Mechanism of suppression of cardiac L-type Ca(2+) 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 and reperfusion. Mepacrine, a phospholipase A(2) inhibitor, has been shown to 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 Ca(2+) current (I(Ca,L)) in rat single ventricular myocytes. Extracellular application of mepacrine significantly inhibited I(Ca,L) in a tonic- and use-dependent manner. The inhibition was also concentration-dependent with an IC(50) of 5.2 microM. Neither the activation nor the steady-state inactivation of I(Ca,L) was altered by mepacrine. The mepacrine-induced inhibition of I(Ca,L) was reversible after washout of the inhibitor. Addition of 1 microM AA partially reversed the mepacrine-induced inhibition of I(Ca,L). Intracellular dialysis, with 2 mM cAMP, significantly increased I(Ca, L), but did not prevent the mepacrine-induced inhibition of I(Ca,L). In addition, extracellular application of isoproterenol or membrane permeable db-cAMP did not reverse the mepacrine-induced inhibition of I(Ca,L). Biochemical measurement revealed that incubation of ventricular myocytes with mepacrine significantly 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,L). While mepacrine is a phospholipase A(2) inhibitor and reduces cAMP production, its inhibitory effect on I(Ca,L) mainly results from a direct block of the channel. Therefore, we speculate that the protective effect of mepacrine during myocardial ischemia and reperfusion mostly relates to its blockade of Ca(2+) channels.