent and confocal microscopy. Granular GFP fluorescence was observed nearby in the apical membrane of PT cells featuring distribution over the cytoplasm. GFP was internalized into endosomes and lysosomes as determined by immunocytochemistry in frogs. The tubular uptake and accumulation of GFP were dose- and time-dependent in both rats and frogs. Intralymphatic sac injection of arginine vasotocin (AVT) decreased the uptake of GFP in hydrated frogs. A high negative correlation between the AVT dose and the uptake of GFP was revealed. The effect of AVT was inhibited by a V(1)-receptor antagonist. A noted decrease in the average number of fluorescent PT profiles per kidney section and their irregular distribution after AVT injections suggest that not all of the glomeruli or preglomerular vessels are equally responsive to AVT. GFP may serve as a good marker for tubular uptake and intracellular traffic in the amphibian kidney for use in in vivo studies.