Rat peritoneal mast cells, which had been sensitized two days earlier by an intraperitoneal injection of rat monoclonal IgE antibodies, were purified by density gradient centrifugation with 60 % Percoll (cell purity >95 %). Histamine release from the purified mast cells (PMC) was then compared to that of a non-purified preparation (peritoneal exudate cells; PEC). Both PEC and PMC released similar amounts of histamine upon stimulation with calcium ionophore A23187 and compound 48/80. In contrast, antigen-induced histamine release from PMC was very low compared to that of PEC. PEC released up to 30 % of total histamine upon challenge with 1 mug/ml of antigen, whereas histamine release from PMC was only one third or less than that of PEC. When PEC was suspended in 60 % Percoll and treated for a period needed for purification, the reduction of antigen-induced histamine release was negligible. Mast cells purified by centrifugation on a metrizamide gradient released only small amount of histamine similar to Percoll-purified mast cells. Non-mast cells (NMC) recovered from the interface of the 60 % Percoll potentiated the antigen-induced histamine release from PMC concentration- and time-dependently. The supernatant of the NMC suspension which was incubated at 37-degrees-C for 60 min, however, failed to potentiate histamine release in PMC. We concluded therefore that separation media such as Percoll and metrizamide do not cause the low antigen-induced histamine release in PMC, but that the separation of mast cells from other cells present in the peritoneal cavity itself causes it. Antigen-induced mast cell histamine release is potentiated through a direct interaction between mast cells and NMC, and some cell surface molecules also seem to be involved.
