Hyposmotic swelling leads to the expression of trypsin-like activity by rat peritoneal fluid mast cells.

Rat peritoneal fluid mast cell present parallel increases in cell area (swelling), and in hydrolytic activity on the trypsin substrate p-tosyl arginine methyl ester (TAME), when placed in Tris buffers of concentrations between 0.15 and 0.03 M. Under these conditions, cells do not degranulate and preserve their trypsin-like enzyme activity after low speed centrifugation. Exposure to more dilute Tris buffers, between 0.015 and 0.003 M, leads to cell rupture accompanied by progressive degranulation and loss of activity on TAME. Protamine, a heparin antagonist prevented this loss when added to mast cells prior to hyposmotic lysis, or lysis by sonication or repeated periods of freezing and thawing. Enzyme activity released in the presence of protamine was fully recovered in supernates of cell lysates submitted to low speed centrifugation. Controlled swelling of mast cells propitiates the expression of trypsin-like activity, possibly by facilitating enzyme-substrate interaction. Cell lysis on the contrary, leads to inactivation of such activity, possibly by enzyme binding to heparin in exposed mast cell granules.