Relationship between the stereoselective negative inotropic effects of verapamil enantiomers and their binding to putative calcium channels in human heart

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Abstract
Ventricular preparations from patients with mitral disease and hypertrophic obstructive cardiomyopathy (HOCM) were set up to contract isometrically. Ventricular membrane particles were also prepared and putative calcium channels were labelled with [3H]‐nimodipine. Positive staircase was induced by varying the rate of stimulation of isolated strips from 6 min−1 to 120 min−1 in the presence of 6–60 μm (—)‐adrenaline or (—)‐noradrenaline. (—)‐Verapamil 3–5 μm or (+)‐verapamil 20–30 μm reversed the force‐frequency relationship (i.e. caused negative staircase) in preparations from patients with mitral disease or HOCM. In subendocardial strips of ventricular septum from 5 patients with HOCM paced at 60 min−1, both (—)‐verapamil and (+)‐verapamil caused cardiodepression. Half‐maximal cardiodepression was observed with 0.4 μm (—)‐verapamil and with 3 μm (+)‐verapamil. [3H]‐nimodipine bound to ventricular membrane particles in a saturable, reversible fashion to a high affinity site with an equilibrium dissociation constant of 0.23 nm. The density of these sites was 95 fmol mg−1 of membrane protein. Binding of the tritiated 1,4‐dihydropyridine was stereoselectively inhibited by 1,4‐dihydropyridine enantiomers and nifedipine. (—)‐Verapamil and (+)‐verapamil inhibited high affinity [3H]‐nimodipine binding in a negative heterotropic allosteric manner with (—)‐verapamil being 5 times more potent than (+)‐verapamil on an IC50 basis. At a given [3H]‐nimodipine concentration, (+)‐verapamil inhibited a greater fraction of specific [3H]‐nimodipine binding. The allosteric mode of (+)‐verapamil inhibition of [3H]‐nimodipine binding was confirmed by kinetic studies. (—)‐Verapamil shifted (+)‐verapamil‐binding inhibition curves to the right in an apparently competitive fashion. The inversion of staircase caused by both verapamil enantiomers suggests that they cause a use‐dependent channel blockade. The similar potency ratios for binding and for cardiodepression are indicative of a common locus of action for both verapamil enantiomers within the calcium channel.

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