Abstract
Stimulation of postconfluent Swiss 3T3 cells in serum‐free medium with 4.3 mM Ca^2+^ results in marked increases in both released and cell‐associated plasminogen activator (PA). Increased release of PA commenced approximately 10 to 12 hours post‐stimulation and continued to increase steadily until 48 hours at which time the stimulated cells (4.3 mM Ca^2+^) released approximately 14 times more PA than control cells (1.8 mM Ca^2+^). Sr^2+^, like Ca^2+^, also stimulates PA synthesis/release either in the presence or in the absence of 1.8 mM Ca^2+^ whereas an excess of Mg^2+^ inhibits Ca^2+^ stimulation. Supranormal [Pi] in the medium stimulates PA synthesis/release in the presence of 1.8 mM Ca^2+^. Further, optimal stimulation by 4.3 mM Ca^2+^ requires a normal level of Pi (1.0 mM). Elevation of medium [Ca^2+^] or [Pi] results in an enhanced uptake of Ca^2+^. The facts that cycloheximide treatment completely abolishes the Ca^2+^ stimulatory effect and that an increase in cell associated PA precedes release indicate that PA release is coupled to synthesis of new PA. Ca^2+^ stimulation of PA synthesis/release also requires continuous energy production and RNA as well as protein synthesis. A hypothesis is proposed to explain the relationship between stimulation of PA production and its enhanced release from cells stimulated by elevated [Ca^2+^] or [Pi] in the media. The possibility that PA release may be an example of the phenomenon of membrane shedding as opposed to secretion is discussed.