The effects of phorbol esters on choline phospholipid hydrolysis in heart and brain

The efflux of choline was determined in rat striatal slices, incubated chicken atria and perfused chicken hearts. 4 beta-Phorbol-12 beta,13 alpha-dibutyrate (PDB) and 4 beta-phorbol-12 beta-myristate, 13 alpha-acetate (PMA) were used to stimulate protein kinase C. The other phorbol esters, 4 beta-phorbol-13 alpha-acetate (PAc) and 4 alpha-phorbol-12 beta,13 alpha-didecanoate (4 alpha PDD), known to be inactive, were tested to evaluate the specificity of the responses. PDB markedly enhanced the efflux of choline in all of the three preparations. The PDB-evoked efflux of choline in incubated chicken atria was equal to the net production of choline and, therefore, was not caused by translocation of intracellular free choline. After inhibition of the cholinesterase activity, PDB linearly increased the efflux of choline in rat striatal slices, but failed to alter the spontaneous efflux of acetylcholine. Thus acetylcholine did not serve as the source of the PDB-evoked efflux of choline. PMA was as effective as PDB, whereas PAc and 4 alpha PDD failed to alter the choline efflux in the perfused heart. Both infusion of a Ca2(+)-free EGTA-containing Tyrode solution and mepacrine reduced the spontaneous efflux of choline by about 40% and blocked the PDB-evoked efflux of choline. In contrast, a Ca2(+)-free solution without EGTA failed to alter the spontaneous and the PDB-evoked choline efflux. It is concluded that phorbol esters stimulate the hydrolysis of choline-containing phospholipids in heart and brain via activation of protein kinase C.