Mathematical modeling of marker influx and efflux in cells

The tumor promoter, phorbol 12-myristate 13-acetate (PMA), affects the processing of fluid that enters a cell from the ambient medium. Previous work showed that marker accumulates to a higher level in PMA-treated than in untreated cells. Since PMA also affects the physical activity of the membrane and stimulates the normal process of taking up extracellular fluid, called endocytosis, it is important to learn whether the perturbations in fluid processing can be attributed entirely to a change in the cell's limiting membrane. To this end, a model for fluid uptake and processing was developed and applied to experiments in which a marker for extracellular fluid was added to cells. From previous work on marker accumulation, it was deduced that there were at least two functional compartments involved in fluid movement. Compartment I is a rapidly filling and rapidly recycling compartment. Compartment II is a slowly filling and emptying compartment. Three routes of vesicle traffic must be considered, one mediating influx from the ambient medium into compartment I, a second, efflux from compartment I to the medium, and a third efflux from compartment I into compartment II. Using earlier models for processing, workers found it difficult to estimate rates of movement through either of the latter routes, as well as the volume of compartment I. The difficulty arises from the fact that only one kinetic constant can be estimated directly from data, namely the instantaneous uptake rate. The remaining data depend on measuring the total mass of marker in the cells. Since the concentration of marker in the cell changes continuously, it is advantageous to employ differential equations to simulate the tracer movement. By applying the model to experimental values, we found estimates for all three rates of fluid movement and the volume of compartment I. It is thought that the model will enable us to determine whether apparent alterations in the time course of uptake arise solely from altered properties of the limiting membrane.