Insulin uptake across the luminal membrane of the rat proximal tubule in vivo and in vitro.
We visualized insulin uptake in vivo across the apical membrane of the rat proximal tubule (PT) by confocal microscopy; we compared it with in vitro findings in a rat PT cell line (WKPT) using fluorescence microscopy and flow cytometry. Surface tubules were observed in vivo with a 633-nm single laser-illuminated real-time video-rate confocal scanning microscope in upright configuration for optical sectioning below the renal capsule. Fields were selected containing proximal and distal tubules; Cy5-labeled insulin was injected twice (the second time after approximately 140 min) into the right jugular vein, and the fluorescence signal (at 650-670 nm) was recorded. Fluorescence was detected almost immediately at the brush-border membrane (BBM) of PT cells only, moving inside cells within 30-40 min. As a measure of insulin uptake, the ratio of the fluorescence signal after the second injection to the first doubled (ratio: 2.11 +/- 0.26, mean +/- SE, n = 10), indicating a "priming," or stimulating, effect of insulin on its uptake mechanism at the BBM. This effect did not occur after pretreatment with intravenous lysine (ratio: 1.03 +/- 0.07, n = 6; P < 0.01). Cy2- or Cy3-labeled insulin uptake in a PT cell line in vitro was monitored by 488-nm excitation fluorescence microscopy using an inverted microscope. Insulin localized toward the apical membrane of these cells. Semiquantitative analysis of insulin uptake by flow cytometry also demonstrated a priming effect (upregulation) on insulin internalization in the presence of increasing amounts of insulin, as was observed in vivo; moreover, this effect was not seen with, or affected by, the similarly endocytosed ligand beta2-glycoprotein.
Kolman
Pavel
P
Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic.
Pica
Angelo
A
Carvou
Nicolas
N
Boyde
Alan
A
Cockcroft
Shamshad
S
Loesch
Andrew
A
Pizzey
Arnold
A
Simeoni
Mariadelina
M
Capasso
Giovambattista
G
Unwin
Robert J
RJ
D000818:Animals; D002460:Cell Line; D004705:Endocytosis / Q000502:physiology; D004847:Epithelial Cells / Q000166:cytology / Q000378:metabolism / Q000648:ultrastructure; D005434:Flow Cytometry; D007004:Hypoglycemic Agents / Q000493:pharmacokinetics*; D066298:In Vitro Techniques; D007328:Insulin / Q000493:pharmacokinetics*; D007686:Kidney Tubules, Distal / Q000166:cytology / Q000378:metabolism*; D007687:Kidney Tubules, Proximal / Q000166:cytology / Q000378:metabolism*; D008297:Male; D018613:Microscopy, Confocal / Q000379:methods*; D008854:Microscopy, Electron; D008856:Microscopy, Fluorescence / Q000379:methods; D051381:Rats; D011921:Rats, Inbred WKY; D017207:Rats, Sprague-Dawley
D007004:Hypoglycemic Agents; D007328:Insulin