Fig. 9
The influence of ABC transporters on the movements of lipophilic substances. The substance is presumed to be able to enter and leave the endothelial cells by diffusion with rate constant kdiff., which for simplicity in this example is assumed to be same on both sides. The substance is expelled from the cell by ABC transporters on the luminal side at a rate, kactiveccell. With these assumptions the effect of the ABC transporters on influx can be calculated by setting cisf = 0 and the effect on efflux by setting cplasma = 0. In both sets of equations, the first line states that at steady-state the net flux into the cell on one side must equal the net flux out of the cell on the other. From the next to the last lines of both sets of equations, if the rate of ABC mediated expulsion from the cell is small or zero, the rate constants for both influx and efflux are (kdiff./2). By contrast from the last lines if the rate of ABC mediated expulsion is large, influx to the brain, \(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\rightarrow}$}}{J}\), becomes very small, while efflux from the brain, \(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\leftarrow}$}}{J}\), is doubled compared to the efflux with no ABC transporter