Fluid‐phase endocytosis by isolated rat adipocytes

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Abstract
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/106 cells‐h at 37°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 < 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.

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