HOW DOES THE CONTRACTILE VACUOLE OF PARAMECIUM-MULTIMICRONUCLEATUM EXPEL FLUID - MODELING THE EXPULSION MECHANISM

To examine the forces needed for discharge of the fluid contents from the contractile vacuole of Paramecium multimicronucleatum, the time co urse of the decrease in vacuole diameter during systole (the fluid-dis charging period) was compared with that of various vacuole discharge m odels. The observed time course did not fit that predicted by a model in which contraction of an actin-myosin network surrounding the vacuol e caused discharge nor that predicted by a model in which the surface tension of the lipid bilayer of the vacuole caused discharge. Rather, it fitted that predicted by a model in which the cell's cytosolic pres sure was responsible for discharge. Cytochalasin B, an effective inhib itor of actin polymerization, had no effect on the in vivo time course of systole, An injection of a monoclonal antibody raised against the proton pumps of the decorated spongiomes (now known to be the locus of fluid segregation in P. multimicronucleatum) disrupted the decorated spongiomes and reduced the rate of fluid segregation, whereas it did n ot alter the time course of systole. We conclude that in P. multimicro nucleatum the internal pressure of the contractile vacuole is caused p redominantly by the cytosolic pressure and that the fluid-segregation mechanism does not directly affect the fluid-discharge mechanism. Elim ination of this cytosolic pressure by rupturing the cell revealed the presence of a novel fluid-discharge mechanism, apparently centered in the vacuole membrane. The involvement of tubulation of the vacuole mem brane as the force-generating mechanism for fluid discharge in disrupt ed cells is discussed.