Fluid-phase endocytosis by isolated rat adipocytes.
We have developed an assay, which uses radiolabeled sucrose as the marker, to measure the rate of fluid-phase endocytosis in isolated rat adipocytes. In addition, the assay was adapted to allow measurement of the release of sucrose from previously loaded cells (fluid-phase exocytosis). Adipocytes take up sucrose at an approximately linear rate for at least 1.5 hours. A portion of the pinocytosed sucrose is rapidly (half-time about 20 minutes) returned to the medium. The minimal value for fluid uptake by endocytosis is 57 nl/10(6) cells-h at 37 degrees C; this value corresponds to the formation of 110,000 endocytic vesicles of 100-nm diameter per cell per hour and the internalization of about 20% of the plasma membrane per hour. Insulin caused a small and variable increase in the rate of sucrose uptake. The average increase of 31% from 11 experiments is statistically significant at the level of P less than 0.01. A small insulin effect upon the uptake of the calcium complex of [14C]EDTA was also observed. Since this complex was taken up at 2.5 times the rate of sucrose, it probably entered by a combination of fluid-phase and adsorptive pinocytosis. Insulin did not elicit a significant change in the rate of sucrose release from preloaded cells.
D000273:Adipose Tissue / Q000166:cytology / Q000378:metabolism*; D000818:Animals; D002250:Carbon Radioisotopes; D002478:Cells, Cultured; D004705:Endocytosis / Q000187:drug effects; D005089:Exocytosis / Q000187:drug effects; D007328:Insulin / Q000494:pharmacology; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D013395:Sucrose / Q000378:metabolism
D002250:Carbon Radioisotopes; D007328:Insulin; D013395:Sucrose