Since pinocytosis has only been recently recognized in polymorphonuclear leukocytes (PMNs), little is known about the fate of pinosomes. Here we report that pinosomes can fuse with the cytoplasmic granules of PMNs. We also find that at least for a short period of time after formation, pinosomes can fuse with the plasma membrane and release their contents to the outside. We present a morphological description and biochemical data on the kinetic parameters of a steady state pool of reversible pinosomes in PMNs. In addition, we have developed conditions under which pinosomes continue to form and fuse with the plasma membrane but fail to fuse with the cytoplasmic granules, i.e., only "reversible" pinocytosis occurs. This inhibition of fusion with the granules is not due to an inability of the pinosomes to move from the surface since under these conditions pinosomes labeled with an electron-dense marker can be seen in the cell interior.

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