Mechanism of suppression of cardiac L-type Ca2+ currents by the phospholipase A(2) inhibitor mepacrine

Phospholipase A(2) plays a crucial role in the release of arachidonic acid (AA) from membrane phospholipids and in myocardial injury during ischemia a nd reperfusion. Mepacrine, a phospholipase A(2) inhibitor, has been shown t o protect the heart from ischemic injury. In order to examine the mechanism of this protection, we investigated the effects of mepacrine on the L-type Ca2+ current (I-Ca,I-L) in rat single ventricular myocytes. Extracellular application of mepacrine significantly inhibited I-Ca,I-L in a tonic- and u se-dependent manner. The inhibition was also concentration-dependent with a n IC50 of 5.2 mu M. Neither the activation nor the steady-state inactivatio n of I-Ca,I-L was altered by mepacrine. The mepacrine-induced inhibition of I-Ca,I-L was reversible after washout of the inhibitor. Addition of 1 mu M AA partially reversed the mepacrine-induced inhibition of I-Ca,I-L. Intrac ellular dialysis, with 2 mM cAMP, significantly increased I-Ca,I-L but did not prevent the mepacrine-induced inhibition of I-Ca,I-L. In addition, extr acellular application of isoproterenol or membrane permeable db-cAMP did no t reverse the mepacrine-induced inhibition of I-Ca,I-L. Biochemical measure ment revealed that incubation of ventricular myocytes with mepacrine signif icantly reduced intracellular cAMP levels. The mepacrine-induced reduction of cAMP production was abolished by addition of AA. Our results demonstrate that mepacrine strongly inhibits cardiac I-Ca,I-L. While mepacrine is a ph ospholipase A, inhibitor and reduces cAMP production, its inhibitory effect on I-Ca,I-L mainly results from a direct block of the channel. Therefore, we speculate that the protective effect of mepacrine during myocardial isch emia and reperfusion mostly relates to its blockade of Ca2+ channels. (C) 2 000 Elsevier Science B.V. All rights reserved.