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

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.

Mode of action of (‐)‐pindolol on feline and human myocardium

“…Data of Clark et al (1982) support our explanation because the dependence on (-)-pindolol concentration of increases in heart rate and myocardial contractile force is similar in spinal cats. An in vitro observation of Ferry et al (1985) supports the staircase concept in man. They increased the pacing rate of isolated ventricular preparations of man between 12 and 120 contractions per minute and observed a proportional increase in contractile force with pacing frequency.…”

“…Membrane particles were prepared as described by Kaumann & Birnbaumer (1974), as validated for human cardiac tissues Ferry et al, 1985;Gille et al, 1985). The protein content was determined by the method of Lowry et al (1951) using bovine serum albumin as a standard.…”

The mechanism of action of the optical enantiomers of verapamil against ischaemia‐induced arrhythmias in the conscious rat

“…Since hepatic clearance of (-)-verapamil was much greater than that of( + )-verapamil, then, in rats, the more rapid metabolic rate may serve to reduce the potency ratio in terms of dose. In human tissue the potency ratio of the enantiomers in vivo for prolonging PR interval, in terms of unbound plasma concentration (Echizen et al, 1985), and the negative inotropic potency ratio in vitro (Ferry et al, 1985) have been found to be 4.5 and 8 in favour of (-)-verapamil, respectively. These values correspond almost exactly with the antiarrhythmic and in vitro negative inotropic potency ratios in raised K+ solution, respectively, described here.…”

“…To test this hypothesis, we have compared the antiarrhythmic actions of the optical enantiomers of verapamil. The hypothesis predicts an antiarrhythmic potency ratio equal to the calcium antagonist potency ratio in the ventricular myocardium, based on the reported potency difference between the enantiomers for calcium antagonism (Bayer et al, 1975;Nawrath et al, 1981;Ferry et al, 1985).…”

Pharmacologically Active Drug Metabolites: Impact on Drug Discovery and Pharmacotherapy

“…61). The metabolite has been shown to be active in vivo and an estimate of its intrinsic potency at the Ca 2+ channel places it approximately 5-fold less active (Ferry et al, 1985;Johnson et al, 1991). It circulates at a greater concentration than parent and is slightly less protein-bound in plasma (Yong et al, 1980;Powell et al, 1988), thus it can be estimated that it contributes about a third of the activity of the parent drug.…”