FLUID-PHASE ENDOCYTOSIS IN THE AMEBAS OF THE CELLULAR SLIME-MOLD DICTYOSTELIUM-DISCOIDEUM - MATHEMATICAL-MODELING OF KINETICS AND PH EVOLUTION

Fluid-phase endocytosis (pinocytosis) is a very active nutritive proce ss in axenic strains of Dictyostelium discoideum amoebae. Fluorescein- labelled dextran was used as a dual fluid-phase and pH marker to chara cterize endocytosis pathway regarding the kinetics of fluid internalis ation and vesicular pH. Continuous loading and pulse-chase experiments have been conducted. In the latter case, marker transit and efflux wa s monitored on cells loaded for short periods of time, washed then sus pended in marker-free medium. To account simultaneously for fluid-phas e endocytosis kinetics and pH evolution we have developed a mathematic al model consisting of interconnected delay stages and compartments. A delay stage is defined as a directional path in which access to the f ollowing endocytic space occurs only after a characteristic transit ti me. In contrast to this behaviour, exit probability from a compartment is independent of the time spent in the compartment. The model descri bes correctly both the kinetics and pH changes in Dictyostelium amoeba endocytic pathway. Useful estimates were obtained for kinetic constan ts, pH and fractional apparent volume of endocytic spaces. This modell ing work provides a theoretical basis on which to design further exper imental studies. It is of general applicability and could be useful to examine the characteristics of fluid-phase endocytosis in other eukar yotes. (C) 1997 Academic Press Limited