The membrane attack complex of complement induces permeability changes via thresholds in individual cells.

Flow cytometry was used to quantify the fluorescence of propidium iodide in rat polymorphonuclear leucocytes (PMN) attacked by the membrane attack complex (MAC) in order to establish the existence of permeability and lytic thresholds in individual cells, a 'threshold' being defined as a cellular event involving the rapid transition of cells from one state to another under physiological conditions. Activation of the complement pathway resulted in PMN being attacked by MAC within 5 min. Approximately 30-40% of the cell population subsequently became permeable to small molecules and macromolecules. Individual PMN passed through 'thresholds' of cell permeability and cell lysis, or recovered from complement attack at different times. In the flow cytometer, three distinct populations of PMN were identified: cells that had recovered before the permeability 'threshold', cells that had recovered after the permeability 'threshold' but before the lytic 'threshold', and cells that failed to recover from complement attack. Individual PMN attacked by MAC passed through permeability and lytic thresholds at different times after an initial lag of 7.5 +/- 2.5 min and 11.5 +/- 1.0 min, respectively. Adenosine, an activator of adenylate cyclase, inhibited removal of MAC from the cell surface. Consequently, more cells passed through the permeability and lytic 'thresholds', resulting in an increased percentage of lysed cells.