Phospholipase D in heart: basal activity and stimulation by phorbol esters and aluminum fluoride

Abstract

Evidence for a general role of phospholipase D in signal transduction is accumulating. In the present study, the activity of the enzyme was investigated in heart tissue under basal conditions and after addition of phorbol esters or aluminum fluoride (AlF-4; 10 mM NaF plus 10 microM AlCl3). Atria of rats and chickens were incubated with [3H]-myristic acid in order to label preferentially phosphatidylcholine. Under basal conditions, the tissues generated choline and phosphatidic acid (PtdOH), the primary catalytic products of phospholipase D. When 0.5 or 2.0% ethanol was present, [3H]-phosphatidylethanol (PETH) was rapidly formed at the expense of [3H]-PtdOH. This transphosphatidylation reaction is specific for phospholipase D activity. The basal formation of PETH was not inhibited by a Ca(2+)-free, EGTA-containing medium. The phorbol ester 4 beta-phorbol-12 beta, 13 alpha-dibutyrate (PDB), which is known to activate protein kinase C, enhanced the net formation of choline, whereas the inactive 4 beta-phorbol-13 alpha-acetate (PAc) was ineffective. PDB (0.2 microM), in contrast to PAc, also increased the formation of [3H]-PtdOH and, in the presence of ethanol, of [3H]-PETH. The PDB-evoked formation of PETH occurred again at the expense of PtdOH. Treshold and maximum effective concentrations of PDB were 10 nM and 0.2-0.6 microM, respectively. The effects of PDB on either choline efflux and generation of PETH showed the same Ca(2+)-dependency, i.e., both effects were blocked by a Ca(2+)-free, EGTA-containing medium, but not by a Ca(2+)-free medium without EGTA.(ABSTRACT TRUNCATED AT 250 WORDS)