Structural determinants of phorbol ester binding in synaptosomes: pharmacokinetics and pharmacodynamics

Abstract

The present study used structurally distinct phorbol esters to investigate the relationship between their pharmacokinetics of binding to protein kinase C (PKC) in rat brain cortex synaptosomes, their affinity for PKC in synaptosomes and ability to enhance noradrenaline release from rat brain cortex. Affinity binding studies using [ 3H]phorbol 12,13-dibutyrate (PDB) yielded a rank order of potency for the phorbol esters in binding to synaptosomal PKC of phorbol 12-myristate-13-acetate (PMA)>deoxyphorbol 13-tetradecanoate (dPT)=PDB≫12-deoxyphorbol 13-acetate (dPA)=phorbol 12,13-diacetate (PDA). In intact synaptosomes PDB, dPA and PDA rapidly displaced bound [ 3H]PDB whereas PMA and dPT were comparatively slow. However, the displacement rates for all the phorbol esters were equally rapid in synaptosomal membranes or synaptosomes permeabilised with Staphylococcus α-toxin. These results suggest that the lipophilic phorbol esters (dPT and PMA) are slower to displace [ 3H]PDB binding because they are hindered by the plasma membrane. In brain cortex slices it was found that the rate of displacement of [ 3H]PDB binding was closely correlated with the degree of elevation of transmitter noradrenaline release. Thus kinetic characteristics may determine biological responses and this may be particularly evident in events which occur rapidly or where there is fast counter-regulation.