EFFECT OF PERFUSATE PH ON THE INFLUX OF 5-5'-DIMETHYL-OXAZOLIDINE-2,4-DIONE AND DISSOCIATION OF EPIDERMAL GROWTH-FACTOR FROM THE CELL-SURFACE RECEPTOR - THE EXISTENCE OF THE PROTON DIFFUSION BARRIER IN THE DISSE SPACE

The influx clearance (PSinf.MID) of the weak acid 5,5'-dimethyl-oxazol idine-2,4-dione (DMO) was determined by the multiple indicator dilutio n method with the isolated perfused rat liver under various perfusate pH conditions, ranging from 6.4 to 7.6. Although the pH partition theo ry predicted an increase in influx clearance of ten times in proportio n to the change in the unionized fraction of DMO, there was no measura ble change in this value. The effect of medium pH on the steady-state cell/medium concentration ratio (C/M) ratio of DMO was also investigat ed using isolated hepatocytes. The C/M ratio increased while medium pH decreased, but this change was less marked than predicted by the pH p artition theory. Finally the pH dependency of the dissociation rate co nstant (k(off)) of epidermal growth factor from its receptor was also investigated using both isolated rat hepatocytes and the perfused rat liver. When the extracellular pH was changed from 6.4 to 5.6, the k(of f) value of isolated hepatocytes increased 44 times, while that of the perfused rat liver increased only 9 times. Therefore, the effect of c hanging the extracellular pH on pH-dependent dissociation of epidermal growth factor from its cell-surface receptor was less in the perfused liver than in isolated hepatocytes. These findings, in addition to th e well-known existence of the Na+-H+ exchanger on the sinusoidal membr ane and the possible existence of the unstirred water layer in the Dis se space, seem to suggest the existence of the proton diffusion barrie r in the rat liver, which remains stronger in the perfused liver than in isolated hepatocytes. (C) Journal of Hepatology.