IMPORTANCE OF ATP-DEPENDENT H-PUMP FOR STABILIZING ELECTROGENESIS AT PLASMALEMMA IN CUCURBITA-PEPO L. CELLS()

Plasma membrane (PM) vesicles were isolated from stems of 10- to 12-da y-old pumpkin (Cucurbita pepo L.) seedlings. The competence of vesicle s to maintain the membrane potential (Delta psi(m)) was assessed for i ts relevance to the operation of electrogenic H+-pump (H+-ATPase) in t he plasmalemma. The Delta psi(m) was assessed by measuring the changes in 1-anilinonaphthalene-8-sulfonate fluorescence. The proton permeabi lity of PM was assayed with a 9-aminoacridine probe by following the d issipation kinetics of an artificially imposed pH gradient across the vesicular membrane. The K+-diffusion potential (Delta psi(D)) was indu ced by the addition of valinomycin; this potential declined by about 3 0% within a 10-min period. When the ATP-dependent component of membran e potential (Delta psi(p)) was generated by the H+ pump and contribute d to Delta psi(m), the membrane potential Delta psi(m) was maintained at a steady-state level. The level of Delta psi(p) in the absence of D elta psi(D) was also stable. Cooling the vesicles from 24 to 6 degrees C reduced the membrane H+ permeability but did not affect the stabili ty of Delta psi(p) in PM vesicles. It is concluded that the operation of electrogenic ATP-dependent H+ pump at the plasmalemma is important for stabilizing Delta psi(m) at a steady-state level.