Ca2+-antagonists inhibit the N-methyltransferase-dependent synthesis of phosphatidylcholine in the heart.

Abstract

Evidence indicates that, in addition to the L-type Ca2+ channel blockade, Ca2+-antagonists target other functions including the Ca2+-pumps. This study was conducted to test the possibility that the reported inhibition of heart sarcolemmal (SL) and sarcoplasmic reticular (SR) Ca2+-pumps by verapamil and diltiazem could be due to drug-induced depression of phosphatidylethanolamine (PE) N-methylation which modulates these Ca2+-transport systems. Three catalytic sites individually responsible for the synthesis of PE monomethyl (site I), dimethyl (site II) and trimethyl (phosphatidylcholine (PC), site III) derivates were examined in SL and SR membranes by employing different concentrations of S-adenosyl-L-methionine (AdoMet). Total methyl group incorporation into SL PE, in vitro, was significantly depressed by 10(-6)-10(-3) M verapamil or diltiazem at site III. The catalytic activity of site I was inhibited by 10(-3) M verapamil only, whereas the site II activity was not affected by these drugs. The inhibition induced by verapamil or diltiazem (10(-5) M) was associated with a depression of the Vmax value without any change in the apparent affinity for AdoMet. Both drugs decreased the SR as well as mitochondrial PE N-methylation at site III. A selective depression of site III activity was also observed in SL isolated from hearts of rats treated with verapamil in vivo. Furthermore, administration of [3H-methyl]-methionine following the treatment of animals with verapamil, reduced the synthesis of PC by N-methyltransferase. Verapamil also depressed the N-methylation-dependent positive inotropic effect induced by methionine in the isolated Langendorff heart. Both agents depressed the SL Ca2+-pump and although diltiazem also inhibited the SR Ca2+-pump, verapamil exerted a stimulatory effect. In addition, verapamil decreased SR Ca2+-release. These results suggest that verapamil and diltiazem alter the cardiac PE N-methyltransferase system. This action is apparently additional to the drugs' effect on L-type Ca2+ channels and may serve as a biochemical mechanism for the drugs' inhibition of the cardiac Ca2+-pumps and altered cardiac function.