The morphology and behavior of polymorphonuclear leukocytes (PMNs) were studied after rapid changes in the concentration of a chemotactic factor N-formylnorleucylleucylphenylalanine (f-NorleuLeuPhe) (Schiffmann et al., 1975, Proc. Natl. Acad. Sci. U. S. A. 72:1059--1062). After an increase in peptide concentration, the cells round, form lamellipodia or ruffles over most of their surface, and stop locomotion. These changes are transient. After a delay, the cells, still in the presence of peptide, withdraw most of the ruffles and resume locomotion, forming ruffles only at their front. Cells repeat the transient generalized ruffling upon further increase in peptide concentration. The behavioral changes occur over the same dose range as binding to a saturable receptor. The duration of the transient response after a concentration increase is roughly proportional to the increase in the number of cell receptors occupied as a result of the concentration change. Decreasing the concentration of peptide causes the cells to round transiently and form blebs before they recommence locomotion. The transient nature of these aspects of the cell's responsiveness to chemotactic factors appears to be due to adaptation by the cells. The ability to adapt to the concentration of a chemotactic factor may be important in leukocyte chemotaxis.

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