Accumulation of propranolol in cultured rat fibroblasts

In order to clarify the involvement of phosphatidylserine (PhS) in the cellular accumulation of propranolol, we have characterized the binding of 3H-propranolol to cultured rat fibroblasts and to liposomes containing PhS. The properties of propranolol binding to the cells and liposomes were analyzed by means of a Scatchard plot. The cells contained at least two classes of propranolol binding sites, one site of high affinity/low capacity and the second site of lower affinity/higher capacity, while the liposomes contained only one class of binding site. The values of the association constant (K) and number of binding sites (n), given on a PhS basis for the propranolol binding site in the liposomes, were both very close to those of corresponding binding parameters for the high affinity/low capacity binding site in the cells. Cell death, caused by various toxic reagents, resulted in a marked decrease in propranolol accumulation in the cells. Kinetic analysis of the drug binding to dead cells showed one binding site with binding parameters comparable to those of the low affinity/high capacity binding site in the intact cells. Polar cations, methylamine and NH4Cl, completely inhibited propranolol binding to the liposomes. On the other hand, these cations partially inhibited propranolol accumulation in intact cells and failed to inhibit the drug binding to dead cells. These results suggest that PhS in cytomembranes represents the high affinity/low capacity propranolol binding site in cultured rat fibroblasts.