Activation of rat serosal mast cells by chymase, an endogenous secretory granule protease

Chymase, the major neutral protease of the rat serosal mast cell (RMC) secretory granule, causes RMC to release their secretory granules and to oxidatively metabolize endogenous arachidonic acid to prostaglandin D2 (PGD2). The granule markers, endogenous beta-hexosaminidase and exogenously added [3H]serotonin, were released from 2.5 X 10(5) RMC in 50 microliters in parallel and in dose-response fashion, reaching a maximum net percent release of approximately 50% with 0.5 to 1.0 units chymase (15 U/mg)/ml. With incremental concentrations of chymase, the release of granule markers occurred with a shorter lag period and in a greater maximal net percent, whereas the release of PGD2 was dose-related without a reduction in latency to detectable generation. Inhibition of the esterase activity of chymase with lima bean trypsin inhibitor decreased the subsequent mast cell response, indicating that the active site of chymase was required to initiate granule secretion and PGD2 generation. The monophasic indomethacin-resistant rise in cellular cAMP at 15 to 45 sec coincident with the onset of chymase-induced mediator release and PGD2 secretion is similar to that observed with IgE receptor-initiated coupled activation-secretion. The ability of heparin to block the activation function of chymase without inhibition of esterase activity reveals a possible physiologic regulatory mechanism for limiting the potential action of secreted chymase.