Influence of phorbol esters on ionophore-mediated calcium exchange-diffusion in liposomes

The interference of phorbol esters upon the process of A23187-mediated calcium exchange diffusion was examined in multilamellar liposomes formed of different types of lipids and incubated at variable temperatures. Phorbol esters facilitated the process of calcium ionophoresis in liposomes formed of dipalmitoylphosphatidylcholine (DPPC) or dimyristoylphosphatidyl-choline (DMPC) and incubated below transition temperature. The magnitude of this facilitating action was negatively correlated with the tumor-promoting capacity of the phorbol esters. The phorbol esters also facilitated calcium ionophoresis in liposomes formed of a mixture of DPPC and cholesterol, provided that the temperature exceeded 34 degrees C. The magnitude of the latter facilitating action was positively correlated with both the temperature and the tumor-promoting potency of the phorbol esters. Thus, the existence of a parallelism between the biological potency of phorbol esters and their biophysical effect in this artificial system tightly depended on such factors as the lipid composition of the liposomal matrix and the ambient temperature.