PLASMA-MEMBRANE H-ATPASE ACTIVITY IN RESPONSE TO MECHANICAL STIMULATION OF BRYONIA-DIOICA INTERNODES()

Mechanical stimulation which reduced elongation of young Bryonia dioic a internodes readily induced a short-lived decrease in ATPase activity followed by a more prolonged stimulation. ATP-dependent proton transp ort measured as decrease in acridine orange absorbance in plasma membr ane vesicles also displayed an early and transient decline by 93% 10 m in after rubbing and a stimulation lasting for at least one hour. Thes e two phases were closely correlated with changes in the apparent K-m for MgATP attesting an initial reduction of the enzyme's affinity for its substrate rapidly followed by an increase. The variations in ATPas e activity were also accompanied by changes in the pool of free ATP wh ich showed a depletion after a temporary rise. The results indicate th at the plasma membrane H+-ATPase might play a key role in signal trans duction by its contribution to the ionic disequilibrium with cessation of activity and by its subsequent direct participation in the restora tion of ionic balances. The data are discussed in relation to calcium compartmentation, cytoplasmic pH regulation and the control of plasma membrane enzyme activities by the lipid environment.