Changes in (Na+ + K+)-ATPase activity associated with stimulation of thymocytes by concanavalin A

Preincubation of rabbit thymocytes and lymph node cells for 20 h in 10% rabbit serum and subsequent exposure of these cells to concanavalin A resulted in a 70-100% increase in (Na+ + K+)-ATPase activity. The (Na+ + K+)-ATPase activity of rabbit lymph node cells and spleen lymphocytes was increased while that of erythrocytes did not show any response to concanavalin A exposure. Since only inhibition (50%) was observed when crude microsomes from rabbit thymocytes were incubated with concanavalin A, activation of (Na+ + K+)-ATPase appeared to require intact cells. Incubation of rabbit thymocytes in 10% rabbit serum for 20 h resulted in slightly increased fluidity of the innermost lipids of the cell membranes, as measured by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene. Fluidity changes did not appear to be the mechanism of activation of ATPase by concanavalin A, since incubation of cells with concanavalin A for 1 h had no effect on membrane fluidity in either freshly isolated or 20-h preincubated thymocytes. Compared with fresh thymocytes, preincubated cells had about twice the number of concanavalin A binding sites. Preincubated thymocytes became temporarily permeable to trypan blue after concanavalin A exposure. The percentage of cells taking up dye reached a maximum of 65% at 45-60 min, but decreased to 18% after a total of 3 h of incubation.