Does the Organic Calcium Channel Blocker D600 Act from Inside or Outside on the Cardiac Cell Membrane?

The effects of extra- and intracellularly applied D600 (methoxyverapamil) and D890 (a quarternary derivative) on the action potentials of isolated guinea pig myocytes were compared. We also studied the extracellular myocytes were compared. We also studied the extracellular effects of these drugs on the calcium current (hybride sucrose gap) and contractile force of right ventricular trabeculae of the cat heart. The following results were obtained: 1. In ventricular trabeculae D600 suppressed the calcium current, tension and the plateau of the action potential. In contrast, D890 even in a 50 times higher concentration did not display any effect on these parameters. 2. In single isolated cells external application of D890 did not alter the configuration of the action potential. In contrast, external application of D600 suppressed the plateau and shortened the action potential in a dose-dependent way. 3. Intracellular injection of D600 or D890 strongly lowered the height of the plateau and abbreviated the action potential. The onset of the effects of both drugs was more rapid on intracellular application than that of external D600. Whereas the effect of an intracellular injection of D600 was reversible, that of D890 was not. These results support the hypothesis that the organic calcium channel blocker D600 enters the cell in the uncharged lipid soluble form and reaches its receptor associated with the calcium channel from inside. Because of its inability to pass the hydrophobic cell membrane, D890 is ineffective from outside but displays blocking effects on intracellular application.