Internalization and apical to basolateral transport of folate in
rat kidney proximal tubule.
Birn, Henrik, S[phi]ren Nielsen, and Erik Ils[phi] Christensen.
Department of Cell Biology, Institute of Anatomy, University of
Aarhus, DK-8000 Aarhus C, Denmark
APStracts 3:0183F, 1996.
Folate derivatives are filtered in the glomeruli and reabsorbed within
the nephron. The amount filtered largely exceeds the minimum daily
requirements. Thus, folate reabsorbed within the kidney must be
returned to the circulation. To establish whether renal proximal
tubule can accomplish this by transport of 3H-folate across the cell,
microperfusion of rabbit proximal tubule with 3H-folate and 14C
-inulin was performed. Transtubular transport of 3H-folate was 5+/-1%
(=0.25 +/-0.07 fmol x min-1) of perfused amount per mm tubule and
remained constant during a 2 hour perfusion period. An accumulation
of 15+/-4% (=0.8+/-0.3 fmol x min-1) of perfused amount per mm tubule
was observed during the same period. Furthermore, to determine
whether endocytosis may be involved in the initial process of folate
uptake in proximal tubule cells we performed light microscopy
autoradiography on cryosections of rat kidney cortex incubated with
3H-folate. Folate binding sites were located apically as well as
intracellularly similar to the location of 3H-folate when injected
into the abdominal aorta and visualized by light microscopy
autoradiography. Thus, folate binding sites as well as internalized
folate is localized both apically and intracellularly. Micropuncture
of rat proximal tubules with folate-coupled collodial gold particles
showed significantly increased endocytosis of folate-gold when
evaluated quantitatively and compared to controls injected with non
coupled gold particles (0.22+/-0.08 vs. 0.03+/-0.01 gold particles
per [mu]m2 tubule cell). The results show that kidney proximal tubule
cells are capable of transcellular transport of 3H-folate with
limited capacity. Folate gold particle uptake suggests that folate
can be internalized by endocytosis.
Received 24 January 1996; accepted in final form 19 September
1996.
APS Manuscript Number F27-6.
Article publication pending Am. J. Physiol. (Renal Fluid Electrolyte
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996